The Oceans

The Blue Planet

Seen from a million kilometres away, Earth is a blue marble with a few smudges of brown and white. The World Ocean covers about 71 per cent of the surface, some 361 million square kilometres, and holds roughly 97 per cent of all the planet's water. Its average depth is around 3,700 metres and its volume about 1.33 billion cubic kilometres. Everything else is small change by comparison: ice caps and glaciers hold under 2 per cent, groundwater a similar share, and all the lakes, rivers and clouds together a sliver of 1 per cent.

Why blue? Water is not quite colourless. Its molecules faintly absorb red light, so a few metres of it tint white sunlight towards blue, and the deeper the water, the bluer the light that scatters back out. Reflected sky adds to the effect towards the horizon. Where the sea turns green or brown, something is mixed in: phytoplankton with their chlorophyll, or silt from rivers such as the Yangtze and the Amazon.

We have seen the ocean whole only recently. In 1968 the crew of Apollo 8 photographed Earthrise over the Moon; in December 1972 Apollo 17 took The Blue Marble, one of the most reproduced photographs in history, with Africa and the Antarctic ice cap framed by sea. In 1990 Voyager 1 looked back from about 6 billion kilometres and caught the planet as Carl Sagan's "pale blue dot". Today satellites such as Sentinel-6 and SWOT measure the height of the sea surface to within centimetres, while nearly 4,000 Argo floats drift through the depths, reporting temperature and salinity.

The water is spread unevenly. The Southern Hemisphere is around 81 per cent ocean, and a globe turned to centre on the South Pacific shows almost nothing but sea. The Pacific alone is larger than all the land put together. Yet for all its size the ocean is thin: an average of 3.7 kilometres on a planet 12,742 kilometres across, proportionally thinner than the skin on an apple.

Did you know? Gathered into a single drop, all the water on, in and above Earth would make a ball only about 1,385 kilometres across, far smaller than the Moon.

That thin film runs the planet. Since the 1970s the ocean has absorbed around 90 per cent of the extra heat trapped by greenhouse gases, and it takes up roughly a quarter of the carbon dioxide we emit. Its plankton make about half of the oxygen produced on Earth, and its slow currents steady the climate. We live on land, but in every way that matters we live on a water planet.

Where the Water Came From

Earth formed about 4.54 billion years ago from dust and rock circling the young Sun, close enough that water should have struggled to survive as ice. So where did an ocean's worth of it come from? Two broad answers compete, and the evidence increasingly says both played a part. Water may have been locked inside the rocky building blocks and outgassed by volcanoes as the planet heated, or it may have been delivered by water-rich asteroids and comets falling in from the cold outer Solar System.

The key witness is hydrogen's heavy twin, deuterium. The ratio of deuterium to ordinary hydrogen, the D/H ratio, works as a fingerprint for where water formed: the colder the birthplace, the more deuterium it tends to hold. Earth's oceans contain about 156 deuterium atoms per million hydrogen atoms. Carbonaceous chondrites, meteorites that come from asteroids, match that figure closely. Most comets do not. Halley's Comet and Hale-Bopp carry around twice as much deuterium, and in 2014 the Rosetta spacecraft found that comet 67P/Churyumov–Gerasimenko has more than three times Earth's ratio. Comet Hartley 2, measured by the Herschel space telescope in 2011, was an Earth-like exception, so comets are not ruled out, but they look like minor contributors.

Sample-return missions have sharpened the picture. Japan's Hayabusa2 brought back grains of the asteroid Ryugu in 2020, and NASA's OSIRIS-REx delivered dust from Bennu in 2023; both contain clays and other minerals that formed in liquid water. Meanwhile a 2020 study of enstatite chondrites, meteorites thought to resemble the rock of the inner Solar System, found they hold enough hydrogen to have supplied several oceans' worth of water from the very start.

The timing comes from tiny crystals. Zircons from the Jack Hills of Western Australia, some about 4.4 billion years old, carry oxygen isotope ratios suggesting they grew from rock that had been in contact with liquid water near the surface. That means oceans existed within roughly 150 million years of Earth's formation, and after the giant impact thought to have made the Moon. By around 3.8 billion years ago, pillow lavas in the Isua belt of Greenland show lava erupting under water.

Did you know? Some of the water in your glass may be older than the Sun: modelling suggests a share of Earth's water formed as ice in the cold molecular cloud that gave birth to the Solar System.

The question matters far beyond Earth. If water arrives routinely with rocky planets, ocean worlds may be common around other stars; if it needed a lucky rain of asteroids, they could be rare. Every grain from Ryugu and Bennu is a vote in that argument, and the ocean we swim in is its first and best piece of evidence.

One World Ocean

Atlases name five oceans, but there is really only one. The Pacific, Atlantic, Indian, Southern and Arctic are basins of a single connected body of water, the World Ocean, and a drop of seawater can wander through all of them. Water from the North Atlantic can sink off Greenland, creep south along the seabed, circle Antarctica and resurface in the Pacific many centuries later. The borders drawn between the basins are human conveniences, not walls.

The giant is the Pacific, about 165 million square kilometres, larger than all the land on Earth combined and home to the deepest point, the Challenger Deep. The Atlantic, about 106 million, is the young S-shaped ocean still widening between the Americas and Europe and Africa; its deepest spot lies in the Puerto Rico Trench at about 8,400 metres. The Indian Ocean, about 70 million, is the warmest, walled in by Asia to the north and ruled by the monsoon. The Southern Ocean rings Antarctica, and the Arctic, smallest and shallowest at about 14 million square kilometres, is a near-enclosed sea capped by drifting ice.

Who decides where one ends? Since 1921 the International Hydrographic Organization, based in Monaco, has coordinated the world's nautical charts, and its publication Limits of Oceans and Seas, first issued in 1928, draws the official lines. The Atlantic and Indian meet along the meridian of Cape Agulhas, Africa's southern tip; the Indian and Pacific along a line south from Tasmania. The Southern Ocean has had a bumpy career. It appeared in the 1937 edition, was dropped in 1953, and a 2000 draft restoring it south of 60° South was never formally ratified. Scientists used it anyway, and National Geographic added it to its maps on World Oceans Day, 8 June 2021.

What makes the Southern Ocean real is the Antarctic Circumpolar Current, the only current that flows all the way round the globe unbroken by land. Driven by the westerly winds of the Roaring Forties and Furious Fifties, it carries well over 100 times the combined flow of all the world's rivers and stitches the three great northern basins together.

Did you know? The Sargasso Sea is the only sea with no coastline: it is bounded entirely by ocean currents, the four arms of the North Atlantic gyre.

Thinking of one ocean is more than pedantry. Plastic from distant coasts piles up on uninhabited Henderson Island in the South Pacific; heat absorbed in the tropics melts ice at the poles; whales and eels cross basins without passports. The 2023 High Seas Treaty treats the ocean beyond national waters as a single shared space, which is exactly what it has always been.

Why the Sea Is Salty

Taste the sea and you taste the continents. Rain is very slightly acidic, because it dissolves carbon dioxide from the air, and as it trickles over rock it slowly breaks minerals apart. This chemical weathering frees calcium, sodium, magnesium and potassium, which rivers carry to the sea. River water tastes fresh only because these ions are dilute. The ocean concentrates them, because evaporation removes water but leaves the salts behind.

The other half of sea salt comes from below. Chloride, the most abundant ion in seawater, is scarce in rocks; it arrived mostly as hydrochloric acid gas from volcanoes early in Earth's history. Today hydrothermal vents along the mid-ocean ridges keep trading chemicals with seawater, adding some elements and stripping out others, notably magnesium. The result averages about 35 grams of salt per kilogram of seawater, or 3.5 per cent. Chloride and sodium make up about 86 per cent of it, with sulfate, magnesium, calcium and potassium most of the rest.

The proportions are remarkably constant. In the 1880s the chemist William Dittmar analysed 77 water samples gathered by HMS Challenger from every ocean and confirmed that, however salty a sample, its major ions keep the same ratios. That made salinity easy to measure: once by titrating chloride, now by electrical conductivity under the Practical Salinity Scale of 1978. Argo floats record it down to 2,000 metres, and satellites such as Europe's SMOS have mapped surface salinity from orbit since 2009.

If rivers keep bringing salt, why is the sea not getting saltier? Because it roughly balances. Salt leaves through vents, by sticking to clays that sink into sediment, and in vast evaporite beds when shallow seas dry out. Around 6 million years ago the Mediterranean was cut off from the Atlantic and largely evaporated in the Messinian salinity crisis, leaving salt layers more than a kilometre thick. Each element has a residence time: sodium lingers for tens of millions of years, iron and aluminium for only centuries. In 1899 John Joly divided the ocean's sodium by the yearly supply from rivers and estimated Earth's age at about 90 million years. He was out by a factor of fifty, because the sea is in balance, not filling up.

Did you know? If all the salt in the sea were spread evenly over the land, it would form a layer about 166 metres thick, as tall as a 40-storey building.

Salinity still varies from place to place. The Red Sea, hot and almost riverless, reaches about 40 grams per kilogram; the Baltic, fed by rivers and rain, falls to just a few grams in the Gulf of Bothnia, too fresh for many marine species. The landlocked Dead Sea is nearly ten times saltier than the ocean. These differences in salt, as much as in heat, drive the currents that stir the whole ocean.

The Shape of the Seafloor

Drain the ocean and the planet would show two great levels. Plot how much of Earth's surface lies at each height, a graph called the hypsometric curve, and two peaks appear: one near sea level, the continents, and a larger one between 4,000 and 5,000 metres down, the deep ocean floor. The difference is not erosion but rock. Continents are thick, light granite floating high, while ocean crust is thin, dense basalt riding low.

Walk out from a typical coast and the floor changes in steps. First comes the continental shelf, the drowned edge of the continent, shallow and gently sloping, averaging about 80 kilometres wide and ending at a break around 140 metres deep. Its width varies hugely: off Siberia the shelf runs for well over 1,000 kilometres, while off Chile and Peru it barely exists. During the last ice age, with sea level about 120 metres lower, much of it was dry land, including Doggerland between Britain and the rest of Europe.

Beyond the shelf break the continental slope drops away at a few degrees, steep by seafloor standards, and is slashed by submarine canyons such as Monterey Canyon off California, as deep as the Grand Canyon. Sediment pours down them in turbidity currents, underwater avalanches. In 1929 one set off by an earthquake off the Grand Banks of Newfoundland snapped a dozen transatlantic telegraph cables one after another, letting scientists clock its speed. At the foot of the slope these deposits pile into the continental rise, which grades into the abyssal plain, between 3,000 and 6,000 metres deep and among the flattest places on Earth, where fine sediment has buried the old volcanic hills.

Mapping all this has been slow. Early sailors used weighted lines; the German Meteor expedition of 1925 to 1927 pioneered echo sounding across the South Atlantic. In the 1950s Marie Tharp, working with Bruce Heezen at Columbia University, turned thousands of soundings into the first physiographic maps of the ocean floor and spotted the rift valley running down the Mid-Atlantic Ridge. Satellites now infer the seafloor from tiny bulges in the sea surface caused by the gravity of underwater mountains, but only at a resolution of kilometres.

Did you know? The surfaces of Mars and the Moon are mapped in far finer detail than our own seafloor.

The Seabed 2030 project, launched in 2017 by the Nippon Foundation and GEBCO, aims to map the entire floor with modern sonar by the end of the decade. When it began, only about 6 per cent had been surveyed that way; by the mid-2020s the figure was around a quarter. Better maps matter for tsunami warnings, cable routes, fisheries and the fierce debate over deep-sea mining. Most of the planet's surface is still, in effect, a blank on the chart.

Plate Tectonics and Ocean Basins

In 1912 a German meteorologist named Alfred Wegener stood up in Frankfurt and argued that the continents move. South America and Africa fit like torn halves of a page; fossils such as the reptile Mesosaurus and the fern Glossopteris turn up on both sides of the South Atlantic; coal seams lie under Antarctic ice. In The Origin of Continents and Oceans (1915) he proposed a lost supercontinent, Pangaea. Most geologists rejected him, chiefly because he could not explain what force could plough continents through solid ocean floor. He died on the Greenland ice cap in 1930, his idea still unaccepted.

The answer came from the sea. Wartime sonar and post-war surveys revealed a world-encircling volcanic ridge and deep trenches. In 1962 the Princeton geologist Harry Hess, who had run an echo sounder while commanding a US Navy transport ship, proposed seafloor spreading: new crust forms at mid-ocean ridges, moves away on both sides like a pair of conveyor belts and sinks back into the mantle at trenches. Continents do not plough through the floor; they ride on it. Magnetic stripes on the seafloor and the ages of drilled sediments soon confirmed the idea, and by the late 1960s plate tectonics had become the unifying theory of geology.

One consequence is startling: the ocean floor is young. The oldest surviving oceanic crust, in the western Pacific, is only about 180 million years old, while continental rocks reach four billion years. Oceans are temporary. In 1966 the Canadian geophysicist J. Tuzo Wilson asked whether the Atlantic had closed and then opened again, pointing to an older ocean, now called Iapetus, whose closure stitched Scotland to England. His insight became the Wilson cycle: a continent rifts, a new ocean opens and widens, its old edges begin to subduct, and it finally closes in a mountain-building collision.

Every stage can be seen today. East Africa is splitting along the Great Rift Valley; the Red Sea is a newborn ocean; the Atlantic is a mature one, widening by about 2.5 centimetres a year, roughly as fast as a fingernail grows, a rate now measured directly by satellites. The Pacific, ringed by trenches, is shrinking. The Mediterranean is a remnant of the vanished Tethys Ocean, squeezed as Africa pushes north, and the Himalaya mark where the ocean in front of India closed completely.

Did you know? Columbus's crossing of the Atlantic in 1492 would be about 13 metres longer today, because the ocean has widened since.

Plate tectonics controls where earthquakes strike, where volcanoes rise and how ocean currents can flow between basins. Some geologists project that in 200 to 250 million years the continents will gather into a new supercontinent, though they argue over its shape. Oceans, it turns out, are born, grow old and die.

The Mid-Ocean Ridge

The longest mountain range on Earth is almost entirely under water. The mid-ocean ridge winds through every ocean like the seam on a tennis ball, about 65,000 kilometres long, rising 2,000 to 3,000 metres above the surrounding floor, with its crest typically about 2,500 metres below the waves. It is where the planet makes new crust. As plates pull apart, hot mantle rock rises, partly melts as the pressure on it drops, and erupts as basalt along the ridge axis, cooling into pillow lavas over a layer of vertical sheeted dykes.

Ridges come in different styles. The Mid-Atlantic Ridge spreads slowly, about 2 to 5 centimetres a year in total, and has a deep central rift valley. The East Pacific Rise spreads several times faster, up to about 15 centimetres a year, and forms a broad, smooth swell. Slowest of all is the Gakkel Ridge under the Arctic ice, opening at about a centimetre a year or less. Along every ridge seawater circulates through hot rock and gushes out as hydrothermal vents, first seen in 1977 by the submersible Alvin on the Galápagos Rift.

Iceland is where the ridge comes ashore. A hotspot beneath it supplies so much magma that the ridge rises above sea level, and visitors can walk through the rift at Þingvellir or snorkel between the North American and Eurasian plates in the Silfra fissure. The island is busy: Surtsey rose from the sea between 1963 and 1967, Eldfell buried part of Heimaey's town in 1973, and since 2021 a string of eruptions on the Reykjanes Peninsula has threatened the town of Grindavík.

The proof of spreading lies in the rock itself. Basalt records the direction of Earth's magnetic field as it cools, and the field flips at irregular intervals, often a few hundred thousand years apart. In 1963 Fred Vine and Drummond Matthews at Cambridge, and independently the Canadian Lawrence Morley, explained the puzzling magnetic stripes that ship-towed magnetometers had found on the seafloor: bands of normal and reversed magnetism laid down at the ridge and carried outwards. If they were right, the stripes should be mirror images on either side of the ridge. A survey of the Reykjanes Ridge south of Iceland, published in 1966, showed exactly that.

Did you know? Morley's paper on the stripes was rejected by journals, with one reviewer reportedly saying such ideas were better suited to cocktail-party conversation.

The last full reversal, about 780,000 years ago, makes a clear marker in the pattern, and the stripes now let geologists date the seafloor without drilling. The ridge also sets the ocean's chemistry and hosts its strangest ecosystems. Every year it adds roughly three square kilometres of new seafloor, a slow birth that never stops.

Trenches and the Challenger Deep

The deepest place on Earth lies at the southern end of the Mariana Trench, a curving gash about 2,500 kilometres long in the western Pacific. Its lowest point, the Challenger Deep, is about 10,935 metres down; estimates differ by tens of metres depending on the sonar and the corrections applied. Set Mount Everest, 8,849 metres tall, on the bottom and its summit would still lie more than two kilometres below the surface. The pressure there is over 1,000 times that at sea level, like the weight of an elephant on every postage stamp.

Trenches mark subduction zones, where one plate dives beneath another. Ocean crust grows colder, thicker and denser as it ages, and at the Mariana Trench some of the oldest crust in the Pacific, around 150 million years old or more, bends down beneath the Philippine Sea Plate and sinks into the mantle. Water squeezed from the descending slab lowers the melting point of the rock above, feeding the volcanoes of the Mariana Islands. Repeated around the Pacific, the same process builds the Ring of Fire, a 40,000-kilometre horseshoe of trenches and volcanoes from Chile to Alaska, Japan and New Zealand that hosts about three quarters of the world's active volcanoes and around 90 per cent of its earthquakes. The great Chilean earthquake of 1960, magnitude 9.5, and the 2011 Tōhoku earthquake off Japan both struck on these boundaries.

The Deep got its name from ships. In 1875 the crew of HMS Challenger lowered a weighted line near the trench and recorded about 8,180 metres, a world record at the time. In 1951 the survey ship HMS Challenger II used echo sounding to find much greater depths, and the deepest spot took her name. On 23 January 1960 Jacques Piccard and Don Walsh descended in the bathyscaphe Trieste, a steel sphere slung beneath a float of petrol, and spent about 20 minutes on the bottom. Nobody returned until 2012, when the film-maker James Cameron dived solo in Deepsea Challenger. Since 2019 the explorer Victor Vescovo's submersible Limiting Factor has made repeated dives, carrying passengers including Kathy Sullivan, in 2020 the first woman to reach the bottom.

Did you know? Before 2019 more people had walked on the Moon than had visited the Challenger Deep: twelve against three.

Other trenches run nearly as deep. The Tonga Trench's Horizon Deep reaches about 10,800 metres, and the Kermadec and Philippine trenches pass 10,000. The hadal zone below 6,000 metres covers only a tiny fraction of the seafloor, yet amphipods, sea cucumbers and snailfish live there, and so does our litter: a plastic bag has been filmed near the bottom of the Mariana Trench. Trenches are where the ocean floor goes to die, carrying crust, water and carbon back into the planet's interior.

Seamounts and Hotspots

Scattered across the ocean floor are tens of thousands of volcanoes that never broke the surface. A seamount is an undersea mountain rising at least 1,000 metres above the surrounding floor. Satellite gravity maps have so far picked out tens of thousands of them, and smaller knolls may number in the millions. Most are unnamed and unvisited. In 2005 the nuclear submarine USS San Francisco ran at speed into one near Guam that was missing from its charts; one sailor died.

Many seamounts grow over hotspots, places where unusually hot mantle rises from deep in the Earth and melts through the plate above. The plate moves; the hotspot, roughly, stays put. The result is a conveyor belt of volcanoes, youngest over the hotspot and older the further they have travelled. The Canadian geophysicist J. Tuzo Wilson proposed the idea in 1963, and W. Jason Morgan linked it to deep mantle plumes in 1971.

The classic example is the Hawaiian–Emperor chain, about 6,000 kilometres of islands and seamounts across the North Pacific. Kīlauea and Mauna Loa on the Big Island sit over the hotspot now. Kauaʻi is about 5 million years old, Midway about 28 million. Then the chain bends sharply north at an age of about 47 million years and continues as the drowned Emperor Seamounts to Meiji Seamount, around 82 million years old, which is slowly being fed into the trenches off Kamchatka. The bend was long read as a change in the Pacific Plate's direction, but ocean drilling in 2001 showed the hotspot itself drifted south during the Emperor stage, and geologists still debate the balance between the two. Already a new volcano, Kamaʻehuakanaloa, formerly called Lōʻihi, is growing off the Big Island's south coast, its summit about 1,000 metres below the waves.

Old seamounts sink as the plate beneath them cools and subsides. Some had their tops planed flat by waves before they went under, becoming guyots, flat-topped peaks identified by Harry Hess in the 1940s and named after the Swiss-born geographer Arnold Guyot. In warm seas coral can grow upwards as the volcano sinks, building a ring-shaped atoll, as Charles Darwin argued in the 1830s.

Did you know? Measured from its base on the seafloor, Mauna Kea is around 10,000 metres tall, more than a kilometre taller than Everest.

Seamounts are biodiversity hotspots too. They deflect currents and lift nutrients, drawing in plankton, fish, sharks and whales, while cold-water corals and sponges cling to their flanks. Off New Zealand and Tasmania they once supported booming fisheries for orange roughy, a fish that can live well over 100 years, until bottom trawling stripped many seamounts bare. Their corals may need centuries to recover. Protecting these islands of life is now a test of whether conservation can reach beyond the coast.

Sediments: the Ocean's Memory

Every year a fine rain of dust, shells and debris settles on the ocean floor, and in places it has been falling undisturbed for more than 100 million years. These marine sediments form the most complete record of Earth's history anywhere, a library written in mud. Near the continents they are mostly terrigenous: sand, silt and clay washed off the land or blown out to sea, such as Saharan dust carried across the Atlantic. Further out, where little land debris reaches, the remains of living things dominate.

When the skeletons of plankton make up more than 30 per cent of a sediment it is called an ooze. Calcareous ooze is built from the chalky shells of foraminifera and the tiny plates of coccolithophores; the White Cliffs of Dover are a Cretaceous ooze lifted into the air. It survives only above the carbonate compensation depth, roughly 4,000 to 5,000 metres, below which cold, high-pressure water dissolves calcite. Siliceous ooze, from glassy diatoms and radiolarians, gathers beneath the productive waters around Antarctica and along the equator. In the deepest, most remote basins only red clay accumulates, perhaps a millimetre every thousand years, studded with manganese nodules and even micrometeorites.

The first worldwide survey came from HMS Challenger, whose dredged samples John Murray and Alphonse Renard described in 1891. The great leap came with drilling. From 1968 the Deep Sea Drilling Project's ship Glomar Challenger drilled through hundreds of metres of sediment and showed that it grows older and thicker away from the mid-ocean ridges, just as seafloor spreading predicted; in 1970 it found salt beneath the Mediterranean, evidence that the sea had once dried out. Its successor, JOIDES Resolution, drilled for the Ocean Drilling Program and its international successors from 1985 until its final expedition in 2024, recovering hundreds of kilometres of core now stored in repositories in Texas, Bremen and Kōchi.

Cores reveal past climates through chemistry. Foraminifera build their shells from seawater, and the ratio of heavy oxygen-18 to oxygen-16 in them rises when ice sheets lock up light water and the ocean cools. In 1955 Cesare Emiliani used this to show that the ice ages came in cycles; in 1976 James Hays, John Imbrie and Nicholas Shackleton matched them to wobbles in Earth's orbit. Cores also capture catastrophes: the iridium-rich layer of the asteroid impact 66 million years ago, and the dark, shell-poor band of the Palaeocene–Eocene Thermal Maximum about 56 million years ago, when a pulse of carbon warmed and acidified the ocean.

Did you know? The Bengal Fan, built from Himalayan debris carried by the Ganges and Brahmaputra, is the largest pile of sediment on Earth, more than 16 kilometres thick in places.

That ancient warming is our closest natural parallel to today's carbon emissions, though we are releasing carbon far faster. The mud of the deep sea remembers what the climate did last time, and it is telling us what comes next.

How Waves Work

Stand on a beach and the sea seems to march towards you in ranks. It does not. A wave is energy passing through water, not water travelling across the ocean. Out at sea each parcel of water traces a near-circle as a crest goes by: up and forward under the crest, down and back under the trough, ending almost where it began. A gull resting on the swell bobs in loops rather than riding to shore. These orbits shrink with depth and fade to almost nothing about half a wavelength down, which is why a submarine at 100 metres can cruise through a gale in near calm.

Most waves are made by wind, and their size depends on three things: how hard it blows, how long it blows, and the fetch, the stretch of open water it blows across. A breeze over a pond makes ripples; a gale blowing for two days across 1,000 kilometres of the Southern Ocean builds seas of 10 metres or more. Once waves outrun the storm that raised them they sort themselves by period into long, smooth swell. In deep water a crest's speed in metres per second is roughly 1.56 times its period in seconds, so the crests of a 15-second swell race along at about 84 kilometres an hour, while the energy itself travels at half that pace.

Much of this was worked out under pressure. During the Second World War Harald Sverdrup and Walter Munk devised the first practical surf forecasts for Allied landings in North Africa and later Normandy. In 1963 Munk and colleagues set up stations from New Zealand to Alaska and followed swell born in storms near Antarctica for more than 10,000 kilometres. Today moored buoys such as the Datawell Waverider record every rise and fall, and radar altimeters on satellites measure wave heights across the whole globe.

A wave ends its journey when it feels the bottom. In water shallower than about half its wavelength it slows, the crests bunch up and the height grows until, at a height of roughly four-fifths of the water depth, the crest outruns its base and topples. Gentle beaches make spilling breakers; steep reefs such as Pipeline on Oahu's North Shore and Teahupo'o in Tahiti throw hollow, plunging barrels. At Nazaré in Portugal a submarine canyon around 5 kilometres deep funnels Atlantic swell straight to the shore and focuses it, and in 2020 the German surfer Sebastian Steudtner rode a wave there measured at 26.21 metres, the largest ever surfed according to Guinness World Records.

Did you know? The tallest waves in the ocean are hidden inside it: internal waves rolling along boundaries between layers of different density in the South China Sea reach around 170 metres high.

Understanding waves now matters more than ever, for ports, offshore wind farms and eroding coasts, and satellite records suggest the Southern Ocean's biggest waves have grown slightly as its winds have strengthened. Every breaker on a beach is the last chapter of a story that began with wind, often half a world away.

Rogue Waves

For centuries sailors told of walls of water rising out of an ordinary sea, and for centuries scientists filed the stories alongside sea serpents. Standard wave theory said a wave twice the height of its neighbours should be vanishingly rare. Then, on New Year's Day 1995, a downward-pointing laser on the Draupner E gas platform in the North Sea recorded a wave 25.6 metres from trough to crest, in a sea whose significant wave height, the average of the highest third of the waves, was around 12 metres. The Draupner wave became the first instrumental record of a rogue wave that nobody could explain away, and it turned folklore into physics.

Oceanographers now define a rogue wave as one at least twice the significant wave height. Some arise by simple chance, when wave trains of different lengths happen to line up their crests. Others are helped along. Where swell runs against a strong current, as off South Africa in the Agulhas Current, waves are squeezed and steepened, and tanker crews have long reported sudden giant crests and deep holes in the sea there. In the 1960s Brooke Benjamin and Jim Feir showed that a regular train of waves is unstable and can let one wave steal energy from its neighbours. In 2019 researchers in Edinburgh recreated the Draupner wave in a round wave tank by crossing two groups of waves at an angle of about 120 degrees.

Seen from space, rogues turn out to be rare but not exotic. In 2001 the European Space Agency's MaxWave project scanned three weeks of radar images from its ERS satellites and found more than ten individual waves over 25 metres high. The ocean is simply very large, and a one-in-many-thousands wave happens somewhere every day.

Ships have paid for the old disbelief. The German cargo ship München vanished in the North Atlantic in December 1978 with all 28 crew; a recovered lifeboat, stowed about 20 metres above the waterline, had been torn from its fittings. In September 1995 the liner Queen Elizabeth 2 met a wave estimated at 29 metres during Hurricane Luis, and her captain compared it to the White Cliffs of Dover. In 2001 the cruise ships Bremen and Caledonian Star both had bridge windows smashed by waves of around 30 metres in the South Atlantic, within days of each other.

Did you know? In November 2020 a buoy off Ucluelet on Vancouver Island recorded a 17.6-metre wave in seas of about 6 metres, almost three times the height of its neighbours, the most extreme ratio on record.

Today ship and platform designers test their structures against rogue scenarios, and forecasting centres issue indices for the sea states most likely to breed them. The sailors were right all along; the mathematics simply needed time to catch up.

The Tides

Twice a day along most coasts the sea creeps up the shore and slips away again. The tide is the longest wave in the ocean: from one crest to the next it stretches halfway round the planet. Isaac Newton explained it in his Principia of 1687. The Moon's gravity pulls harder on the side of the Earth facing it than on the Earth's centre, and harder on the centre than on the far side. That difference, the tidal force, stretches the oceans into two bulges, one under the Moon and one on the opposite side. As the Earth turns, a coast passes through both, giving two high tides about 12 hours and 25 minutes apart, because the Moon moves on along its orbit while we spin.

The Sun raises tides too, but although it is vastly more massive it is so far away that its tidal effect is a little under half the Moon's. When Sun, Moon and Earth line up, at new and full moon, their pulls add together and the range is largest: these are spring tides, a name that has nothing to do with the season. A week later, at the quarter moons, the Sun pulls at right angles to the Moon and the range shrinks to the modest neap tides.

Real oceans complicate the tidy picture. Continents block the bulges, so tides slosh around each basin as great rotating waves that pivot on amphidromic points, places where the range is almost nothing; the North Sea has three. Where a basin's natural sloshing period matches the tide's, the effect is amplified. The Bay of Fundy in Canada rocks back and forth in about 13 hours, close to the 12.4-hour tide, and at Burntcoat Head in Nova Scotia the range reaches about 16 metres, the largest in the world. The Severn Estuary in Britain manages more than 14 metres, while much of the Mediterranean sees only a few tens of centimetres.

Predicting all this was a triumph of Victorian engineering. William Thomson, later Lord Kelvin, broke each port's tide into a sum of simple waves, or constituents, each with a period set by astronomy. In 1872 he designed a tide-predicting machine of pulleys and a wire that added ten constituents together and drew a year's tides in a few hours. Its descendants, refined by Arthur Doodson in Liverpool, produced the tide tables for the Normandy landings in 1944, and machines like them were used until computers took over in the 1960s.

Did you know? The solid Earth has tides too: twice a day the ground beneath your feet rises and falls by a few tens of centimetres, too smoothly to notice.

Tides do more than move water. Their friction is slowly braking the Earth's spin, lengthening the day by around two thousandths of a second per century, and pushing the Moon away by about 3.8 centimetres a year, measured by lasers bounced off mirrors left by Apollo astronauts. The tides are a slow tug of war between three worlds, and the Moon is being nudged away for its trouble.

Tsunamis

The word tsunami is Japanese for "harbour wave": fishermen came home to find their harbours wrecked by a wave they never noticed out at sea. Despite the old name, it has nothing to do with the tides. A tsunami begins when a huge volume of water is suddenly shoved out of place, most often by a megathrust earthquake at a subduction zone, where the seabed can jump upwards by several metres along hundreds of kilometres of fault. Landslides, volcanic eruptions and, very rarely, asteroid impacts can do the same.

The result is a wave unlike any wind wave. Its wavelength can be 100 kilometres or more, far longer than the ocean is deep, so the whole water column moves and the wave's speed depends only on depth. Over abyssal plains 4,000 metres deep it travels at about 200 metres a second, more than 700 kilometres an hour, as fast as a jet airliner, yet it may be less than a metre high and pass beneath ships unnoticed. As it reaches shallow water it slows sharply, and its energy is squeezed into a shorter, much taller wave. Often the trough arrives first and the sea draws back, exposing reefs. What follows is not a curling breaker but a fast-rising flood that keeps coming for minutes, in a series of waves over several hours.

On 26 December 2004 a magnitude 9.1 earthquake ruptured about 1,300 kilometres of fault off Sumatra. The tsunami killed around 230,000 people in 14 countries and reached Somalia, some 5,000 kilometres away, seven hours later. The Indian Ocean had no warning system. On a beach in Phuket, ten-year-old Tilly Smith recognised the retreating sea from a geography lesson and raised the alarm, and the beach was cleared in time. On 11 March 2011 the magnitude 9.0 Tōhoku earthquake sent water surging up to around 40 metres above sea level near Miyako, overtopping sea walls, killing about 18,000 people and crippling the Fukushima Daiichi nuclear plant.

Warning systems grew out of disaster. The Pacific Tsunami Warning Center was set up in Hawaii in 1949, after a tsunami from the Aleutian Islands killed more than 150 people there in 1946, and an Indian Ocean system followed in 2006. Their key instrument is the DART buoy: a pressure sensor on the seabed that can feel the passing tsunami as a change of a centimetre or so in the weight of water above, and relays it by satellite through a buoy on the surface.

Did you know? In 1958 a rockslide into Lituya Bay, Alaska, sent water surging 524 metres up the opposite slope, higher than the Empire State Building, stripping the forest to bare rock.

Technology helps, but the oldest advice still saves the most lives: if the ground shakes hard and long, or the sea suddenly retreats, head for high ground without waiting. On the hillside above the village of Aneyoshi an old stone tablet warns people not to build below it; in 2011 the waters stopped just short of the village. The best warning system ever built is still a person who knows to run uphill.

Surface Currents and Gyres

The top few hundred metres of the ocean are dragged by the winds into currents that circle whole basins. In the tropics the trade winds blow from east to west; in the middle latitudes the westerlies blow the other way. Between them they spin the water into vast loops called gyres. There are five great subtropical gyres, in the North and South Atlantic, the North and South Pacific and the Indian Ocean. In the Northern Hemisphere they turn clockwise; in the Southern, anticlockwise.

The twist comes from the planet's rotation. On a spinning Earth anything moving over long distances is deflected to the right in the north and to the left in the south: the Coriolis effect. During the drift of his ship Fram across the Arctic in 1893–96, Fridtjof Nansen noticed that the pack ice moved 20 to 40 degrees to the right of the wind. He passed the puzzle to a young Swedish physicist, Vagn Walfrid Ekman, who in 1905 showed that each layer of water drags the one beneath it, a little slower and turned a little further, in a corkscrew now called the Ekman spiral. Averaged over the wind-driven layer, the water moves at right angles to the wind. This Ekman transport heaps water into the middle of each gyre, raising its centre by up to about a metre, and the currents flow around that low hill.

The gyres are lopsided. Currents on their western sides, such as the Gulf Stream, the Kuroshio off Japan, the Brazil Current and the East Australian Current, are narrow, fast and deep, while those on their eastern sides, like the California and Canary currents, are broad and sluggish. In 1948 Henry Stommel showed that this western intensification follows from the Coriolis effect growing stronger towards the poles. Around Antarctica no continent stands in the way, so the Antarctic Circumpolar Current runs right round the globe, moving well over 100 million cubic metres of water a second, the largest current on Earth.

Oceanographers once relied on ships' logs and drift bottles; today more than a thousand satellite-tracked drifting buoys and radar measurements of sea-surface height map the currents daily. Some of the best data came by accident. On 10 January 1992 a container ship lost cargo in a North Pacific storm, releasing 28,800 plastic bath toys: yellow ducks, red beavers, green frogs and blue turtles. Curtis Ebbesmeyer and James Ingraham used beachcombers' finds and a computer model of surface currents to follow them. The first came ashore near Sitka, Alaska, about ten months later, and others circled the gyre for years.

Did you know? The Sargasso Sea, in the heart of the North Atlantic gyre, is the only sea on Earth with no coastline, bounded entirely by currents.

The same slow carousels that carried the ducks now gather floating plastic into great patches, such as the one between Hawaii and California. They also carry heat, larvae and nutrients, and once carried sailing ships along the trade routes. A rubber duck, it turns out, is a better oceanographer than it looks.

The Gulf Stream

The Gulf Stream is a river of warm water within the Atlantic. It pours through the Straits of Florida, runs north along the American coast to Cape Hatteras, then peels away into the open ocean towards the Grand Banks of Newfoundland, where it becomes the North Atlantic Current. It is around 100 kilometres wide, reaches down a kilometre or more, and at the surface can flow faster than 2 metres a second. It is the western edge of the North Atlantic gyre, squeezed narrow and fast by the Earth's rotation, as Henry Stommel showed in 1948.

Europeans met it early. In April 1513 the ships of Juan Ponce de León, sailing off Florida, found themselves carried backwards despite a fair wind. His pilot, Antón de Alaminos, took note, and in 1519 he used the current to speed a ship from Mexico towards Spain. For two and a half centuries Spanish treasure fleets rode the stream home through the Florida Straits.

The first printed chart came from a postal complaint. As deputy postmaster for the American colonies, Benjamin Franklin wondered why mail packets from Falmouth took about two weeks longer to reach New York than merchant ships took to reach Rhode Island. His cousin Timothy Folger, a Nantucket whaling captain, explained that whalers hunted along the edges of the stream and often saw the British packets battling against it. Folger sketched its course, and Franklin had the map engraved around 1769–70; British captains largely ignored it. On later voyages Franklin lowered thermometers over the side and found the stream markedly warmer than the sea around it.

Its flow is measured in sverdrups, after the Norwegian oceanographer Harald Sverdrup: one sverdrup is a million cubic metres of water a second. Through the Florida Straits the current carries around 30 sverdrups; by the time it passes Newfoundland it has drawn in so much surrounding water that it carries roughly 150. All the world's rivers together discharge little more than one sverdrup into the sea.

Did you know? Since 1982 the Florida Current has been measured by an old submarine telephone cable to the Bahamas: seawater flowing through the Earth's magnetic field generates a voltage along the cable.

The stream does carry tropical heat north, and it is often credited with keeping Britain and Norway mild enough for palm trees in Cornwall and ice-free harbours beyond the Arctic Circle. Yet in 2002 Richard Seager and colleagues argued that most of the winter warmth separating western Europe from eastern Canada comes from the atmosphere: westerly winds blowing off an ocean that stores summer heat, and the way the Rocky Mountains steer the jet stream. Other researchers stress the ocean's role, especially through the wider Atlantic overturning that the stream belongs to, and the argument continues. Europe owes the Gulf Stream a debt; scientists are still arguing over how large it is.

The Global Conveyor Belt

Beneath the wind-driven currents runs a slower circulation driven by density. Cold water is denser than warm, and salty water denser than fresh, so this thermohaline circulation (from the Greek for heat and salt) is set by both. Only in a few places does surface water grow dense enough to sink all the way to the abyss. In the Greenland, Iceland and Norwegian Seas and the Labrador Sea, winter winds chill salty Atlantic water until it plunges, forming North Atlantic Deep Water. Around Antarctica, in the Weddell and Ross Seas, freezing sea ice squeezes out its salt, leaving water below 0 °C that is the densest in the ocean: Antarctic Bottom Water.

From the North Atlantic the deep water creeps south at depths of two to four kilometres, crosses the equator and reaches the Southern Ocean, where the Antarctic Circumpolar Current mixes it and spreads it into the Indian and Pacific Oceans. There it slowly warms and rises over centuries, then returns near the surface through the Indonesian islands, around the Cape of Good Hope and back up the Atlantic in the Gulf Stream, to sink once more. In 1987 the geochemist Wallace Broecker drew the whole system as a single ribbon and called it the great ocean conveyor. One full circuit takes something like a thousand years.

How do we know the age of water? Chemistry dates it. Radiocarbon in dissolved carbon decays steadily once water leaves the surface, and the deep North Pacific turns out to hold the oldest water in the ocean, last in contact with the air well over a thousand years ago. Industrial chlorofluorocarbons, released only since the 1930s, trace freshly sunk water spreading south in decades. Since 2004 the RAPID array of moorings strung across the Atlantic at 26.5 degrees north has measured the Atlantic Meridional Overturning Circulation, or AMOC, continuously. It averages around 17 sverdrups, but swings widely: in the winter of 2009–10 it dropped by about a third.

The ribbon is a cartoon. Real deep flows break into eddies and run along the western sides of basins, and much of the deep water returns to the surface in the Southern Ocean, hauled up by the fierce westerly winds rather than by slow mixing alone. The cartoon has a history of switching, too. Around 12,900 years ago, during the Younger Dryas, the Northern Hemisphere plunged back to near-glacial cold for about 1,200 years, probably after meltwater from retreating ice sheets freshened the North Atlantic and weakened the sinking; the trigger is still debated.

Did you know? A parcel of water that sinks off Greenland today may not see daylight again for something like a thousand years, far beyond the year 3000.

Today Greenland's melt and heavier rainfall are freshening the North Atlantic again. Some studies infer that the AMOC has weakened since the mid-twentieth century; direct measurements are too short to be sure. The IPCC's 2021 assessment judged it very likely to weaken this century but, with medium confidence, not to collapse abruptly before 2100. The conveyor moves slowly, which is exactly why changes to it last so long.

Upwelling

Most of the sunlit ocean is a blue desert. Nutrients sink with dead plankton and droppings into the dark water below, out of reach of the drifting algae that need sunlight to grow. Upwelling brings them back. Along certain coasts, winds blowing parallel to the shore towards the equator, combined with the Earth's rotation, drive the surface water offshore through Ekman transport. Cold water from 100 to 300 metres down rises to take its place, often by only a few metres a day, carrying nitrate, phosphate and silicate into the light. Satellites see the result as bands of cool water hugging the coast, turned green by blooms of phytoplankton.

The richest examples are the four great eastern boundary upwelling systems: the California Current, the Humboldt Current off Peru and Chile, the Canary Current off north-west Africa and Iberia, and the Benguela Current off Namibia and South Africa. Together they cover only around one per cent of the ocean's surface, yet they supply something like a fifth of the world's wild-caught fish. Upwelling also happens along the equator, where the trade winds pull surface water apart, in the Arabian Sea when the summer monsoon blows off Somalia and Oman, and around Antarctica.

The Humboldt is the champion. Alexander von Humboldt measured its coolness off Peru in 1802, and it feeds the Peruvian anchoveta, a small, silvery fish that supports the largest single-species fishery on Earth. In 1970 the catch topped 12 million tonnes. The same fish feed vast colonies of guanay cormorants, boobies and pelicans, whose droppings, piled metres deep on islands such as the Chinchas, were mined as fertiliser in the nineteenth-century guano boom. In 1972 an El Niño switched off the upwelling while fishing pressure stayed high, and the anchoveta stock crashed.

Elsewhere the riches bring other surprises. Off Namibia the cold Benguela water chills the air above it, wrapping the Skeleton Coast in fog that sustains beetles and plants in the Namib Desert. So much plankton dies and sinks on the shelf that bacteria strip the water of oxygen. Off California, upwelling fed a sardine boom that filled the canneries of Monterey, immortalised in John Steinbeck's Cannery Row of 1945, before the fishery collapsed around the late 1940s.

Did you know? Off Namibia, decaying plankton on the seabed sometimes releases bursts of hydrogen sulphide that turn the sea milky green and make the coast smell of rotten eggs.

In 1990 the oceanographer Andrew Bakun suggested that global warming would strengthen upwelling winds, because land heats faster than sea and sharpens the pressure difference between them. The evidence so far is mixed, with stronger winds in some regions and expanding low-oxygen zones in others. Either way, the world's fish supply depends on a few windswept coasts. Where the deep sea surfaces, the ocean lays its richest tables.

Eddies and Whirlpools

Look at a satellite map of sea-surface height and the ocean is not a set of smooth rivers but a field of swirls. These mesoscale eddies, spinning lenses typically 50 to 200 kilometres across, drift for weeks, months or even years. They are the ocean's weather, and they hold most of the kinetic energy in the sea. A warm eddy domes the surface up and a cold one dips it, by a few tens of centimetres, which is enough for radar altimeters on satellites such as TOPEX/Poseidon, launched in 1992, to track them across the globe. The SWOT satellite, launched in December 2022, now maps eddies far smaller than its predecessors could see.

Many eddies are born when a great current meanders and pinches off a loop. The Gulf Stream sheds rings: warm-core rings that wander north of the stream spinning clockwise, and cold-core rings to its south spinning the other way, each carrying its own plankton for months. Off South Africa the Agulhas Current runs down the coast, overshoots the tip of the continent and doubles back into the Indian Ocean. At the turn it throws off several Agulhas rings a year, some 300 kilometres across, which drift into the Atlantic carrying warm, salty Indian Ocean water. Deeper down, meddies are lenses of salty Mediterranean water spinning about a kilometre below the surface, sometimes for years.

Whirlpools are smaller and tidal: they form where strong tidal currents race through narrow straits over an uneven seabed. The original maelstrom is the Moskstraumen off the Lofoten Islands in Norway, made famous by Edgar Allan Poe's story "A Descent into the Maelström" of 1841 and by Jules Verne, who sent Captain Nemo's Nautilus into it. Nearby Saltstraumen, near Bodø, drives some of the strongest tidal currents in the world, up to about 40 kilometres an hour. In Japan the Naruto whirlpools, in the strait between Shikoku and Awaji, can reach 20 metres across at spring tides. Between the Scottish islands of Jura and Scarba, the Gulf of Corryvreckan roars as the flood tide strikes a rock pinnacle rising from the seabed.

The legends exaggerate. No whirlpool drills a funnel to the seabed or swallows ships whole as Charybdis does in Homer's Odyssey. The real dangers are steep standing waves, overfalls and boats losing steering in the swirl, though these are quite enough to drown the unwary.

Did you know? In 1947 George Orwell, then writing Nineteen Eighty-Four on Jura, capsized in the Corryvreckan with his son and nephews; they scrambled onto a rock and were rescued by a passing lobster boat.

Eddies matter far beyond spectacle. They move heat, salt, carbon and nutrients, so climate models must be fine enough to resolve them, and tagged white sharks have been found diving deep inside warm Gulf Stream eddies to hunt. The sea's grandest spirals are invisible from the deck, and its most famous ones are far less hungry than the legends say.

El Niño and La Niña

Fishermen on the coasts of Peru and Ecuador long ago noticed that around Christmas a warm current arrived from the north and the fishing slackened. They called it El Niño, the Christ Child. Every few years the warming came stronger and stayed for months, emptying the nets and bringing torrential rain to desert coasts, and the name came to mean those great events. Its cold opposite was later named La Niña. Together with a swing in the atmosphere they form ENSO, the El Niño–Southern Oscillation, the strongest natural year-to-year fluctuation in the Earth's climate, recurring every two to seven years.

In a normal year the trade winds blow from east to west along the equator, piling warm surface water up in the west, so the sea near Indonesia stands about half a metre higher than off Ecuador and the western Pacific warm pool tops 28 °C. In the east the thermocline, the boundary between warm surface water and cold deep water, sits close to the surface, and upwelling brings cold, rich water to the coast of South America. Air rises over the warm pool, dropping heavy rain, flows east high up and sinks over the cool eastern Pacific. In 1969 Jacob Bjerknes named this loop the Walker circulation, after Sir Gilbert Walker, who in the 1920s found the Southern Oscillation, a seesaw of air pressure between Darwin and Tahiti, while searching for a way to forecast failures of the Indian monsoon.

Bjerknes saw that ocean and air feed each other. If the trade winds weaken, warm water sloshes back east along the equator, the thermocline off Peru deepens, upwelling brings up only warm water, the eastern Pacific warms further and the winds weaken still more. That is El Niño: the rains move to the central and eastern Pacific, and the anchoveta and the seabirds that eat them starve. La Niña is the normal state exaggerated, with stronger trades and a colder east.

The 1997–98 El Niño was one of the strongest on record, alongside those of 1982–83 and 2015–16. Floods tore through Peru and Ecuador, drought and fires blanketed Indonesia in smoke, East Africa flooded, and around a sixth of the world's coral reefs died in the bleaching of 1998. Scientists saw it coming. After the 1982–83 event took them by surprise, they built the TAO array of around 70 moored buoys across the equatorial Pacific, completed in 1994, which with satellites let forecasters watch the 1997 event build months ahead.

Did you know? Sir Gilbert Walker, the mathematician who discovered the Southern Oscillation, was also an expert on the flight of boomerangs.

El Niño years tend to be the warmest globally, and the 2023–24 event helped push 2024 to the hottest year then recorded. La Niña brought the floods that drowned much of Queensland in 2010–11. How a warming world will change ENSO itself is still uncertain. What is certain is its reach: a shift in the wind over the Pacific can empty a Peruvian net, flood a Kenyan valley and set an Indonesian forest smouldering.

The Sunlit Zone

The ocean is lit like a room with a single lamp in the ceiling. Sunlight pours into the top few metres, then fades fast, so that by around 200 metres only about one per cent of the surface light remains. That bright upper skin is the epipelagic or sunlit zone, and although it holds only around a twentieth of the ocean's volume, it is where nearly all the sea's photosynthesis happens. Its lower edge, the base of the euphotic zone, is set by the point where plants can no longer make more food than they burn.

The plants are almost all invisible. Phytoplankton, single-celled algae and bacteria drifting in their trillions, carry out roughly half of all the photosynthesis on Earth. Diatoms build glassy shells of silica; coccolithophores armour themselves with chalk plates; and the smallest of all, Prochlorococcus, was only discovered in 1988 by Sallie Chisholm, Rob Olson and colleagues. It is now thought to be the most abundant photosynthetic organism on the planet.

Wind keeps the top layer stirred into a mixed layer of nearly even temperature, a few tens of metres deep in summer and hundreds of metres in a North Atlantic winter. Its depth matters. In 1953 the Norwegian oceanographer Harald Sverdrup showed that a spring bloom can only begin once the mixed layer becomes shallow enough for plankton to stay in the light longer than they sink out of it. Each spring that switch flips across the North Atlantic, and the sea turns green from Newfoundland to Norway.

Did you know? In 1986 a white Secchi disc lowered into the Weddell Sea off Antarctica stayed visible to a depth of about 80 metres, a record for clear water.

On this pasture grazes a busy food web. Copepods the size of rice grains eat the algae; anchovies off Peru and sardines off South Africa eat the copepods; tuna, dolphins, marlin and seabirds eat the fish. Flying fish skim the tropical Pacific, the ocean sunfish basks at the surface to warm up, and in the Sargasso Sea whole floating forests of Sargassum weed shelter crabs, shrimps and young turtles.

We now watch this zone from orbit. Since NASA's SeaWiFS satellite went up in 1997, ocean-colour sensors have measured the green of chlorophyll across the whole planet every few days, mapping blooms off Patagonia, the blue deserts of the central gyres and the slow spread of those deserts as surface waters warm. Warmer water sits more stubbornly on top of the cold, rich water below, starving the sunlit zone of nutrients. What happens in this thin layer sets the menu for the entire ocean beneath it.

The Twilight Zone

Below 200 metres the light does not switch off; it dims to a blue dusk that lasts for the next 800 metres. This is the mesopelagic or twilight zone, too dark for photosynthesis but bright enough, near its top, for an animal to be seen against the glow from above. Life here is shaped by that single problem: how to eat without being eaten in a place where every silhouette gives you away.

The zone announced itself by accident. During the Second World War, American sonar operators noticed a sound-reflecting layer hundreds of metres down that looked like a false bottom. Odder still, it rose at dusk and sank at dawn. In 1945 the marine biologist Martin Johnson argued that this deep scattering layer must be alive, and he was right: it is made of fish, shrimps, krill and squid in such numbers that they echo like solid ground.

That daily rise and fall is diel vertical migration, and by mass it is the largest migration on Earth. As the sun sets, animals that have hidden in the gloom all day swim up hundreds of metres to graze and hunt in the dark surface waters, then retreat before sunrise. A lanternfish a few centimetres long may climb several hundred metres in an evening, every night of its life, across every ocean from the Arabian Sea to the Southern Ocean.

Did you know? The barreleye fish Macropinna microstoma, filmed off California by MBARI scientists, has a transparent head through which its tubular green eyes can swivel to look straight up.

The cast is extraordinary. Some 250 species of lanternfish wear rows of light organs like portholes. Bristlemouths of the genus Cyclothone, no longer than a finger, may be the most numerous vertebrates on the planet. Hatchetfish hide their shadows with belly lights, and Humboldt squid rise from the depths off Peru and Mexico to hunt at night. Estimates of the fish alone have soared: trawls once suggested about a billion tonnes, but a 2014 acoustic survey by Xabier Irigoien and colleagues argued for ten times that or more, because many fish see nets coming and dodge them.

The commute matters far beyond the zone. Migrators eat at the surface and release waste at depth, carrying carbon down below the reach of the atmosphere, a living part of the biological carbon pump. Projects such as Woods Hole's Ocean Twilight Zone study now measure that flow, while some nations eye lanternfish as feed for fish farms. A fishery here could take the fish that help bury our carbon, and we still barely know how many there are.

The Midnight Zone

At about 1,000 metres the last trace of sunlight is gone. From here to roughly 4,000 metres stretches the bathypelagic or midnight zone, the largest living space on Earth by volume. It is cold, around 4 °C, pressed by hundreds of atmospheres and almost foodless. The only light is the light animals make, and in the open water a remarkable share of them make it: when Séverine Martini and Steven Haddock analysed years of video from MBARI's remotely operated vehicles off California, they found that about three-quarters of the animals they recorded could glow.

Food arrives mostly as marine snow, a slow drizzle of dead plankton, mucus and faecal pellets falling from the sunlit zone above. Japanese scientists Noboru Suzuki and Kenji Kato named it in 1953 for its likeness to falling snowflakes. A particle may take weeks to sink this far, and much of it is eaten on the way, so the animals of the midnight zone are built to spend almost nothing and to miss no chance.

That economy shapes their bodies. Many are black or red, which is invisible here, with weak muscles and watery flesh that cost little to maintain. The deep-sea anglerfish dangles a lure lit by glowing bacteria and waits; in some species the tiny males latch onto a female and fuse with her for life, since finding a partner twice in the dark is too much to ask. The gulper eel is little more than a vast pouch-like mouth on a whip of a tail, and the black swallower, Chiasmodon niger, can gulp prey more than twice its own length, its stomach stretching like a balloon.

Did you know? The vampire squid neither drinks blood nor hunts: it trails sticky filaments to gather marine snow, as Hendrik Hoving and Bruce Robison showed in 2012.

Not everything here is small. Sperm whales dive to 2,000 metres from the Azores to Kaikōura in New Zealand, clicking to find squid in total darkness, and southern elephant seals hunt at similar depths in the Southern Ocean. The first humans to glimpse this world were William Beebe and Otis Barton, who reached 923 metres in their steel bathysphere off Bermuda in 1934 and described lights like distant stars.

We still know only a sliver of the zone. Each deep ROV dive off Monterey, Hawaii or the Mid-Atlantic Ridge turns up animals no one has named. As fishing, mining and warming reach deeper, the planet's biggest habitat is changing before it has even been surveyed.

The Abyss

Between about 4,000 and 6,000 metres down lie the abyssal plains, the broad floors of the ocean basins. They are among the flattest places on Earth, smoother than any prairie, because a steady fall of sediment has buried the rough volcanic crust beneath. Taken together, these plains cover roughly half of the planet's surface, more than all the continents combined, and almost none of it has been seen.

The sediment arrives with geological patience. Far from land it is mostly fine red clay and the shells of microscopic plankton, settling at a few millimetres per thousand years. The water is cold, around 1 to 3 °C, and the pressure is several hundred atmospheres. Food comes only as marine snow from kilometres above or, now and then, a windfall: a dead whale, a sunken tree trunk or a mat of plankton that sank in one go after a bloom.

In this slow world the sea cucumber is king. On many abyssal plains, holothurians make up most of the weight of large animals on the seafloor, ploughing through the mud and digesting the thin film of food on it. Scotoplanes, the "sea pig", walks on stubby tube feet in herds. Tripod fish perch on three stiff fins, facing the current to catch drifting prey, while xenophyophores, single cells the size of a fist, build fragile homes of mud.

Did you know? The Challenger expedition dredged the first manganese nodules from the deep floor in 1873, near the Canary Islands.

Those polymetallic nodules, potato-sized lumps rich in manganese, nickel, cobalt and copper, grow by only a few millimetres per million years. They carpet parts of the Clarion-Clipperton Zone, an area of the central Pacific of around 4.5 million square kilometres between Hawaii and Mexico, and mining companies want them for batteries. The International Seabed Authority in Kingston, Jamaica, has spent years drafting rules for mining the international seabed. Many of the creatures living there have never been named, and some, such as certain sponges and corals, grow only on the nodules themselves.

How fast would the abyss recover? In 1989 German scientists ran the DISCOL experiment in the Peru Basin, ploughing an 11-square-kilometre patch of seafloor to imitate mining. When researchers returned in 2015, 26 years later, the plough tracks looked almost fresh and many animals had still not come back. On the abyssal plain, a mark made in an afternoon lasts for generations.

The Hadal Zone

The deepest parts of the ocean do not lie under the open sea but in narrow gashes where one tectonic plate dives beneath another. Below about 6,000 metres begins the hadal zone, named after Hades, the Greek underworld. It spans nearly half of the ocean's full depth range yet covers less than two per cent of the seafloor, scattered across some thirty trenches, mostly around the Pacific rim from the Kermadecs to the Kurils.

Pressure is the defining fact. At the bottom of the Challenger Deep in the Mariana Trench, about 10,935 metres down, it exceeds 1,000 atmospheres. Water at that pressure squeezes the shapes of proteins out of true, so hadal animals load their cells with protective molecules called piezolytes. The best known is TMAO, trimethylamine N-oxide, the chemical behind the smell of fish. In 2014 Paul Yancey, Alan Jamieson and colleagues showed that TMAO in fish muscle rises steadily with depth, and predicted that at around 8,200 metres a fish would need so much of it that its body fluids would pull in seawater dangerously. Fish, they suggested, simply run out somewhere near there.

The prediction has held up well. In September 2022 a baited camera lowered into the Izu-Ogasawara Trench south of Japan filmed a young snailfish of the genus Pseudoliparis at 8,336 metres, the deepest fish ever recorded. Snailfish are the hadal success story: soft, tadpole-shaped and scaleless, they hunt small crustaceans at the bottom of trenches across the Pacific. The Mariana snailfish, described in 2017, lives around 7,000 to 8,000 metres.

Did you know? Amphipods collected from the Mariana and Kermadec trenches in 2017 carried PCB pollution up to 50 times higher than crabs from one of China's most polluted rivers.

Below the fish limit, crustaceans take over. Amphipods, shrimp-like scavengers, swarm onto bait within minutes even at the bottom of the Challenger Deep, where Hirondellea gigas lives at nearly 10,900 metres. Sea cucumbers, worms and single-celled xenophyophores live on the trench floors too, fed by sediment that slumps down the steep walls and gathers in the axis like silt in a gutter.

People have reached the bottom only a handful of times. Jacques Piccard and Don Walsh descended in the bathyscaphe Trieste on 23 January 1960; James Cameron dived alone in Deepsea Challenger in 2012; and in 2018 and 2019 Victor Vescovo's Limiting Factor reached the deepest point of all five oceans. Even here they found plastic and pollution, proof that nowhere in the sea is out of our reach.

Light and Colour Underwater

Dive with a red-striped fish and watch it change. At the surface the stripes glow; by 20 metres they look brown; deeper still they turn grey or black. Nothing has happened to the fish. Water is simply eating the light, and it eats the colours in order. Water molecules absorb long, red wavelengths most strongly, so in clear ocean water red light is largely gone within about 10 metres, orange by 20 or so, yellow by about 50 and green by around 100. Blue light, which water absorbs least, reaches 200 metres and more.

That selective absorption is the main reason the sea is blue. For a long time people assumed it simply reflected the sky. In 1921, sailing home from Europe aboard the SS Narkunda, the Indian physicist C. V. Raman watched the deep blue of the Mediterranean, tested it with a polarising prism and argued that the colour came from light scattered by the water itself. His work on scattering led to the Raman effect and a Nobel Prize in 1930. Later physicists added the final piece: water's own vibrations absorb red, so what scatters back to our eyes is the blue that survives. Where plankton thrive, chlorophyll tints the water green, which is why coastal seas off Namibia or Newfoundland look olive while the Sargasso Sea is a deep sapphire.

Animals have exploited these rules for millions of years. Because red light never reaches the deep, red is the colour of invisibility there: deep-sea shrimps, jellyfish and fish are often scarlet, safely black in the blue gloom. The dragonfish Malacosteus breaks the rule by shining its own red searchlight and seeing it with a pigment borrowed from chlorophyll in its food, a private beam no prey can detect.

Did you know? The silver spinyfin, a deep-sea fish, carries 38 copies of the rod opsin gene, more light-sensing genes for dim light than any other vertebrate known.

In the twilight zone the danger is a shadow against the light from above, so many animals practise counter-illumination: hatchetfish and lanternfish line their bellies with light organs that match the downwelling glow, erasing their silhouettes. The Hawaiian bobtail squid does the same with glowing Vibrio bacteria. Most deep-sea fish have eyes tuned to a single colour, blue near 480 nanometres, because little else is left to see.

Understanding this colour chemistry matters beyond the beauty of it. Satellites read the ocean's colour to measure plankton, divers carry lights to see true colours, and engineers choose blue-green lasers for underwater communication, because in the sea blue is the colour that lasts.

Pressure

Every 10 metres of seawater adds about one atmosphere of pressure, the weight of the whole sky pressing on you at sea level. A diver at 10 metres feels two atmospheres, at 40 metres five. At the bottom of the Challenger Deep, nearly 11 kilometres down, the pressure is about 1,100 atmospheres, like the weight of a small car pressing on every square centimetre.

The popular image of the deep is of things being crushed flat. The truth is subtler. Water is barely compressible, so an animal made mostly of water feels pressure from inside and outside equally and is not squashed at all. The danger is air. Boyle's law says a gas halves its volume each time the pressure doubles, so a free diver's lungs at 10 metres are half their surface size. When Herbert Nitsch reached 214 metres in 2007, his lungs were squeezed to about a twenty-second of their surface volume. Submarines, bathyspheres and human lungs are in trouble because they hold gas; jellyfish are not.

Gas causes a second problem on the way up. Under pressure, extra nitrogen dissolves into a diver's blood and tissues; rise too fast and it comes out as bubbles, like a shaken bottle of lemonade. This is decompression sickness, the bends. It crippled workers digging the foundations of the Brooklyn Bridge in the 1870s. The French physiologist Paul Bert identified nitrogen bubbles as the cause in 1878, and in 1908 John Scott Haldane gave the Royal Navy the first staged decompression tables.

Did you know? The blobfish, voted the world's ugliest animal in 2013, looks like a normal fish at home around 1,000 metres; its famous sag is damage caused by hauling it up.

Deep-sea animals avoid air altogether. Many deep fish have no gas-filled swim bladder, relying on fatty livers or watery, gelatinous tissue to float. Sperm whales diving to 2,000 metres let their lungs collapse and store oxygen in their blood and muscles instead, and a Cuvier's beaked whale off California was tracked to 2,992 metres. At a molecular level, deep animals carry protective molecules that keep their proteins working, the same chemistry that sets the limit for fish in the trenches.

Oceanographers enjoy a simple demonstration. Polystyrene cups, decorated by students, are tied to instruments lowered thousands of metres down. The air in the foam is squeezed out, and the cups come back as perfect miniatures, proof that the deep crushes air rather than water. The lesson runs through all deep-sea engineering: from the bathysphere that carried William Beebe to 923 metres in 1934 to today's titanium submersibles, the hard part is keeping a bubble of air alive.

Temperature and the Thermocline

Swim down through a tropical sea and you pass through three worlds of temperature. At the top is the warm mixed layer, stirred by wind and waves, often 25 °C or more and nearly uniform for tens of metres. Below it the temperature plunges through the thermocline, a band where the water may cool by 15 or 20 degrees across a few hundred metres. Beneath that lies the deep ocean, a vast reservoir of water between about 0 and 4 °C, unchanged by summer or winter. Roughly nine-tenths of the ocean's water sits below the thermocline, and the average temperature of the whole ocean is only about 4 °C.

The layering exists because warm water is lighter than cold. The sun heats the surface from above, so the ocean is stable, like oil floating on vinegar, and the thermocline acts as a lid. That lid is a blessing and a curse. It keeps the deep cold, but it also traps nutrients below, out of reach of the plankton in the light, which is why the clear blue tropics are often biological deserts. In temperate seas a seasonal thermocline forms each summer and breaks down in winter storms, while in polar seas the water is cold from top to bottom and there is barely a thermocline at all.

There are local exceptions. The deep Mediterranean stays around 13 °C because warm, salty water sinks there, and the bottom of the Red Sea is a balmy 21 °C or so. In the deep basins of the Red Sea there are even hot brine pools above 60 °C, heated from the crust below.

Did you know? The ocean has absorbed more than 90 per cent of the extra heat trapped by greenhouse gases since the 1970s, according to the IPCC.

Measuring all this used to be painstaking. The Challenger expedition of 1872 to 1876 lowered thermometers on ropes, one depth at a time. In 1938 Athelstan Spilhaus invented the bathythermograph, which drew a temperature trace as it fell, and navies used it to find thermoclines where submarines could hide from sonar. Today the work is done by Argo, a fleet of around 4,000 robotic floats launched from 2000 onwards. Each float drifts at 1,000 metres for about nine days, sinks to 2,000 metres, then rises to the surface measuring temperature and salinity, and radios its profile to a satellite.

Argo has transformed climate science, recording the steady warming of the upper ocean basin by basin. Deep Argo floats now reach 6,000 metres, because the cold reservoir at the bottom is where the long memory of our warming will be stored.

Sound in the Sea

Light dies within a few hundred metres of the surface, but sound travels astonishingly well in seawater. It moves at about 1,500 metres per second, more than four times faster than in air, and low notes can cross whole ocean basins. For the animals of the sea, and for the people who study them, the ocean is above all a world of hearing.

The first measurement came in 1826, when the Swiss physicist Daniel Colladon and the mathematician Charles Sturm struck an underwater bell in Lake Geneva and timed the sound across about 14 kilometres, getting 1,435 metres per second, close to the modern figure for fresh water. After the Titanic sank in 1912, the Canadian inventor Reginald Fessenden built an electric oscillator that could send sound through water and listen for echoes; in 1914 it detected an iceberg about three kilometres away. From that line came sonar, which hunted submarines in two world wars and later mapped the seafloor.

Sound speed depends on temperature and pressure. Near the surface, warm water makes sound fast; deep down, pressure speeds it up again. In between, at about 1,000 metres in mid-latitudes, sits a layer of minimum speed. Sound entering it bends back towards the slow axis like light in an optical fibre, and can travel thousands of kilometres. In 1944 Maurice Ewing and J. Lamar Worzel proved this SOFAR channel existed by detonating small charges and hearing them hundreds of kilometres away. In 1991 the Heard Island Feasibility Test sent sounds from the southern Indian Ocean that were picked up off Bermuda and the west coast of North America.

Did you know? The "Bloop", a mysterious roar recorded by NOAA hydrophones across the Pacific in 1997, was finally identified in 2012 as the sound of an Antarctic icequake.

Whales were using the ocean's acoustics long before we were. Navy engineer Frank Watlington recorded strange sounds off Bermuda, and Roger Payne and Scott McVay showed that humpback whales sing long, repeating, structured songs; the 1970 album Songs of the Humpback Whale helped launch the campaign to save the whales. Blue and fin whales call at deep frequencies near 20 hertz that can carry hundreds of kilometres. Snapping shrimps crackle so loudly on reefs that they can mask submarines.

Today the sea is getting louder. Ship propellers, seismic airguns and sonar fill the same low frequencies whales use, and studies off California found deep-water noise rising sharply over the late twentieth century. For animals that find food, mates and each other by sound, a noisy ocean is like a fog that never lifts.

The Polar Oceans

The two polar oceans are mirror images. The Arctic Ocean, about 14 million square kilometres, is the smallest and shallowest of the world's oceans, a basin almost enclosed by North America, Greenland and Eurasia, open to the Pacific only through the narrow Bering Strait and to the Atlantic through the gap beside Greenland. The Southern Ocean is the reverse: open water circling an ice-covered continent, with no land to stop it anywhere around the globe. The National Geographic Society formally recognised it as the fifth ocean in 2021.

Both are defined by sea ice, frozen seawater rarely more than a few metres thick that grows each winter and shrinks each summer. Around Antarctica the swing is enormous, from about 3 million square kilometres in February to around 18 million by September, more than doubling the continent's size. In the Arctic, satellites have tracked a steep decline since 1979. In September 2012 the summer ice fell to about 3.4 million square kilometres, the lowest on record, roughly half the area typical in the 1980s, and it has grown thinner too. Polar bears, walruses and ringed seals depend on that ice as a hunting platform and nursery.

Around Antarctica flows the mightiest current on Earth, the Antarctic Circumpolar Current. Driven east by the westerly winds of the Roaring Forties and Furious Fifties, it is the only current that circles the globe unbroken, carrying well over a hundred million cubic metres of water every second through the Drake Passage. It also isolates Antarctica from warmer water to the north, helping keep it frozen.

Did you know? Antarctic icefish have no red blood cells or haemoglobin at all, a discovery made by the Norwegian biologist Johan Ruud in 1954.

Polar seas are cold but rich. Strong winds and upwelling bring nutrients up, and the summer sun runs almost round the clock. In the Southern Ocean that fuels Antarctic krill, finger-long crustaceans whose total weight has been estimated at around 380 million tonnes, feeding penguins, seals, albatrosses and the great baleen whales. Notothenioid fish survive water below 0 °C thanks to antifreeze proteins in their blood, studied by Arthur DeVries from the late 1960s.

Explorers learned these seas the hard way. Fridtjof Nansen let his ship Fram freeze into the Arctic pack in 1893 and drifted for three years, proving the ice moves across a deep ocean rather than resting on land. In 1915 the pack in the Weddell Sea crushed Ernest Shackleton's Endurance; its wreck was found in 2022, 3,008 metres down and remarkably intact. Today the poles are warming faster than almost anywhere, and what happens to their ice will be felt on every coast.

Plankton: the Drifting Multitude

Plankton is not a kind of creature but a way of life. The word, coined by the German physiologist Victor Hensen in 1887 from the Greek for "wanderer", covers anything that cannot swim against a current. That includes bacteria a few thousandths of a millimetre across, the eggs and larvae of crabs and cod, and a lion's mane jellyfish with tentacles longer than a tennis court. What unites them is that the sea, not their own muscle, decides where they go.

Biologists split the multitude in two. Phytoplankton are the plants of the drift, single cells that photosynthesise in the sunlit top 200 metres. Diatoms build glassy shells of silica shaped like pillboxes and stars; coccolithophores armour themselves in chalk plates; dinoflagellates whirl along on two whip-like flagella. Zooplankton are the animals that graze them. The most important are copepods, crustaceans the size of a rice grain that may be the most numerous animals on Earth. Some zooplankton drift for life, while others, like fish larvae, are only passing through.

We know so much about them thanks partly to a remarkably durable machine. In 1931 the marine biologist Alister Hardy began towing his Continuous Plankton Recorder behind merchant ships. Water flows through a small opening and the plankton are trapped on a slowly winding band of silk, which preserves a ribbon-like record of every mile. The survey, now run from Plymouth, has towed its recorders for millions of nautical miles. It has shown that warm-water copepod communities in the north-east Atlantic shifted about ten degrees of latitude north between the 1960s and the 2000s, while the cold-water Calanus finmarchicus, a staple for young cod, retreated.

Plankton make spectacle as well as statistics. Blooms of the coccolithophore Emiliania huxleyi turn whole seas milky turquoise in satellite images, from the Barents Sea to the coast of Patagonia. Night-time ripples of blue light off the Maldives and in Puerto Rico's Mosquito Bay come from dinoflagellates flashing when disturbed. Every night the largest migration on the planet takes place as countless zooplankton rise hundreds of metres from the twilight depths to feed at the surface, then sink again before dawn.

Did you know? The White Cliffs of Dover are made largely of coccoliths, the microscopic chalk plates shed by plankton in a warm sea some 70 to 100 million years ago.

Plankton sit at the base of almost every marine food chain, they help draw carbon down into the deep, and their dead shells pile up into the rocks of the future. As the ocean warms, the plankton map is being redrawn, and everything that eats them will have to follow. The smallest drifters set the menu for the whole sea.

The Breath of the Ocean

Take two breaths, and one of them was probably made at sea. Microscopic phytoplankton carry out roughly half of all the photosynthesis on Earth, fixing around 50 billion tonnes of carbon a year in the sunlit surface layer. Inside each cell, chlorophyll catches sunlight and uses its energy to split water, building sugar from carbon dioxide and releasing oxygen as a by-product. The ocean's share rivals that of every forest, grassland and field put together.

It is the oldest breath of all. Ancient marine cyanobacteria began pumping out oxygen billions of years ago, and around 2.4 billion years ago enough had built up to transform the atmosphere in the Great Oxidation Event. Their descendants still dominate. Prochlorococcus, the smallest and probably the most abundant photosynthetic organism on the planet, was only recognised in 1988, when Sallie Chisholm, Robert Olson and colleagues spotted its faint red glow with a flow cytometer, a laser instrument that counts cells one by one. At around 0.6 thousandths of a millimetre across, it numbers in the octillions across the warm open ocean.

Measuring the breath of a whole planet needs a view from orbit. Since NASA's SeaWiFS satellite began work in 1997, ocean-colour sensors have mapped chlorophyll by reading the subtle green tint it gives the sea. The maps show blazing productivity in upwelling zones off Peru, Namibia and California, spring blooms sweeping across the North Atlantic, and vast "ocean deserts" in the centres of the subtropical gyres, where nutrients run short and the water is the clearest blue on Earth.

There is a popular myth to correct here. The ocean is not a lung exporting spare oxygen for us to breathe. Plankton, fish and above all bacteria consume almost all of what phytoplankton make, and over a year the sea's net contribution to the air is close to zero. The oxygen we breathe today is a reservoir built up over hundreds of millions of years, partly because a small share of marine carbon was buried in sediments before it could be respired. What the plankton really sustain day to day is the ocean itself.

Did you know? A single millilitre of sunlit tropical seawater can hold more than 100,000 cells of Prochlorococcus, yet no one knew it existed until 1988.

That ocean is losing its breath. Measurements suggest the global ocean has lost around 2 per cent of its dissolved oxygen since 1960, because warmer water holds less gas and mixes less with the depths, and low-oxygen zones off West Africa and in the eastern Pacific have spread. Every breath on land is a gift from ancient plankton; every fish now depends on the living ones.

The Ocean Food Web

A food web is the ocean's ledger of who eats whom. At its base are the producers, phytoplankton and seaweeds that turn sunlight into living tissue. Above them come grazers such as copepods and krill, then small fish and squid, then the predators that eat them, up to orcas, great white sharks and tuna. Each step is a trophic level, from the Greek for nourishment, and each one is expensive. Only about a tenth of the energy at one level, on average, ends up in the bodies of the next; the rest is burned in living, swimming and keeping warm.

That rule of thumb traces back to the American ecologist Raymond Lindeman, who in 1942 described energy flowing through a Minnesota bog lake. In the sea it explains a great deal. Short chains are efficient: off Peru, the Humboldt Current feeds diatoms that anchoveta eat directly, which is why that one coast has supported the world's largest single-species fishery. Long chains, such as those of the open subtropical ocean, can support only a few top predators. Scientists now trace who eats whom using stable isotopes of nitrogen, which become slightly heavier with each step up the web.

The tidy pyramid hides a hidden economy. In 1983 Farooq Azam and colleagues described the microbial loop: a large share of what plankton make leaks out as dissolved organic matter, which only bacteria can use. Tiny protists eat the bacteria and are in turn eaten by zooplankton, returning energy to the main web. Viruses join in, bursting vast numbers of microbes every day and spilling their contents back into the water. In the open ocean, much of the carbon fixed each day passes through microbes rather than through fish.

Some species matter far more than their numbers suggest. In the 1960s the ecologist Robert Paine pulled ochre sea stars off the rocks at Mukkaw Bay in Washington State; mussels took over and many other species vanished. He called such animals keystone species. In the Aleutian Islands sea otters play the same role, eating sea urchins that would otherwise mow down kelp forests. Whales are now seen as keystones too: by feeding at depth and defecating at the surface, they return iron and nitrogen to sunlit water and fertilise the plankton.

Did you know? Viruses are by far the most numerous biological entities in the sea, and by one widely cited estimate they kill around a fifth of all ocean microbial biomass every day.

Humans are now the dominant predator in many marine webs. In 1998 the fisheries scientist Daniel Pauly showed that global catches were shifting from large predators to smaller fish lower down, a process he called "fishing down marine food webs". Pull one thread, and the whole web moves.

Jellyfish and Their Kin

Jellyfish have pulsed through the sea for at least 500 million years, since long before the first dinosaurs. They belong to the Cnidaria, a group of around 11,000 species that also includes corals, sea anemones and hydroids. A jellyfish has no brain, no heart and no bones, and is roughly 95 per cent water. Its body plan is a pulsing bell with a single opening that serves as both mouth and anus, ringed by tentacles armed with stinging cells.

Those cells, called cnidocytes, are among the most remarkable weapons in biology. Each holds a coiled, barbed thread inside a capsule under great pressure. When touched, the capsule fires in around a millionth of a second, one of the fastest movements known in nature, and injects venom. Most stings are harmless to people, but the box jellyfish Chironex fleckeri of northern Australia and South-east Asia can kill an adult within minutes. Box jellyfish also have 24 eyes, some with lenses, which help them steer around mangrove roots.

Many cnidarians lead double lives. A typical jellyfish starts as a free-swimming larva that settles on the seabed and becomes a polyp, like a tiny anemone. The polyp then buds off stacks of baby jellyfish, called ephyrae, which grow into the adult medusa. In the 1990s researchers in Italy noticed that Turritopsis dohrnii, a hydrozoan jellyfish smaller than a fingernail, could do something astonishing: when stressed or injured, the adult turns back into a polyp and starts over. It is often called the immortal jellyfish, though in the wild most are simply eaten.

Not everything that looks like a jellyfish is one. The Portuguese man o' war, Physalia physalis, is a siphonophore: a colony of specialised individuals, one forming the gas-filled float that sails before the wind, others the fishing tentacles that trail for 10 metres or more. Comb jellies, which shimmer with rainbow light, belong to a separate group altogether. Jellyfish can also be giants: Nomura's jellyfish in the seas around Japan can reach 2 metres across and 200 kilograms, and the lion's mane jellyfish trails tentacles tens of metres long.

Did you know? In 2011 a swarm of moon jellyfish clogged the cooling-water intakes of Torness nuclear power station in Scotland and forced it to shut down its reactors.

Jellyfish blooms seem to have grown more frequent in some places, from the Sea of Japan to the Mediterranean. Overfishing removes their competitors and predators, warm water speeds their growth, and harbours give polyps new surfaces to settle on, though scientists still debate how widespread the trend is. Brainless they may be, but jellyfish are very good at waiting for their moment.

Octopus, Squid and Cuttlefish

Cephalopods, the "head-footed" molluscs, are snails that gave up their shells for speed and wits. Around 800 living species include octopuses, squid, cuttlefish and the shelled nautiluses. All have arms around a parrot-like beak, a siphon that squirts water for jet propulsion, three hearts and copper-based blood that turns blue in air. Most live fast and die young: many octopuses survive only a year or two and die soon after breeding.

Their intelligence is the stuff of legend, and of laboratory record. A common octopus has around 500 million neurons, comparable to the cortex of a dog, and roughly two-thirds of them sit in its arms, which can taste, feel and react largely on their own. Octopuses solve mazes, unscrew jars and slip out of aquarium tanks. In 2009 researchers in Indonesia filmed veined octopuses, Amphioctopus marginatus, carrying coconut shells across the seabed to use later as a shelter, one of the first documented cases of tool use in an invertebrate. In 2022 the UK's Animal Welfare (Sentience) Act recognised cephalopods as sentient beings after a review of the evidence.

Their skin is a living screen. Thousands of chromatophores, tiny elastic sacs of red, yellow and brown pigment, are stretched open or squeezed shut by muscles under direct nerve control, changing a pattern in a fraction of a second. Beneath them, reflective cells add iridescent blues and bright whites. A cuttlefish can play waves of dark stripes across its body to mesmerise a crab, and match a chessboard placed under its tank. Strangely, most cephalopods seem to have only one type of light receptor in their eyes, and how they match colours so well remains under study.

When camouflage fails, there is ink: a cloud of melanin and mucus. Some squid squirt a blob roughly their own size, a "pseudomorph" that holds together as a decoy while they flee. The mimic octopus, Thaumoctopus mimicus, discovered off Sulawesi in 1998, goes further and impersonates venomous lionfish, flatfish and sea snakes. At the other extreme of size, the giant squid Architeuthis was first filmed alive in its deep-sea home in 2012 near Japan's Ogasawara Islands, while the colossal squid of the Southern Ocean is the heaviest invertebrate known, with one specimen weighing about 495 kilograms.

Did you know? An octopus has three hearts: two pump blood through the gills and the third through the rest of its body, and that main heart stops beating while it swims.

Cephalopods seem to be gaining from changing seas, with numbers rising in many fisheries as their fish competitors decline. The most alien minds on Earth may yet inherit the ocean we are emptying.

Crabs, Lobsters and Krill

Crustaceans are the insects of the sea: jointed-legged animals in a hard outer skeleton, astonishingly varied and almost everywhere. Around 67,000 species have been described, from copepods smaller than a full stop to the Japanese spider crab, whose legs can span more than 3.5 metres. Crabs, lobsters, shrimps, barnacles and woodlice all belong to the group. Their armour, the exoskeleton, is made of chitin stiffened with calcium carbonate, and it is both their great strength and their great problem.

Armour does not stretch, so to grow a crustacean must moult. Hormones signal the start of the process, called ecdysis: the animal reabsorbs some minerals from its old shell, grows a soft new one underneath, then splits the old casing along a seam and backs out of it, legs, gills, eyestalks and all. Freshly moulted, it gulps water to swell to its new size and hides for days while the new shell hardens. A soft-shell crab on a menu is simply one caught in that vulnerable window.

Lobsters show how long the strategy can run. European and American lobsters keep moulting throughout life, and large individuals are estimated to be 50 years old or more. Their cells keep the enzyme telomerase active, which spawned an internet myth that lobsters are biologically immortal. They are not: each moult needs more energy than the last, and many old lobsters die during the effort or stop moulting and succumb to shell disease. At the other extreme, the peacock mantis shrimp of the Indo-Pacific strikes with a club-like limb so fast, as Sheila Patek and colleagues measured in 2004, that it boils the water in front of it into collapsing bubbles.

Then there is Antarctic krill, Euphausia superba, a shrimp-like animal about 6 centimetres long. Its total biomass is estimated at around 300 to 500 million tonnes, which may make it the most abundant wild animal species on Earth by weight. Krill graze algae under the sea ice and in open water, gathering in swarms that can stain the sea red, and they feed blue and humpback whales, penguins, seals and fish. Britain's Discovery Investigations of the 1920s and 1930s first mapped their life cycle while studying the whaling grounds.

Did you know? When food runs short through the Antarctic winter, krill can moult into a smaller body and shrink, surviving for months without eating.

Krill are now a fishing target too, managed since 1982 by the international commission CCAMLR, while warming and shrinking sea ice around the Antarctic Peninsula appear to be shifting their range south. A crab's armour is its castle; a krill swarm is a continent's lunch.

The World of Fishes

Fishes are the most diverse vertebrates on Earth. More than 36,000 species have been described, more than all mammals, birds, reptiles and amphibians combined, and the great majority are bony fishes, with skeletons of true bone rather than cartilage. Almost all of these are ray-finned fishes, whose fins are webs of skin held up by slender spines. Their lineage reaches back more than 400 million years, and from that common plan evolution has produced seahorses, flounders, eels, tuna and the fish that fishermen still call "the one that got away".

Life in water starts with breathing it. Water enters the mouth and flows over the gills, where blood in thin filaments runs the opposite way. This counter-current exchange keeps blood meeting water that is always a little richer in oxygen, and lets a fish extract a far larger share of the dissolved oxygen than our lungs take from air. Many bony fish also carry a swim bladder, a gas-filled bag that they inflate or deflate to hover without effort, which is why a cod can hang in midwater like an airship.

Fish sense the sea in ways we cannot. The lateral line, a row of pores along each flank, detects tiny changes in water pressure, letting a fish feel the wake of a predator or the movements of its neighbours. That sense makes schooling possible. Herring, sardines and anchovies can wheel in unison by the million; each winter off South Africa's east coast the sardine run draws dolphins, sharks, gannets and Bryde's whales into one of the great feeding frenzies on the planet.

The range of size is staggering. The stout infantfish of the Great Barrier Reef matures at about 7 millimetres, and dwarf gobies of the Indian and Pacific oceans are barely larger. At the other end, the ocean sunfish and its relatives are the heaviest bony fishes: a giant sunfish found off the Azores in 2021 weighed 2,744 kilograms. A single female ocean sunfish may carry around 300 million eggs. Some fish surprise us even about their existence: in 1938 the museum curator Marjorie Courtenay-Latimer spotted a coelacanth, a lobe-finned fish thought extinct for 66 million years, in a trawler's catch off East London, South Africa.

Did you know? The heaviest bony fish ever recorded was not a tuna or a marlin but a giant sunfish, found off the Azores in 2021 and weighing 2,744 kilograms.

Fish are also the world's most important wild food, and the most visibly overused. When the Grand Banks cod off Newfoundland collapsed, Canada closed the fishery in 1992 and some 30,000 people lost their work overnight. The sea's richest vertebrate kingdom is also its most heavily harvested.

Sharks and Rays

Sharks swam the seas before there were trees on land. Their lineage, the chondrichthyans, dates back more than 400 million years and today numbers around 1,200 species of sharks, rays and chimaeras. Their skeletons are made of cartilage, lighter and more flexible than bone, and their skin is covered in tiny tooth-like scales called dermal denticles, which smooth the flow of water and inspired swimsuits and ship coatings. Rays are flattened sharks at heart, with gill slits on the underside and pectoral fins grown into wings.

Sharks are famous for their senses. Smell picks up blood and prey in the water, and the lateral line feels movement. Most remarkable are the ampullae of Lorenzini, jelly-filled pores on the snout first described by the Italian anatomist Stefano Lorenzini in 1678. They detect the faint electric fields given off by the muscles of living animals, so a hammerhead can find a stingray buried in sand. Its wide head spreads the sensors like a metal detector. Many species may also use the Earth's magnetic field to navigate across open ocean.

Some sharks break every record. The whale shark, Rhincodon typus, is the largest living fish, reaching around 18 metres, and it feeds on plankton and fish eggs by filtering seawater, gathering each year off Ningaloo Reef in Western Australia, in the Gulf of Mexico and off the Maldives. The giant oceanic manta ray spans up to 7 metres from wingtip to wingtip and has one of the largest brains for its body size of any fish. And in 2016 a team led by Julius Nielsen dated Greenland sharks by measuring radiocarbon in the lenses of their eyes, which form before birth: the oldest was estimated at around 392 years, give or take 120, making it the longest-lived vertebrate known.

The popular image of sharks as hunters of people is a myth worth correcting. Unprovoked bites number fewer than a hundred a year worldwide, while people kill sharks in the tens of millions. Many die for finning, in which fins are sliced off for soup and the body is often thrown back. A 2021 study in Nature found that numbers of oceanic sharks and rays had fallen by about 71 per cent since 1970, with the oceanic whitetip, once among the most common large animals in the tropical ocean, among the worst hit.

Did you know? A Greenland shark grows only about a centimetre a year and does not reach maturity until it is around 150 years old.

Protection is slowly arriving, through finning bans, sanctuaries and trade limits under the CITES convention. A family that survived several mass extinctions now depends on decisions made in a few decades.

Sea Turtles

Sea turtles have navigated the oceans for more than 100 million years, and just seven species survive: the green, loggerhead, hawksbill, Kemp's ridley, olive ridley, flatback and leatherback. All breathe air, all spend almost their whole lives at sea, and every female must still crawl up a beach to lay her eggs, the one moment that ties these ocean wanderers to land.

Many females return to nest on or near the beach where they themselves hatched decades earlier, a feat called natal homing. How they find it has long been one of biology's puzzles. Research led by Kenneth Lohmann at the University of North Carolina suggests hatchlings imprint on the unique magnetic signature of their home coast, a combination of field strength and angle, and use the Earth's magnetic field as a map. Tagging, begun on a large scale by Archie Carr at Tortuguero in Costa Rica in the 1950s and continued today with satellite transmitters, has shown loggerheads hatched in Japan crossing the Pacific to feed off Baja California before returning.

On the beach, a female digs a flask-shaped nest with her rear flippers and lays around a hundred leathery eggs. Then she leaves. The eggs incubate for roughly two months, and the warmth of the sand decides each hatchling's sex: this is temperature-dependent sex determination. Near 29 °C the sexes are roughly equal; warmer nests produce mostly females, cooler ones mostly males. On the warm northern Great Barrier Reef, home to the vast Raine Island rookery, a 2018 study found that 99 per cent of young green turtles were female.

Hatchlings dig out together, usually at night, and race for the brightest horizon, which on a natural beach is the sea. Only a small fraction, often estimated at around one in a thousand, survive to adulthood. The leatherback is the giant of the family: it can exceed 600 kilograms, keeps its body warmer than the surrounding water, dives deeper than 1,000 metres and crosses whole oceans to feed on jellyfish. Olive ridleys, by contrast, nest in mass arrivals called arribadas, when tens of thousands come ashore within days at beaches such as Ostional in Costa Rica and Rushikulya in Odisha, India.

Did you know? The heaviest sea turtle ever recorded was a male leatherback found on the beach at Harlech in Wales in 1988, weighing 916 kilograms.

Six of the seven species are listed as threatened. Fishing nets, plastic bags mistaken for jellyfish, beachfront lights that lure hatchlings inland and warming sands all take their toll. Turtles that have outlived the dinosaurs' extinction now need dark beaches, turtle-excluder devices and a cooler world.

Seabirds

Around 350 species of birds make their living from the sea, from storm petrels no heavier than a sparrow to emperor penguins taller than a toddler. Seabirds include albatrosses, petrels, shearwaters, gannets, cormorants, gulls, terns, auks and penguins. They share salt glands above the eyes that let them drink seawater and sneeze out the excess, dense waterproof plumage and long, slow lives: many do not breed until they are five or more years old and then raise a single chick a year.

The wandering albatross has the longest wingspan of any living bird, up to about 3.5 metres, and it barely needs to flap. Using dynamic soaring, it swoops down into the slower air near the waves and climbs back into the faster wind above, gaining energy from the difference in wind speed, as the physicist Lord Rayleigh first suggested in 1883. Tracking devices have shown albatrosses covering hundreds of kilometres a day, and in 2005 researchers led by John Croxall reported a grey-headed albatross that flew round the whole Southern Ocean in 46 days. The oldest known wild bird, a Laysan albatross named Wisdom, was ringed on Midway Atoll in 1956 and was still raising chicks more than 65 years later.

Others go further still. In 2010 Carsten Egevang and colleagues fixed tiny geolocators to Arctic terns in Greenland and Iceland and found they flew to the Weddell Sea and back, a round trip averaging about 70,900 kilometres, the longest migration known of any animal. Because they follow summer between the poles, Arctic terns may see more daylight than any other creature.

At the opposite extreme are the divers. The Atlantic puffin flies underwater with its short wings, reaching depths of around 60 metres, and returns to its burrow carrying a beakful of sand eels held in place by its spiny tongue. Penguins have given up flight altogether: an emperor penguin has been recorded diving to 564 metres and staying under for over 27 minutes. As a broad rule the better a bird dives, the worse it flies, because wings that grip water are too small to carry it far through air.

Did you know? Seabird droppings were once worth fighting over: in the 19th century guano from Peru's Chincha Islands was shipped worldwide as fertiliser and helped spark a war with Spain in the 1860s.

Seabirds are now the most threatened group of birds. One analysis estimated that monitored seabird populations fell by about 70 per cent between 1950 and 2010, from longline bycatch, invasive rats on nesting islands, plastic and dwindling fish. When seabirds go hungry, it is often the first sign that the sea beneath them is in trouble.

Whales

Whales are mammals that went back to the sea. They breathe air through a blowhole on top of the head, nurse their young on milk and keep a warm body inside a coat of blubber. Around 90 living species make up the order Cetacea, and they fall into two great camps. The baleen whales, about 15 species including the blue, humpback, right and grey whales, have no teeth as adults. Instead, hundreds of fringed plates of keratin, the stuff of fingernails, hang from the upper jaw and strain shrimp-like krill or small fish from enormous mouthfuls of water. The toothed whales, around 75 species, hunt one prey at a time and include the sperm whale, the narwhal, the beaked whales and every dolphin and porpoise.

Their ancestors walked. In 1981 the palaeontologist Philip Gingerich described Pakicetus, a wolf-sized, long-tailed animal from rocks in Pakistan around 50 million years old. What gave it away was its ear: a thick, curled bone shell around the middle ear found only in whales. Fossils from Pakistan and India, many found by Hans Thewissen and Gingerich, then trace the move into water step by step. Ambulocetus, the "walking whale", hunted like a crocodile in coastal shallows around 49 million years ago. Rodhocetus had shortened legs and a stronger tail. By 40 million years ago Basilosaurus, a sea serpent 15 metres long whose bones lie by the hundred in Egypt's Wadi Al-Hitan, the Valley of the Whales, still carried tiny hind legs it could not walk on.

Did you know? The closest living relatives of whales are hippopotamuses: DNA and the double-pulley shape of the ankle bone in early whale fossils both place them in the same branch of hoofed mammals.

People learned to hunt them early. Basque whalers were chasing right whales in the Bay of Biscay by the eleventh century, and by the 1800s American ships out of Nantucket and New Bedford sailed every ocean for sperm whale oil to light lamps. The industrial age was far worse. The exploding harpoon gun developed by the Norwegian Svend Foyn in the 1860s and the factory ships of the 1920s let crews take the fastest and largest species, and a 2014 study estimated that close to three million whales were killed in the twentieth century alone, most of them in the Southern Ocean.

The International Whaling Commission agreed a pause on commercial whaling in 1982, which took effect in 1986. Japan, Norway and Iceland still hunt in their own waters, but many populations are recovering, and humpbacks now crowd coasts from Hawaii to the Antarctic Peninsula. Today the greatest threats are ship strikes, fishing gear and noise rather than harpoons. The sea's biggest animals began as small creatures paddling at the edge of an ancient sea.

The Blue Whale

The blue whale, Balaenoptera musculus, is the largest animal known to have lived. Adults reach around 30 metres, about the length of two and a half double-decker buses, and the heaviest may have weighed somewhere between 150 and 190 tonnes. No dinosaur yet found is known to have been heavier, although palaeontologists keep weighing the evidence for the biggest sauropods. Seen from a ship, a blue whale is a long, mottled blue-grey back that seems to go on for ever, then a blow nine metres tall.

All of that body is built on some of the sea's smallest prey. Blue whales eat krill, shrimp-like crustaceans a few centimetres long. A feeding whale accelerates into a swarm and opens its jaws almost at right angles, while pleated grooves running from chin to belly balloon outwards so the throat fills with a mass of water roughly as heavy as the whale itself. Then it pushes the water out through some 300 or more baleen plates on each side, keeping the krill. A 2021 study using tags on whales off California estimated that a feeding blue whale may eat up to around 16 tonnes of krill a day at the height of summer.

We know this because scientists have climbed onto the whales, in a manner of speaking. Teams led by Jeremy Goldbogen at Stanford attach suction-cup tags carrying cameras, depth gauges and accelerometers, and in 2019 one such tag, stuck beside a blue whale's flipper in Monterey Bay, recorded its heart for the first time. The beat fell to as few as two a minute on deep dives and rose to around 37 when the whale surfaced to breathe. A heart from a whale that died off Newfoundland in 2014, preserved by the Royal Ontario Museum, weighs about 180 kilograms.

Did you know? A newborn blue whale calf is around seven metres long and, feeding on its mother's rich milk, can gain roughly 90 kilograms a day.

Blue whales live in every ocean except the Arctic, from the Antarctic ice edge to the warm waters off Sri Lanka, where a resident population feeds within sight of the shore. Their low, booming calls are among the loudest sounds made by any animal and can carry hundreds of kilometres. That size nearly finished them. Once factory ships and fast catchers came to the Southern Ocean, whalers killed more than 300,000 blue whales in the twentieth century, and the Antarctic population fell to below one per cent of its former size before full protection came in 1966.

Recovery is slow, because a female may calve only every two to three years. Antarctic blue whales are increasing, and surveys around South Georgia now report sightings where for decades there were almost none, but they remain endangered and ship strikes are a serious hazard off busy coasts such as California and Sri Lanka. The biggest animal in history still depends on the sea's ability to grow krill.

Dolphins and Porpoises

Dolphins and porpoises are small toothed whales, and they are not the same animals. The oceanic dolphin family, around 37 species, includes the bottlenose, the spinner and, surprisingly to many, the orca, whose males reach about 9 metres and which is simply the largest dolphin. Most dolphins have a beak, a curved dorsal fin and cone-shaped teeth. The seven species of porpoise are smaller and stockier, with blunt faces, triangular fins and spade-shaped teeth. River dolphins of the Amazon, the Ganges and other great rivers form further, older branches of the family tree.

What makes them remarkable is echolocation. A dolphin produces rapid clicks by forcing air past structures in its nasal passages called phonic lips. The clicks pass through the melon, a fatty lens in the forehead that focuses them into a forward beam. Echoes bounce back from fish, rocks or a buried flatfish and are picked up through a channel of fat in the lower jaw that conducts sound to the inner ear. From the timing and shape of the echoes the animal builds a picture detailed enough to tell apart objects of different materials. Much of this was worked out from the 1950s onwards in experiments with trained bottlenose dolphins, including studies by the US Navy, which still uses them to find mines.

Their social lives are just as sophisticated. Each bottlenose dolphin develops a signature whistle, a call that works much like a name, and long-term studies in Sarasota Bay, Florida, running since 1970, show that dolphins remember the whistles of companions they have not met for decades. In Shark Bay, Western Australia, some females wear marine sponges on their beaks to protect them while digging for fish in the sand, a habit passed from mother to daughter. Orcas live in family groups led by older females and have hunting traditions so distinct that groups living in the same waters may never interbreed: in the north-east Pacific some eat only salmon, while others hunt seals.

Did you know? Female orcas, like humans and a few other toothed whales, live for decades after they stop breeding, and grandmothers lead their families to salmon when food is scarce.

Their intelligence does not protect them from nets. The vaquita, a small porpoise found only in the northern Gulf of California, has collapsed because it drowns in gillnets set for the totoaba, a fish whose swim bladder is smuggled to Asia. Surveys in 2023 found only around ten left. China's baiji, a river dolphin of the Yangtze, was declared probably extinct after a search in 2006 failed to find a single one. In New Zealand the Māui dolphin numbers only a few dozen.

Dolphins are among the most studied animals in the sea, and they are among the clearest proof that the ocean is home to minds as well as bodies. The next question is whether we can keep the quietest of them alive.

Seals, Sea Lions and Walruses

The pinnipeds, or "fin-footed" mammals, are carnivores that hunt at sea but must come ashore or onto ice to rest, moult and give birth. Their closest living relatives are bears and weasels. There are around 33 living species in three families, and you can tell them apart at a glance. True seals, such as the harbour, grey and Weddell seals, have no external ear flaps and swim with sweeps of their hind flippers, which trail behind them so that on land they shuffle on their bellies. Eared seals, the sea lions and fur seals, have small ear flaps, row with long front flippers and can turn their hind flippers forward to walk, even gallop, on rocks. The third family has a single member: the walrus.

True seals are the champion divers. In the 1960s the American physiologist Gerald Kooyman began attaching depth recorders to Weddell seals on the sea ice of McMurdo Sound in Antarctica. They dived to around 600 metres and stayed down for more than an hour, then found their way back to a single breathing hole, which they keep open by grinding the ice with their teeth. Elephant seals go further. Southern elephant seals have been recorded at more than 2,000 metres, and bulls, which can weigh around four tonnes, are the largest of all pinnipeds. A seal manages this by diving on exhaled breath, storing oxygen in blood and muscle rather than in the lungs, and slowing its heart.

Did you know? The walrus's scientific name, Odobenus, means "tooth-walker", because walruses sometimes hook their tusks into the ice to haul their great bodies out of the water.

Those tusks are elongated upper canine teeth that can reach about a metre in large males. They are used for display, fighting and hauling out, but not for digging. A walrus feeds on the seabed of the shallow Arctic shelf, finding clams with around 400 to 700 sensitive whiskers and sucking the soft body out of the shell with a powerful tongue. Pacific walruses once rested on floating sea ice above their feeding grounds; as summer ice retreats, tens of thousands now crowd onto beaches in Chukotka and Alaska, where stampedes can crush the young.

Pinnipeds are spread far wider than the polar ice. Galápagos sea lions sleep on park benches on the islands, Cape fur seals breed by the hundred thousand on the coast of Namibia and Hawaiian monk seals haul out on Pacific beaches. Many were hunted almost to extinction for fur and oil in the nineteenth century; the northern elephant seal fell to perhaps a few dozen animals and has since returned to well over 100,000. Today elephant seals even carry sensors that measure temperature and salinity under Antarctic sea ice, where ships cannot go. Pinnipeds live with one flipper in each world, and they now report back from both.

The Giant Squid

For centuries sailors told of the kraken, a monster off Norway big enough to be mistaken for an island. In 1752 and 1753 the Danish-Norwegian bishop Erik Pontoppidan described it in his Natural History of Norway, half believing the tales of fishermen. The real animal behind the legend is the giant squid, Architeuthis dux. In 1857 the Danish zoologist Japetus Steenstrup named the genus from a beak washed up on a Danish beach and argued that the kraken stories had a living basis. Females, the larger sex, can reach around 12 to 13 metres including their two long feeding tentacles, though most of that length is tentacle, and the body itself is a few metres long.

For a long time the evidence was all dead. In 1861 the crew of the French gunboat Alecton tried to haul one aboard near Tenerife and came away with only a piece of tail, an episode that helped inspire the squid attack in Jules Verne's Twenty Thousand Leagues Under the Seas. In 1873 fishermen in Newfoundland hacked a tentacle off a squid that grabbed their boat, and the following year the naturalist Moses Harvey famously photographed another specimen draped over a rod above his bath. Sperm whales offered more clues: their stomachs held thousands of squid beaks, and their skin bore circular scars from the toothed suckers of their prey.

Did you know? A genetic study of 43 giant squid from around the world, published in 2013, found that they all belong to a single species, with surprisingly little difference between oceans.

Seeing one alive took until the twenty-first century. In 2004 the Japanese zoologists Tsunemi Kubodera and Kyoichi Mori lowered a baited camera to around 900 metres off the Ogasawara Islands and photographed a giant squid attacking the line, the first images of one in its natural home. In 2012 Kubodera, with the American researchers Edith Widder and Steve O'Shea, filmed one from a submersible off Chichijima. Widder's trick was an electronic lure that mimics the flashing of a panicking jellyfish, and in 2019 her camera system filmed another in the Gulf of Mexico. Giant squid have eyes around 27 centimetres across, among the largest in the animal kingdom, probably for spotting the glowing wake of an approaching sperm whale.

Its cousin, the colossal squid of the Southern Ocean, is shorter but much heavier, with swivelling hooks on its arms and tentacles. A specimen caught in the Ross Sea in 2007 weighed around 495 kilograms and is displayed at Te Papa museum in Wellington, and in 2025 a young colossal squid was filmed alive for the first time near the South Sandwich Islands. The kraken turns out to be real, just shy, fragile and far more interested in fish than in sailors.

Bioluminescence

In the deep sea most of the light is made by animals. Bioluminescence is cold light produced by a chemical reaction: a light-emitting molecule called a luciferin is oxidised with the help of an enzyme, a luciferase, and gives off a photon instead of heat. Many marine animals use the same luciferin, coelenterazine, often taken in with their food, and most of their light is blue, around 470 to 490 nanometres, the colour that travels farthest through seawater. Some make the light themselves; others, like anglerfish, farm glowing bacteria in special organs.

How common is it? In 2017 Séverine Martini and Steven Haddock of the Monterey Bay Aquarium Research Institute went through 17 years of video from remotely operated vehicles, covering the water from the surface to nearly 4,000 metres off California. Of the hundreds of thousands of animals they could identify, 76 per cent were bioluminescent. On land the ability is a rarity, found in fireflies, a few fungi and little else. In the ocean it has evolved independently dozens of times, in fish, squid, shrimp, jellyfish, worms and single-celled plankton.

The light has four main jobs. Predators use it as bait: a female anglerfish dangles a glowing lure from a modified spine on her head, and the dragonfish Malacosteus shines a red searchlight that most deep animals cannot see. Prey use it to vanish. Hatchetfish and many squid practise counterillumination, glowing faintly from their bellies to erase their silhouette against the dim light from above. Defenders use it to startle or blind, like the vampire squid that releases a cloud of glowing mucus, or as a burglar alarm: when a copepod disturbs a dinoflagellate, the flash can attract a bigger fish that eats the copepod. And many species use light to talk. Lanternfish carry rows of light organs in patterns that differ between species, and every spring millions of firefly squid rise to spawn in Toyama Bay, Japan, turning the shore blue.

Did you know? In 1995 a ship off Somalia sailed through a "milky sea" of glowing water, and satellite images later showed the patch covered around 15,400 square kilometres, an area roughly the size of Northern Ireland, probably lit by bacteria.

These glowing patches, described by sailors for centuries, were confirmed from space by Steven Miller and colleagues in 2005, and satellites have since spotted larger ones near Java. Ocean light has also transformed medicine. In the 1960s Osamu Shimomura extracted a protein called aequorin and the green fluorescent protein from the jellyfish Aequorea victoria, found off the coast of Washington State. GFP now lights up cells and genes in laboratories worldwide and earned him a share of the 2008 Nobel Prize in Chemistry.

Today biologists film the deep with red light, which most of its inhabitants cannot see, so that the show goes on undisturbed. The darkest habitat on Earth turns out to be full of signals.

Great Ocean Migrations

Some of the longest journeys on the planet are made underwater. A migration is a regular round trip between places that meet different needs: rich feeding grounds, often cold and productive, and safe or warm places to breed. Ocean migrants follow currents, the seasons and the Earth's magnetic field, and some cover more distance in a lifetime than any land animal.

Humpback whales feed on krill in polar waters in summer and travel to the tropics to give birth, fasting for months on the way. Humpbacks from the Antarctic Peninsula reach the coasts of Colombia and Costa Rica, one of the longest regular migrations of any mammal at around 8,000 kilometres each way. In 2011 and 2012 a female grey whale named Varvara, tagged off Sakhalin in the Russian Far East, crossed the Pacific to Mexico and back, around 22,500 kilometres in 172 days. In 2003 and 2004 a great white shark called Nicole swam from South Africa to Western Australia and back again within nine months, a round trip of some 20,000 kilometres that surprised biologists who thought white sharks stayed local.

The strangest story is the European eel. For centuries no one had ever seen an eel egg or a mating eel. The Danish biologist Johannes Schmidt spent nearly two decades, from 1904, towing fine nets across the Atlantic and found that the eel larvae, transparent leaves called leptocephali, became smaller and smaller towards the Sargasso Sea. He concluded that this was where all European eels are born. The larvae drift back towards Europe on the Gulf Stream over many months, before slipping into rivers as glass eels. In 2022 scientists using satellite tags finally tracked adult eels from the Azores towards the Sargasso, though no one has yet watched one spawn.

Did you know? Pacific salmon find their home stream partly by smell: experiments begun by Arthur Hasler in the 1950s showed that young salmon imprint on the particular chemical scent of the river where they grew up.

Salmon also carry a magnetic map. Studies of sockeye returning to the Fraser River in Canada suggest they use the local strength and angle of the Earth's magnetic field to steer towards the right stretch of coast. Atlantic bluefin tuna were tracked across whole oceans for the first time when Barbara Block of Stanford and colleagues began fitting electronic archival tags in the 1990s. The tags showed that fish feeding off New England and Canada belong to two populations, one spawning in the Gulf of Mexico and the other in the Mediterranean, which mix at sea but go home to breed.

Migration makes these animals hard to protect, because no single country owns their whole journey. A tuna caught in the Bay of Biscay may have been born off Texas; a whale protected in Costa Rica may be struck by a ship off Chile. Ocean migrants knit the seas together, and their survival depends on the whole route.

Life at Hydrothermal Vents

In February 1977 the research submersible Alvin dived to around 2,500 metres on the Galápagos Rift, in the eastern Pacific. The team aboard were geologists and chemists, including Jack Corliss and John Edmond, looking for hot springs predicted by the theory of plate tectonics. They found the warm water, and something nobody expected: thickets of white crabs, giant clams, mussels and red-plumed worms, a crowded oasis on a seafloor that was meant to be a desert. There were no biologists on board, and the samples were pickled in Russian vodka because the ship had run short of preservative.

A hydrothermal vent forms where seawater seeps down through cracks in young ocean crust near a mid-ocean ridge, is heated by the magma below and comes back up loaded with dissolved minerals and gases. In 1979 Alvin found the most dramatic kind on the East Pacific Rise: black smokers, chimneys of metal sulphides belching water at around 350 to 400 °C. The pressure is so great that the water does not boil. When it meets the near-freezing sea, the minerals precipitate as black clouds of particles and build the chimney, sometimes by tens of centimetres a day.

The secret of the oasis is chemosynthesis. Instead of sunlight, bacteria use the chemical energy in hydrogen sulphide, oxidising it to make sugars from carbon dioxide. The giant tube worm Riftia pachyptila, which can grow to around two metres, has no mouth and no gut. In 1981 a young Harvard researcher, Colleen Cavanaugh, proposed that it is fed by bacteria living inside a spongy organ called the trophosome, which fills most of its body. Its scarlet plume, coloured by a special haemoglobin, gathers oxygen, carbon dioxide and sulphide and delivers them to its partners.

Did you know? The scaly-foot snail, discovered at vents in the Indian Ocean in 2001, armours its foot with scales and its shell with iron sulphide, making it the only animal known to build a skeleton from iron.

Every vent field has its own residents. The Pompeii worm lives in tubes on the chimney walls of the East Pacific Rise, tolerating water above 40 °C at its tail. The hairy-armed yeti crab, found near Easter Island in 2005, farms bacteria on its bristles. At the Lost City field on the Mid-Atlantic Ridge, discovered in 2000, white carbonate towers up to 60 metres tall vent warm alkaline fluid rather than acidic black smoke, and some researchers think conditions like these may have helped life begin.

More than 600 vent species have now been described, most found nowhere else. The same sulphide chimneys that support them contain copper, zinc and gold, and mining companies and governments are weighing whether to dig them up. The 1977 dive showed that sunlight is not the only foundation for an ecosystem, and that some of Earth's richest communities live in its hottest, darkest places.

Whale Falls

When a great whale dies and sinks, it delivers a feast to one of the hungriest places on Earth. The deep seafloor normally lives on marine snow, a thin drizzle of organic flakes from far above. A 40-tonne carcass dropping onto a patch of abyssal mud brings as much food as that patch might otherwise receive over thousands of years. Such a whale fall becomes an island of life that can last for decades.

The first natural whale fall to be studied was found by accident. In 1987 the crew of the submersible Alvin, using sonar in the Santa Catalina Basin off southern California, came across the skeleton of a blue whale at around 1,240 metres. The oceanographer Craig Smith of the University of Hawaii returned to it again and again, and later sank whale carcasses on purpose so that he and his colleagues could watch what happened from the start. With Amy Baco he described a succession of stages that overlap rather than follow neatly one after another.

First come the mobile scavengers. Within hours hagfish, sleeper sharks, rattail fish and crabs gather, stripping tens of kilograms of flesh a day; the soft tissue is gone within months to about two years. Next is the enrichment opportunist stage, when the bones and the oily sediment around them are colonised by worms, snails and small crustaceans at densities of tens of thousands per square metre. Then comes the long sulphophilic stage. Bacteria break down the fats stored inside the bones, releasing hydrogen sulphide, and chemosynthetic bacteria, mussels and clams similar to those at hydrothermal vents feed on it for 50 years or more. Last, some scientists add a reef stage, when the mineral skeleton remains as a hard surface for filter feeders.

Did you know? Osedax, discovered in 2002 on a grey whale carcass in Monterey Canyon, has no mouth or gut: the female sends green roots into the bone and feeds via bacteria, and the tiny males live inside her tube.

These "bone-eating worms", first described by Greg Rouse, Shana Goffredi and Robert Vrijenhoek in 2004, have since been found from Swedish fjords, where one species is nicknamed the bone-eating snot-flower, to the Antarctic. Whale falls have revealed hundreds of species, many unknown before, and fossil whale skeletons show that similar communities existed tens of millions of years ago. Some biologists think whale falls may have served as stepping stones that helped vent and seep animals spread between distant oases across the deep, though this is still debated.

In 2019 the exploration ship Nautilus livestreamed a freshly found whale fall off central California to thousands of online viewers. Whaling greatly reduced the supply of carcasses in the twentieth century, and as populations recover, more great whales will once again end their lives as feasts. Even in death, a whale goes on feeding the sea.

Living Fossils

Charles Darwin coined the phrase living fossil in On the Origin of Species in 1859 for organisms that seem to have changed little since ancient times. The ocean is full of them. They are not literally unchanged, since every living species has its own recent evolutionary history, but they belong to lineages whose basic body plan has lasted for hundreds of millions of years while most of their relatives vanished.

The most famous is the coelacanth. Known from fossils going back around 410 million years, these lobe-finned fish disappear from the fossil record around 66 million years ago, and scientists assumed they had died out with the dinosaurs. In December 1938 Marjorie Courtenay-Latimer, curator of a small museum in East London, South Africa, was called to the docks by the trawler captain Hendrik Goosen and spotted a large steel-blue fish with fleshy, limb-like fins among the catch. She sketched it and wrote to the ichthyologist J. L. B. Smith, who recognised it at once and named it Latimeria chalumnae in her honour. It took 14 years to find a second specimen, in the Comoro Islands, where local fishermen already knew the fish well. A second species turned up in an Indonesian fish market in 1997.

Did you know? Coelacanths are thought to live for around 100 years and to carry their young for about five years, among the longest pregnancies known in any animal.

Coelacanths live in caves and on steep volcanic slopes at around 150 to 700 metres, drifting slowly and sometimes standing on their heads. Their lobed fins move in the same alternating pattern as the legs of a walking animal, and they are related to the fish that gave rise to the first four-legged land animals.

The nautilus is a survivor of another kind. It is a cephalopod, a cousin of the octopus and squid, but it keeps the external shell its relatives abandoned. The shell is divided into chambers, and by adjusting the gas and liquid inside them through a tube called the siphuncle, the nautilus rises and sinks like a submarine. Nautiloids appeared around 500 million years ago and dominated ancient seas; today a handful of species live on reef slopes in the Indo-Pacific, where they are threatened by the trade in their shells.

Horseshoe crabs are older still as a group, with fossils around 445 million years old, and they are closer to spiders and scorpions than to crabs. Each spring, Atlantic horseshoe crabs spawn on the beaches of Delaware Bay in their hundreds of thousands, and red knots, migrating shorebirds, depend on their eggs. Their pale blue, copper-based blood contains cells that clot on contact with bacterial toxins, the basis of a test used to check vaccines and injected medicines. Living fossils are not failures of evolution but its long-distance champions.

Where Land Meets Sea

The coast is the planet's most restless border, the narrow band where land, sea and air meet and argue. Its shape is set by what the land is made of and what the sea throws at it. Most of today's shorelines are young. When the last great ice sheets melted, sea level rose by around 120 metres, drowning river valleys and coastal plains, and it settled close to its present height only some 6,000 to 7,000 years ago. Almost everything we see at the seaside, from cliffs to sand spits, has been carved or built since then.

How long is a coastline? It sounds like a question with one answer. The English scientist Lewis Fry Richardson noticed that published lengths of the same borders disagreed wildly, and in 1967 Benoit Mandelbrot explained why in a paper titled How Long Is the Coast of Britain? Measure with a 100-kilometre ruler and you skip every bay; shrink the ruler to a metre and you trace each rock, and the total keeps climbing. Coastlines are fractal, with wiggles inside wiggles. This coastline paradox is why estimates of the world's shoreline range from about 350,000 kilometres to more than 1.6 million.

Waves do the carving. Their impact crams air and water into cracks until the rock splits, which geologists call hydraulic action. They hurl pebbles at the foot of the cliff, a process called abrasion, and seawater slowly dissolves chalk and limestone. A cliff is undercut, its overhang collapses and the cliff retreats, leaving a flat wave-cut platform at its foot. Where hard and soft rocks alternate, the soft bands become bays and the hard ones stand out as headlands, which are hollowed into caves, then arches, and finally lone sea stacks. Victoria's Twelve Apostles in Australia are limestone stacks, and one of them collapsed into the Southern Ocean in 2005.

Soft coasts give way fastest. The Holderness coast of East Yorkshire is made of boulder clay dumped by glaciers, and it retreats on average around 1.5 to 2 metres a year, one of the fastest rates in Europe. Around two dozen villages recorded since medieval times now lie beneath the North Sea. Some of the eroded material drifts south to feed Spurn Head, a hooked spit of sand across the mouth of the Humber. On Alaska's Arctic coast, bluffs of thawing permafrost have retreated by more than 10 metres in a single year.

Did you know? Richardson found that Spain and Portugal quoted different lengths for their shared border: 987 kilometres according to Spain and 1,214 according to Portugal.

The edge matters more than ever. Around 40 percent of humanity lives within 100 kilometres of the sea, and rising water and stronger storms are speeding up retreat. Engineers must choose between sea walls, rock armour, rebuilt dunes and planned retreat. The line on the map is only a snapshot; the real coast is always on the move.

Beaches and Sand

A beach is a river of sand that happens to be lying still for the moment. It is a heap of loose grains, from fine sand to cobbles, piled up by waves between the low-tide line and the highest reach of storms. Pick up a handful and you hold a sample of the land behind and the sea in front. On most temperate beaches the grains are quartz, the toughest mineral in granite, weathered out of mountains and carried to the coast by rivers over thousands of years.

Not all sand is golden. Black beaches such as Reynisfjara in Iceland are made of crushed basalt lava. Papakōlea in Hawaii glints green with crystals of olivine. Bermuda's pink beaches owe their blush to the red shells of a single-celled foraminiferan, Homotrema rubrum. On tropical coasts much of the sand is biogenic, made from broken shells, coral and the chalky plates of the green alga Halimeda. A surprising amount passes through fish. Parrotfish scrape algae off dead coral with beak-like fused teeth, grind the rock in their throats and excrete it as fine white sand. A 2015 study in the Maldives led by the geographer Chris Perry found that parrotfish supplied most of the new sand on the reef island it examined.

Sand rarely stays put. Waves usually reach the shore at an angle, so the rush of water up the beach, the swash, carries grains diagonally, while the backwash drags them straight back down the slope. Repeated endlessly, this zigzag moves sand along the coast in longshore drift. Where the coast turns, drift builds spits such as Spurn Head in England or Farewell Spit in New Zealand, more than 25 kilometres long. The wooden groynes that march down many British beaches trap the drift on one side and starve the beach beyond. Beaches also breathe with the seasons, as winter storms pull sand offshore into bars and gentle summer swell pushes it back.

Did you know? A single large parrotfish can produce hundreds of kilograms of sand a year, so some of the whitest tropical beaches are, quite literally, fish droppings.

Coastal scientists follow sand with dyed tracer grains, repeated GPS and drone surveys, and decades of satellite images. A 2018 global study of satellite images led by Arjen Luijendijk found that about a quarter of the world's sandy beaches are eroding.

Sand is also running short. A 2019 report by the UN Environment Programme estimated that humanity uses 40 to 50 billion tonnes of sand and gravel a year, making it the most extracted solid material on Earth. Desert grains are mostly too smooth and round to bind well in concrete, so builders want river and marine sand, and illegal sand mining strips beaches and riverbeds from Morocco to India. Singapore has enlarged its land area by about a quarter since the 1960s, largely with imported sand. The Netherlands tried a gentler idea in 2011: the Sand Motor, about 21.5 million cubic metres dumped off the coast near The Hague for the waves to spread along the shore. Every beach is a budget, and ours is overdrawn.

Rocky Shores and Tide Pools

Twice a day a rocky shore is drowned, then left to bake. Its residents must survive breaking waves at high tide and, a few hours later, sun, wind, rain and hungry birds. The result is one of the clearest patterns in ecology: zonation, horizontal bands of life stacked up the rock like stripes on a flag. Each band marks how long its inhabitants can bear to be out of water.

At the top, in the splash zone, black and orange lichens and tiny periwinkles hang on where only spray reaches. Below come the barnacles, crustaceans cemented head-down to the rock, which close their plates when the tide falls and kick food into their mouths with feathery legs when it returns. Lower still, mussels crowd into beds, limpets graze films of algae and then return to a home scar ground into the rock to fit their shell exactly, and dog whelks drill into their neighbours. At the bottom, uncovered only by the lowest spring tides, lie forests of kelp, sea anemones, sea urchins and starfish.

Why are the bands so sharp? In the 1950s the American ecologist Joseph Connell studied two barnacles on the Isle of Cumbrae in Scotland, publishing his results in 1961. The small Chthamalus lived high on the shore and the larger Semibalanus lower down. Where he removed Semibalanus, Chthamalus thrived lower; where they grew together, the bigger barnacle smothered it or prised it off. The upper limit of a zone, he concluded, is set mostly by drying out, and the lower limit mostly by competitors and predators.

Then came the most famous experiment on any shore. From 1963 Robert Paine of the University of Washington spent years prising ochre starfish, Pisaster ochraceus, off a stretch of rock at Mukkaw Bay and throwing them into the sea. Without their chief predator, California mussels spread across the rock and crowded out almost everything else, and the number of species in his plot fell from 15 to 8. In 1969 Paine coined the term keystone species for a predator whose removal unravels a community, an idea since applied to sea otters, wolves and much else.

Did you know? Limpet teeth are reinforced with fibres of the iron mineral goethite, and in 2015 engineers found they may be the strongest biological material ever tested, outperforming spider silk.

Tide pools are natural laboratories, where anyone with a bucket can watch hermit crabs swap shells and anemones fold up as the water warms. In a sunny afternoon a small pool can grow much warmer and saltier than the sea, and in heavy rain almost fresh. They are also early warnings. From 2013 a sea star wasting disease swept the Pacific coast of North America from Alaska to Mexico, killing millions of starfish; the sunflower star declined by more than 90 percent and is now listed as critically endangered. On many shores, Paine's experiment ran itself. Lift one predator off the rock and the whole shore rearranges itself.

Estuaries and Salt Marshes

An estuary is where a river meets the tide, a partly enclosed body of water in which fresh and salt mix into brackish water. Fresh water is lighter, so it tends to slide seaward over the top while denser seawater creeps inland along the bottom as a salt wedge. In a deep river with a strong flow, like the Mississippi, the layers stay sharp; in a shallow estuary with big tides, like the Severn, currents churn them almost completely together. At any one spot, salinity can swing from nearly fresh to almost full seawater within a single tide.

Life must cope with that swing, and the rewards are huge. Rivers deliver nutrients, tides stir them and shallows warm quickly, so estuaries are among the most productive ecosystems on Earth. They are nurseries: young herring, flounder, sea bass, shrimp and crabs shelter in murky water where larger predators struggle to hunt. Fringing many temperate estuaries are salt marshes, meadows of cordgrass, glasswort and sea lavender that flood at high tide. Their stems slow the water so mud settles, and a healthy marsh can build itself upward by a few millimetres a year as the sea rises.

Chesapeake Bay, the largest estuary in the United States, is about 300 kilometres long but averages only around 6 metres deep, and its watershed covers some 166,000 square kilometres. When the English captain John Smith explored it in 1608, oyster reefs were a hazard to boats. Overharvesting, disease and silt have since cut native oysters to a small fraction of their former numbers, and fertiliser washing off farms feeds algal blooms that leave summer dead zones of oxygen-starved water. Scientists have tracked the bay's health with monitoring stations and survey cruises since the 1980s, and the surrounding states now rebuild oyster reefs by the hectare.

Across the Atlantic, the Wadden Sea stretches some 500 kilometres along the coasts of the Netherlands, Germany and Denmark. It is the largest unbroken system of intertidal sand and mud flats in the world, and it became a UNESCO World Heritage Site in 2009. At low tide guided walkers cross the seabed to islands; at high tide seals haul out on the banks. Each year some 10 to 12 million migrating birds stop to refuel on its worms, snails and shellfish, among them red knots and bar-tailed godwits travelling between the Arctic and Africa.

Did you know? The Río de la Plata between Argentina and Uruguay is often called the widest estuary in the world, about 220 kilometres across at its mouth.

Estuaries also host great ports and cities, from London on the Thames to Shanghai at the mouth of the Yangtze, so they face dredging, pollution and land reclamation. Salt marshes bury carbon quickly in their waterlogged mud, and in England, the Netherlands and California engineers now breach old sea walls on purpose to let marshes return. Where the river slows and the tide arrives, the sea raises its young.

Mangrove Forests

Mangroves are trees that do what most trees cannot: stand with their roots in salt water. Around 70 species from several unrelated plant families have evolved the trick independently, and together they fringe sheltered tropical and subtropical coasts in a belt covering roughly 147,000 square kilometres, an area about the size of Bangladesh. They grow in grey, waterlogged mud that holds almost no oxygen and is flooded by the tide twice a day.

Each kind solves these problems in its own way. Red mangroves of the genus Rhizophora prop themselves up on arching stilt roots and filter salt out at the root surface, keeping most of it from ever entering the tree. Mangroves of the genus Avicennia let more salt in and then sweat it out through salt glands on their leaves, which can be crusted with crystals on a dry day. To breathe, Avicennia sends up thousands of pencil-like pneumatophores, roots that poke out of the mud like snorkels. Many mangroves are also viviparous: their seeds sprout while still on the tree and drop as spear-shaped propagules that can float for months before lodging in new mud.

The greatest of all mangrove forests is the Sundarbans, roughly 10,000 square kilometres of islands and channels in the delta of the Ganges, Brahmaputra and Meghna, shared by Bangladesh and India. It is the only mangrove forest with tigers, which swim between islands and hunt in the tidal mud, and it also shelters saltwater crocodiles, Ganges river dolphins and fishing cats. Elsewhere mangroves line the Florida Everglades, the Niger Delta and the Amazon coast of Brazil, and gnarled stands even survive along the desert shores of the Red Sea.

Mangroves protect the coasts that house them. Their tangled roots slow waves and storm surges, and studies after the 2004 Indian Ocean tsunami and Cyclone Sidr, which struck Bangladesh in 2007, suggest that villages behind wide mangrove belts often suffered less damage, although scientists debate how much any forest can stop the largest waves. The roots also trap sediment and fallen leaves, burying carbon in deep, airless soil. A 2011 survey of Indo-Pacific mangroves led by Daniel Donato found they store several times as much carbon per hectare as most tropical upland forests, most of it in the mud. This is the heart of blue carbon.

Did you know? Red mangrove propagules can survive for a year or more floating at sea, which helps explain how mangroves colonised coasts across entire oceans.

Yet mangroves have been cleared on a vast scale for shrimp ponds, rice fields, timber and coastal building, especially in Southeast Asia, and a large share of the world's mangrove area has been lost since the mid twentieth century. Losses have slowed, and from Indonesia to Senegal communities are replanting, learning that the right species in the right mud matter more than the number of seedlings. A mangrove is a sea wall, a nursery and a carbon vault, and it plants itself for free.

Kelp Forests

Kelp forests are the sea's woodlands, but they are not made of plants. Kelps are large brown algae, more closely related to diatoms than to oak trees. They grow in cool, nutrient-rich water along roughly a quarter of the world's coastlines. A kelp has no true roots: a holdfast grips the rock, a flexible stipe rises like a trunk, and leaf-like blades soak up sunlight, buoyed towards the surface by gas-filled bladders.

The champion is giant kelp, Macrocystis pyrifera, which can grow as much as half a metre in a day in good conditions and reach more than 30 metres in length. Off California, Chile, New Zealand and Tasmania its canopy spreads across the surface like the roof of a forest. Beneath it live garibaldi and rockfish, abalone, crabs and urchins, while sea lions twist between the stipes and grey whales sometimes slip inside to shelter. Off South Africa a different giant, the sea bamboo Ecklonia maxima, forms the Great African Seaforest made famous by the film My Octopus Teacher.

Kelp forests also gave ecology one of its best stories. In the early 1970s James Estes and John Palmisano compared islands in Alaska's Aleutian chain. Where sea otters survived, kelp grew thick. Where the fur trade had wiped otters out, sea urchins had multiplied unchecked and grazed the forest down to bare rock, an urchin barren. Otters eat urchins, and urchins eat kelp: remove the top predator and the forest falls. Ecologists call this a trophic cascade, and Estes and Palmisano's 1974 paper became a classic.

The cascade can run without otters too. From 2013 a wasting disease wiped out sunflower stars, another urchin predator, along the Pacific coast of North America. Then a long marine heatwave nicknamed the Blob starved the kelp of nutrients. Purple urchins exploded, and northern California lost more than 90 percent of its bull kelp canopy along some 350 kilometres of coast. In Tasmania the East Australian Current has pushed warm, nutrient-poor water south, and about 95 percent of the island's giant kelp forests have disappeared since the 1940s, while long-spined urchins have followed the warm water down from New South Wales. In 2012 these forests became the first marine community listed as endangered under Australian national law.

Did you know? Sea otters have no blubber and rely on the densest fur of any mammal, with up to about 150,000 hairs on every square centimetre.

Kelp can bounce back fast when conditions allow. Divers now cull urchins to clear space, farmers grow kelp on ropes for food and fertiliser, and Tasmanian scientists are breeding heat-tolerant giant kelp to replant reefs. The lesson of the Aleutians still holds: look after the predator and the forest looks after itself.

Seagrass Meadows

Seagrasses are the only flowering plants that live fully submerged in the sea. They are not seaweeds: they have roots, veins, leaves, flowers and seeds, and they descend from land plants that returned to the water on at least three separate occasions tens of millions of years ago. Around 70 species form meadows on sandy and muddy shallows along every continent except Antarctica, and at least 300,000 square kilometres of them have been mapped, with more still being found.

Their flowers are pollinated underwater. Eelgrass, Zostera marina, releases long, thread-like pollen that drifts until it snags on a flower. In 2016 researchers working at Puerto Morelos on Mexico's Caribbean coast filmed tiny crustaceans and worms carrying pollen between turtle grass flowers at night, a marine echo of bees. Seagrasses also spread by creeping underground stems called rhizomes, so a meadow can be one plant cloned many times over.

The Mediterranean's own species, Posidonia oceanica or Neptune grass, grows extremely slowly, building a dense mat of living and dead rhizomes, called a matte, that can be several metres thick and take thousands of years to form. Meadows between Ibiza and Formentera are part of a UNESCO World Heritage Site declared in 1999, and genetic studies suggest that some Posidonia clones there could be tens of thousands of years old. Winter storms roll its fibres into Neptune balls that wash up on Mediterranean beaches.

Did you know? In 2022 geneticists showed that a seagrass meadow in Shark Bay, Western Australia, is a single Posidonia australis plant spreading across about 180 kilometres and perhaps 4,500 years old.

Meadows feed large grazers. A dugong eats tens of kilograms of seagrass a day, leaving bare feeding trails through the meadow, and populations in Shark Bay, Moreton Bay and the Arabian Gulf depend on it, as do green turtles across the tropics. When pollution killed vast areas of seagrass in Florida's Indian River Lagoon, more than 1,000 manatees died in 2021, many of them from starvation. Eelgrass beds in cooler seas shelter seahorses, scallops and juvenile cod.

Seagrass is also a carbon store. Its leaves slow the water so that particles settle and are buried in oxygen-poor sediment, where they can stay locked away for centuries. Though meadows cover only about 0.1 percent of the seafloor, studies suggest they account for around a tenth of the organic carbon buried in ocean sediments each year. They are disappearing, however: a 2009 global review led by Michelle Waycott estimated losses of about 110 square kilometres a year since 1980, driven by dredging, pollution, anchors and warming. Restoration can work. In the coastal bays of Virginia, scientists and volunteers have scattered eelgrass seed since 1999, and the meadows they started now cover more than 3,600 hectares. Unseen from the beach, the meadow quietly feeds dugongs and buries carbon.

Coral Reefs

A coral reef is a city built by animals the size of a fingernail. Each reef-building coral is a colony of polyps, small relatives of sea anemones, each with a ring of stinging tentacles around a mouth. Polyps take calcium and carbonate from seawater and lay down skeletons of aragonite beneath themselves, generation after generation, until the limestone of their dead ancestors forms reefs kilometres long and hundreds of metres thick.

The secret of their success lives inside them. In the coral's tissues are vast numbers of single-celled algae called zooxanthellae, dinoflagellates of the family Symbiodiniaceae. They photosynthesise and pass the coral sugars that can meet up to 90 percent of its energy needs, and in return they get shelter and nutrients. Their pigments give corals much of their colour. The partnership needs clear, warm, sunlit water, so tropical reefs thrive in seas poor in nutrients, mostly between about 30 degrees north and south. When the water stays too hot for weeks, the partnership breaks down, the coral expels its algae and turns white: this is bleaching.

Reefs come in three main forms. Fringing reefs grow close to shore, as along much of the Red Sea. Barrier reefs stand farther out beyond a lagoon, as off Belize. Atolls are rings of reef around a lagoon with no high island in the middle at all. Charles Darwin proposed in 1842, in The Structure and Distribution of Coral Reefs, that these are stages of one story: a reef fringes a volcanic island, the island slowly sinks, the reef keeps growing upward towards the light, and finally only a ring remains. The theory was confirmed in 1952 when American geologists drilled through more than 1,200 metres of coral limestone at Enewetak Atoll in the Marshall Islands and struck volcanic basalt.

Did you know? Darwin worked out his theory of atolls on the west coast of South America, before he had ever seen a true coral reef.

Reefs cover well under 1 percent of the ocean floor, yet they shelter roughly a quarter of known marine species. The Coral Triangle, the seas between Indonesia, the Philippines, Malaysia, Papua New Guinea, Timor-Leste and the Solomon Islands, holds about three quarters of all reef-building coral species. Clownfish nestle in anemones, cleaner wrasse run stations where groupers queue to be groomed, and giant clams grow to more than a metre across. Not all reefs are tropical: in cold, dark water off Norway, the coral Desmophyllum pertusum builds reefs such as the Røst Reef, which stretches for some 40 kilometres.

Reefs feed and protect hundreds of millions of people, but warming, acidification, pollution and overfishing now hit them together. In April 2024 scientists at NOAA confirmed the fourth global bleaching event, after those of 1998, 2010 and 2014 to 2017. Built by animals and powered by algae, a reef is the ocean's oldest partnership under strain.

The Great Barrier Reef

The Great Barrier Reef is not one reef but a system of around 2,900 individual reefs and some 900 islands strung for about 2,300 kilometres along the coast of Queensland. The marine park that protects it covers about 344,000 square kilometres, larger than Italy. It is the largest structure built by living things, and it can genuinely be seen from orbit, a claim often wrongly made about the Great Wall of China.

It is both younger and older than it looks. Reefs have grown here for hundreds of thousands of years, dying back or drowning as sea levels fell and rose with the ice ages. The living reef we see has grown largely since the sea reached roughly its present level, some 6,000 to 8,000 years ago. Before that, much of today's reef floor was dry coastal plain where people walked and hunted.

People have known the reef far longer than any map. More than 70 Aboriginal and Torres Strait Islander Traditional Owner groups hold it as sea country, a living web of places, species, laws and stories handed down over thousands of years, and researchers such as the geographer Patrick Nunn have argued that some oral traditions recall the coast drowning as the seas rose. Europeans met the reef the hard way. On 11 June 1770 James Cook's Endeavour ran aground on a reef near what is now Cape Tribulation and was nearly lost. Protection came much later: the Great Barrier Reef Marine Park Act in 1975 and World Heritage listing in 1981.

The reef's spectacles include the mass coral spawning that follows full moons in late spring, when colonies across hundreds of kilometres release bundles of eggs and sperm on the same few nights. Dwarf minke whales visit each winter, and dugongs graze the seagrass in its lagoon. Since the mid 1980s the Australian Institute of Marine Science has surveyed reefs across the system every year, while aerial surveys from planes and helicopters, led in 2016 by the coral scientist Terry Hughes, map bleaching reef by reef.

Did you know? Raine Island, a speck of sand of about 32 hectares in the northern reef, hosts the world's largest nesting colony of green turtles.

The reef suffered mass bleaching in 1998 and 2002, then five times in nine years, in 2016, 2017, 2020, 2022 and 2024. The 2016 heatwave killed around 30 percent of the reef's corals. Coral can recover within a decade if left alone, and cover has rebounded strongly in places, helped by fast-growing plate corals. But those same corals are the first to bleach and the favourite food of the crown-of-thorns starfish, and the gaps between heatwaves are shrinking. Monitoring after the 2024 event recorded some of the steepest annual falls in coral cover since surveys began. The world's largest living structure is now also one of its clearest thermometers.

Islands and Atolls

An island is simply land surrounded by water, but how it got there sorts islands into a few great families. Continental islands sit on continental shelves and were joined to the mainland when sea levels were lower. Great Britain was linked to Europe across a now-drowned land called Doggerland until around 8,000 years ago, and New Guinea was joined to Australia. Others are fragments that drifted away: Madagascar split from India around 88 million years ago and has been alone ever since, which is why most of its plants and animals live nowhere else.

Oceanic islands rise from the deep seafloor and have never touched a continent. Most are volcanoes, some built over hotspots, like Hawaii and the Galápagos, and others strung in curving island arcs above subduction zones, like Japan, the Aleutians and the Lesser Antilles. The Galápagos lie about 1,000 kilometres off Ecuador. When Darwin visited in 1835 he collected mockingbirds and finches that, studied back in London with the ornithologist John Gould, helped point him towards evolution. Life reaches oceanic islands only by flying, swimming or rafting, so they fill with oddities: giant tortoises, marine iguanas and flightless birds such as the dodo of Mauritius.

Coral islands are built by life itself. When a volcano sinks, its reef may grow into an atoll, a ring of reef and low islets around a lagoon. The Maldives consist of around 1,190 coral islands grouped into 26 atolls, with an average height of about 1.5 metres above sea level, making it the lowest-lying country on Earth. Kiritimati in Kiribati is the atoll with the largest land area. The islets themselves are heaps of reef sand and rubble shaped by waves, and a 2018 study of Tuvalu found that most of its islands had held their ground or grown over four decades of rising seas, as long as the reefs that feed them stayed healthy.

We have watched an island being born. On 14 November 1963 the crew of a fishing boat off southern Iceland saw smoke rising from the sea, and within days a new volcanic island, Surtsey, stood above the waves. It erupted until 1967, reaching about 2.7 square kilometres. Iceland declared it off-limits to all but scientists, making it a unique laboratory of colonisation. The first flowering plant, sea rocket, was found in 1965, gulls began to breed there in the 1980s, and dozens of plant species have since arrived by wind, wave and bird. Waves have already cut the island to around half its peak size.

Did you know? A new island formed in Tonga in 2015 between two older volcanic islets, only to be blown apart by the colossal Hunga Tonga–Hunga Haʻapai eruption of January 2022.

Islands are small, but they hold an outsized share of the world's unique species, of its extinctions and of its maritime claims, since even a speck of land can anchor an exclusive economic zone 200 nautical miles wide. Every island is an experiment in isolation, and the sea decides how long it runs.

The Polynesian Wayfinders

The greatest feat of exploration before the compass was made in canoes. Starting around 3,000 years ago, people of the Lapita culture sailed east from the islands near New Guinea and reached Fiji, Tonga and Samoa. After a long pause their descendants pushed on, and in a burst of voyaging roughly a thousand years ago they found almost every habitable speck in the Polynesian Triangle, an expanse of ocean with corners at Hawaiʻi, Rapa Nui (Easter Island) and Aotearoa New Zealand, each side around 7,000 kilometres long. Radiocarbon dates suggest Rapa Nui was settled around AD 1200 and New Zealand, the last large land on Earth to be peopled, around AD 1250 to 1300.

They had no instruments, only knowledge. A wayfinder holds a star compass in his head: the horizon divided into houses where particular stars rise and set. A star that rises in one house sets in its mirror image on the other side, so a sequence of stars through the night gives a steady bearing. Stars that pass straight overhead mark latitude; Arcturus, the star the Hawaiians call Hōkūleʻa, crosses the zenith over Hawaiʻi. By day the navigator reads the swells, long waves raised by distant, steady winds, feeling their rhythm through the hull, and notices when an island bends or blocks them. Clouds piling up over land, the colour of a lagoon reflected on a cloud's underside, and seabirds such as the white tern and the brown noddy, which fish out at sea and fly home at dusk, all widen the target.

For decades some scholars argued that the Pacific had been settled by lucky drift. The answer came from an experiment. In 1976 the double-hulled voyaging canoe Hōkūleʻa, built by the Polynesian Voyaging Society, sailed from Hawaiʻi to Tahiti in about a month, guided without instruments by Mau Piailug, a master navigator from the tiny atoll of Satawal in Micronesia. Thousands of people crowded the shore at Papeete to greet her. Piailug went on to teach the Hawaiian Nainoa Thompson, and between 2014 and 2017 Hōkūleʻa sailed around the world.

Did you know? The famous stick charts of the Marshall Islands, which map how swells bend around atolls, were studied and memorised on land and never carried on a voyage.

Other evidence points the same way. The sweet potato, a South American plant, was growing in Polynesia before Europeans arrived, and a 2020 genetic study found traces of Native American ancestry in some eastern Polynesians dating to around AD 1200. Wayfinding nearly vanished under colonial rule and missionary disapproval; today it is taught again in schools from Hawaiʻi to Aotearoa. The open Pacific was never an empty barrier to the people who read it. It was a highway.

Ships of the Age of Sail

For four centuries the wooden sailing ship was the most complex machine on Earth, and the wind decided where it could go. The ocean's great wind belts, the steady trade winds of the tropics and the westerlies further from the equator, became the motorways of the age, and learning their pattern mattered as much as building the hulls.

The largest fleets of the early fifteenth century were Chinese. Between 1405 and 1433 the admiral Zheng He led seven expeditions from Ming China across the Indian Ocean to Calicut, Hormuz, the Red Sea and the Swahili coast of East Africa. Records describe fleets of more than 200 vessels carrying over 27,000 men. Chinese texts give the biggest treasure ships a length of around 130 metres, but historians debate this, and many think 60 to 75 metres more likely. Either way, they dwarfed anything in Europe. A giraffe, an African animal presented to the emperor in 1414, caused a sensation at court. Then the Ming turned inward and the voyages stopped.

Portugal took a different approach: small ships and patient learning. The caravel, perhaps 20 metres long, carried triangular lateen sails that let it sail close to the wind. Portuguese pilots discovered the volta do mar, swinging far out into the Atlantic to catch favourable winds home. Bartolomeu Dias rounded the Cape of Good Hope in 1488, and Vasco da Gama reached India in 1498. Bigger, square-rigged carracks and later galleons carried cargo and guns; from 1565 Spanish galleons crossed the Pacific between Acapulco and Manila every year, riding the westerlies home.

The defining voyage left Spain in September 1519. Ferdinand Magellan set out with five ships and around 250 men, found a strait through the tip of South America and crossed an ocean so calm he named it the Pacific, a passage of about three months on which men ate leather and sawdust. Magellan was killed in the Philippines in 1521. In September 1522 a single battered ship, the Victoria, reached Spain under Juan Sebastián Elcano with 18 survivors of the first voyage around the world.

Did you know? When the Victoria's crew went ashore in the Cape Verde Islands in July 1522, they found their carefully kept log was a day behind: sailing west around the globe had cost them a day.

The age ended at full speed. Nineteenth-century clippers, long and narrow with towering sails, raced tea from China and wool from Australia; in 1854 the American Flying Cloud sailed from New York to San Francisco in under 90 days. But steamships and the Suez Canal, opened in 1869, the same year the Cutty Sark was launched, made them obsolete. Every sea lane we use today was first traced by wind.

Finding Longitude

A sailor could always find latitude, how far north or south he was, by measuring the height of the Sun at noon or of the Pole Star at night. Longitude, the position east or west, was another matter. The Earth turns 360 degrees in 24 hours, or 15 degrees every hour, so longitude is really a question of time. If you know the time at home and the local time where you are, read from the Sun, the difference gives your position. Every four minutes of error puts you a degree out, which at the equator is about 111 kilometres.

The trouble was keeping home time at sea. Pendulum clocks stopped on a rolling deck, and temperature changes made metal springs run fast or slow. Ships navigated by dead reckoning, estimating speed and heading, and errors could be deadly. In 1707 a British fleet under Sir Cloudesley Shovell, misjudging its position, struck the rocks of the Isles of Scilly; four warships sank and somewhere between 1,400 and 2,000 men drowned.

In 1714 Parliament passed the Longitude Act, offering prizes for a practical method, with the largest for accuracy within half a degree. Astronomers favoured the sky. The Moon moves against the stars by about its own width every hour, so measuring the angle between the Moon and a bright star with a sextant, then looking it up in tables, gives Greenwich time. This lunar distance method became practical when the Astronomer Royal Nevil Maskelyne published the first Nautical Almanac for 1767, but each fix took hours of calculation.

John Harrison, a self-taught carpenter and clockmaker who grew up in Lincolnshire, bet on a machine. He spent decades on three large sea clocks before realising that a small, fast-beating watch would be steadier. His fourth timekeeper, H4, completed in 1759, was about 13 centimetres across. On a trial voyage to Jamaica in 1761 and 1762 it lost only about five seconds in 81 days. The Board of Longitude, dominated by astronomers, demanded more trials and more proof, and Harrison received his full reward only after King George III took his side and Parliament voted him the money in 1773.

Did you know? Before it would pay him, the Board made Harrison take H4 apart in front of a panel of experts and hand over his drawings so that others could copy it.

Copies proved the point. Captain Cook carried a copy of H4 on his second Pacific voyage and praised it highly. By the nineteenth century the marine chronometer was standard, and in 1884 an international conference chose the meridian through Greenwich as longitude zero. Satellites now give a position in seconds, but they work on Harrison's principle: an exact clock is a map.

Captain Cook's Pacific

James Cook was a farm labourer's son from Yorkshire who learned the sea in colliers out of Whitby, joined the Royal Navy and made his name charting the coast of Newfoundland. In three voyages between 1768 and 1779 he filled in more of the map of the Pacific than anyone before him, and set the stage for what came after.

The first voyage, in HMS Endeavour from 1768 to 1771, had two missions. The public one was to observe the transit of Venus across the Sun from Tahiti in June 1769, part of an international effort to measure the distance to the Sun. The sealed orders sent Cook south in search of a great southern continent. He charted the coasts of New Zealand, then the east coast of Australia, where the botanists Joseph Banks and Daniel Solander collected so many new plants at Botany Bay that it was named after them. In June 1770 the ship struck the Great Barrier Reef and was nearly lost.

At Tahiti Cook took aboard Tupaia, a priest and navigator from Raʻiātea, who drew a chart of some 74 islands around Tahiti for the British. Its arrangement baffled Europeans for two centuries; recent studies suggest it records the islands by sailing directions from a central point rather than by European map grid. Tupaia translated for Cook among the Māori, but died at Batavia in 1770, where fever and dysentery killed many of the crew.

The second voyage, in Resolution and Adventure from 1772 to 1775, made the first known crossing of the Antarctic Circle in January 1773 and reached 71 degrees south. Cook found ice, not a fertile continent, and the old idea of a great southern land was finished. He carried a copy of Harrison's chronometer and fought scurvy, the vitamin C deficiency that killed sailors by the thousand, with fresh food gathered at every landfall and strict cleanliness. He credited sauerkraut and malt, though fresh greens and fruit did the real work. The third voyage, seeking a Northwest Passage, brought the first recorded European visit to Hawaiʻi in 1778 and reached the Bering Strait. Cook was killed at Kealakekua Bay on Hawaiʻi in February 1779.

Did you know? The sailing master on Cook's third voyage was a young William Bligh, later captain of the Bounty, and George Vancouver sailed with Cook as a midshipman.

Cook's charts were superb, and they were also a road map. Eighteen years after Botany Bay the First Fleet arrived to found a penal colony, and whalers, traders and missionaries followed across the ocean, bringing new diseases to islands that had no immunity. His voyages remain a high point of navigation and the start of a painful chapter for the peoples of the Pacific. A map is never neutral.

The Challenger Expedition

Modern oceanography began with a warship stripped of most of its guns. In the 1860s the naturalist Charles Wyville Thomson had dredged living animals from depths where many experts expected nothing could survive. He persuaded the Royal Society and the Admiralty to send a ship around the world to study the whole ocean, from its chemistry to its creatures. HMS Challenger, a steam-assisted wooden corvette, had all but two of her guns removed to make room for laboratories, a darkroom and miles of rope.

She left Portsmouth in December 1872 and returned in May 1876, after a voyage of about 127,000 kilometres through the Atlantic, Pacific, Indian and Southern Oceans. Captain George Nares commanded; Wyville Thomson led a small scientific staff that included the naturalists John Murray and Henry Moseley, the chemist John Young Buchanan and the artist John James Wild. At each of 362 stations the crew stopped to measure depth, haul up mud, catch animals, take water samples and record temperatures at different depths.

Sounding was slow, heavy work. A weight on a hemp line was lowered until it hit bottom, marked by a change in how fast the rope ran out, and a deep sounding could take hours. In March 1875, near the Mariana Islands, the line ran out to 4,475 fathoms, about 8,180 metres, in the trench whose deepest point now carries the ship's name: the Challenger Deep. The soundings also revealed a broad rise down the middle of the Atlantic, the first hint of the Mid-Atlantic Ridge.

The haul was enormous. The expedition described 4,717 new species, from glass sponges to deep-sea anglerfish, and found the seafloor carpeted with oozes made of tiny shells and with red clay, studded in places with lumps of manganese. It also killed a famous idea. The biologist Thomas Huxley had named a slimy substance found in preserved mud samples Bathybius haeckelii, thinking it a primordial living layer covering the deep seabed. Buchanan showed it was calcium sulfate, precipitated when alcohol was added to seawater.

Did you know? Both NASA's Space Shuttle Challenger and the lunar module of Apollo 17, which landed on the Moon in 1972, were named after the Victorian survey ship.

Analysing the results took two decades. Murray edited the Report on the Scientific Results, 50 volumes and nearly 30,000 pages published between 1880 and 1895, written by experts across Europe and America. Its measurements are still used as a baseline for how the ocean has warmed. Challenger turned the deep sea from a blank into a subject, and every research cruise since has followed her routine: stop, lower, haul, record.

Diving: Bells to Scuba

People have dived for food, pearls and sunken treasure for thousands of years, mostly on a single breath. The haenyeo of Jeju Island in South Korea, many of them women in their seventies, still gather shellfish and seaweed at depths of 10 metres or more without tanks; their culture was added to UNESCO's heritage list in 2016. The Bajau sea nomads of South-East Asia spend hours a day underwater, and a 2018 study found they have unusually large spleens, an organ that releases extra oxygen-carrying red blood cells during a dive. Competitive free divers go much further: in 2007 the Austrian Herbert Nitsch descended to 214 metres on one breath with the help of a weighted sled.

Staying down longer meant taking air along. A diving bell, open at the bottom, traps air like an upturned glass pushed into water. In 1691 the astronomer Edmond Halley built a bell that was topped up with weighted barrels of air sent down from the surface, and he and several companions spent more than an hour at a depth of around 18 metres. In the nineteenth century the standard diving dress, a copper helmet sealed to a waterproof suit and fed by air pumped down a hose, made underwater salvage and construction routine. The design associated with Augustus Siebe dominated diving for a century.

Depth brought a new danger. Under pressure, nitrogen from the air dissolves into the blood; if a diver rises too fast it forms bubbles, causing decompression sickness, the bends, which crippled bridge-builders and divers alike. The French physiologist Paul Bert identified the cause in 1878, and in 1908 the Scottish scientist John Scott Haldane published staged decompression tables for the Royal Navy.

Did you know? Halley's diving bell had a glass window in the top to let in daylight, and a valve to release the stale, exhaled air.

Freedom came in 1943. In occupied France the naval officer Jacques-Yves Cousteau and the engineer Émile Gagnan adapted a gas valve into a demand regulator that delivers air from a tank at exactly the pressure of the surrounding water, only when the diver breathes in. They called it the Aqua-Lung. Cousteau's book and film The Silent World made scuba famous, and today millions of recreational divers explore reefs worldwide, generally to a limit of about 40 metres, beyond which nitrogen narcosis clouds judgement.

Professional divers go deeper by breathing helium mixtures and living for weeks in pressurised chambers, a technique called saturation diving that services oil rigs in the North Sea and the Gulf of Mexico. In 1988 divers from the French company COMEX worked at 534 metres in the Mediterranean. Even so, the human body remains the most limited submarine of all.