What We Still Do Not Know About the Deep Ocean

There is a tidy way to describe the state of our knowledge of the sea, and it is slightly embarrassing.

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

That line is from my book The Oceans, and it is the right place to start. Most of the solid surface of our planet lies under water, and for most of history we could not see any of it. This article goes through what the deep has shown us so far, and where the honest answer is still "we are not sure".

Most of the planet is a place nobody has looked at

Drain the ocean and the planet would show two great levels. One sits near sea level: the continents. The other, larger one lies between 4,000 and 5,000 metres down: the deep ocean floor.

Between about 4,000 and 6,000 metres 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. Then comes the sentence that matters:

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 reason is practical. For centuries sailors measured depth with a lead line, a weighted rope marked in fathoms. It worked in harbours but was hopeless in the open ocean, where a single deep sounding could take hours. By 1900 the deep ocean had been sounded only a few thousand times. Sonar and satellites have improved matters a great deal since, but seeing the seafloor in detail still means sending something down to look.

Above the plains, the water itself is the bigger unknown. From about 1,000 metres to roughly 4,000 metres stretches the midnight zone, the largest living space on Earth by volume. It is cold, around 4 °C, pressed by hundreds of atmospheres and almost foodless.

Even the deepest point is an estimate

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. That figure comes with a caveat: estimates differ by tens of metres depending on the sonar and the corrections applied. We do not know the depth of the deepest hole on the planet to the nearest metre.

Set Mount Everest, 8,849 metres tall, on the bottom and its summit would still lie more than two kilometres below the surface. Pressure rises by about one atmosphere for every 10 metres of seawater, so at the bottom of the Challenger Deep it is about 1,100 atmospheres.

Very few people have been there. Before 2019 more people had walked on the Moon than had visited the Challenger Deep: twelve against three. In 2020 the former NASA astronaut Kathryn Sullivan dived to it, becoming the first person both to walk in space and to reach the deepest point of the ocean. Getting there, and back, is an engineering problem as hard as spaceflight.

The trenches as a whole are barely sampled. The hadal zone, below about 6,000 metres, 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. Fish give out near 8,200 metres. Amphipods, small crustaceans, reach the floor.

The deep keeps producing life nobody predicted

The best evidence that we do not know what is down there is how often we have been surprised.

In February 1977 the research submersible Alvin dived to around 2,500 metres on the Galápagos Rift. The team aboard were geologists and chemists 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.

At those vents, bacteria turn volcanic chemicals into food. A whole way of making a living, with no sunlight involved, had been missed until someone happened to drive past it.

The surprises have not stopped. The scaly-foot snail, discovered at vents in the Indian Ocean in 2001, armours its foot with scales and its shell with iron sulphide. It is the only animal known to build a skeleton from iron.

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.

The vampire squid neither drinks blood nor hunts. It trails sticky filaments to gather marine snow, the thin drizzle of organic flakes from far above, as Hendrik Hoving and Bruce Robison showed in 2012.

That last one is worth a second look. The animal had a name and a reputation long before anyone knew what it ate.

Familiar monsters are barely better known. The giant squid is the real animal behind the Norse kraken. In 1857 the Danish zoologist Japetus Steenstrup named the genus from a beak washed up on a Danish beach. Females can reach around 12 to 13 metres including their two long feeding tentacles. Yet the animal only stepped out of legend and onto film in 2012.

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. So basic facts about an animal of that size, such as how many species of it there are, were settled only in the last few years.

Light is a similar story. In the deep sea most of the light is made by animals. When Séverine Martini and Steven Haddock analysed years of video from remotely operated vehicles off California, they found that about three-quarters of the animals they recorded could glow. And in 1995 a ship off Somalia sailed through a "milky sea" of glowing water. Satellite images later showed the patch covered around 15,400 square kilometres, roughly the size of Northern Ireland. It was probably lit by bacteria. Note the word "probably".

We have left marks on places we have never seen

One thing we do know about the deep is that being out of sight has not kept it out of reach.

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.

Our chemistry arrived at the bottom of the trenches before most of our instruments did.

The deep floor also heals slowly. In 1989, in an experiment called DISCOL, scientists ploughed tracks across the seafloor of the Peru Basin. When they went back 26 years later, the tracks were still clearly visible, with life only partly recovered.

This matters because of what lies scattered across parts of the abyssal plain: polymetallic nodules, lumps about the size of a potato, rich in manganese, nickel, cobalt and copper. Each grows only a few millimetres every million years. Whether to scoop them up is an open question, and it is being asked about ground that has mostly not been surveyed.

How the gaps are being closed

A research ship can be in only one place at a time, and the ocean is vast. Today most observations come from machines. The modern ocean is watched from five directions at once: by crewed submersibles, by robots on tethers, by free-swimming robots, by drifting floats and from orbit.

There is also a reason for some patience with the subject. The sea is resilient, and when people give it a chance it often recovers faster than expected. Northern elephant seals were hunted to perhaps a few dozen animals by the 1890s. Protected in Mexico and the United States, they now number in the hundreds of thousands.

What to take from this

• The abyssal plains cover roughly half the planet's surface, and almost none of that area has been seen directly.
• The depth of the Challenger Deep is known only to within tens of metres.
• Whole ecosystems, such as hydrothermal vent communities, were unknown before 1977.
• New and strange animals have kept turning up this century, including one that builds armour from iron.
• Pollution has reached the deepest trenches, and disturbed seafloor was still scarred 26 years on.
• Most of what we learn now comes from robots, floats and satellites, not from people going down.

If you want the rest of the sea, from the surface to the trenches, The Oceans: A visual encyclopaedia of the sea covers it in 100 chapters with 100 detailed plates. It is a 302-page PDF and EPUB, and it costs £9 on Gumroad: matsiems.gumroad.com/l/the-oceans.