Science

We have sharper maps of Mars than of most of Earth's deep ocean floor, and the little of it we have reached rewrote biology: in 1977 a submersible found thriving life around hot vents in total darkness, fed by chemistry rather than sunlight

The deep ocean remains so alien that we know Mars better—and the creatures we've discovered in its lightless depths have fundamentally changed what we thought life required to survive.

By Lachlan Brown

Published August 3, 2026 · How we edit

The deep ocean. Most of Earth's seafloor is still unmapped in detail, and the life found around its hydrothermal vents reshaped where scientists think life could exist. Image: Francesco Ungaro / Pexels.

The least-known place on Earth is not some remote patch of desert or ice. It is most of the planet’s own seafloor. We have sharper maps of the surface of Mars than we do of the deep ocean that covers well over half our world.

That sounds like a line invented for effect, but it holds up, and it says something about how genuinely hard the deep ocean is to reach. It is worth taking seriously, not least because what little we have found down there once forced biology to rethink itself.

I am a writer with a long interest in this material, not an oceanographer, so treat this as a careful reading rather than expert testimony.

The least-mapped surface on Earth

The whole seafloor has been sketched at coarse resolution from satellites, which detect its broad shape in the way the sea surface bulges over mountains and sinks over trenches. Fine, direct mapping by sonar is another matter. The Nippon Foundation and GEBCO project Seabed 2030, which aims to map the entire ocean floor in detail by the end of this decade, reported that as of 2025 only about 27 per cent had been mapped to modern standards.

Put the other way, most of the ocean remains unmapped and unobserved, with the US agency NOAA putting the unexplored share at around 80 per cent. The surfaces of the Moon, Mars and Venus have all been charted in finer detail than the bottom of our own sea.

The mapping is speeding up, and Seabed 2030 added an area roughly the size of the Indian subcontinent in a single recent year. But the gap is a fair measure of how stubbornly the deep resists us.

Why it is so hard to reach

The obstacle is not distance but pressure. Water is heavy, and it stacks up fast.

At the bottom of the Mariana Trench, nearly 11 kilometres down, the pressure is over a thousand times what you feel at the surface, something like a tonne bearing down on every square centimetre. It is also permanently dark below about a kilometre, where no sunlight reaches, and close to freezing. Building a craft that can carry a person into that and back is closer in difficulty to spaceflight than to ordinary diving, which is part of why only a handful of people have ever reached the deepest point, starting with the Trieste in 1960.

For most of the deep ocean, our eyes have been robotic ones, lowered on cables or sent down as autonomous vehicles.

The discovery that rewrote biology

In 1977, one of those crewed submersibles, Alvin, was exploring the Galápagos Rift when it came across something no one had predicted. Around cracks in the seafloor venting hot, mineral-rich water sat dense communities of life: fields of giant tube worms, clams and crabs, thriving in permanent darkness far below any sunlight.

This should not, by the textbooks of the time, have been possible. Almost every food chain then known was built on photosynthesis, on energy captured from the sun. Here was an entire ecosystem running on something else. The base of it turned out to be microbes performing chemosynthesis, drawing energy from chemicals in the vent fluid, chiefly hydrogen sulphide, rather than from light.

It was one of those findings that quietly moves a boundary. Life, it turned out, did not require sunlight at all. It required an energy source and liquid water, and a vent on a black seafloor could supply both.

Why this reaches beyond Earth

That last point is where the deep ocean stopped being only an Earth story. If life can run on chemistry at a dark seafloor here, then the same might be possible on other worlds that hide oceans under ice, with no sunlight reaching them either.

This is now central to how the search for life beyond Earth is framed. Saturn’s moon Enceladus is the clearest case: I have written about how it fires jets of its buried ocean into space, and the Cassini spacecraft found chemical hints in those plumes consistent with hydrothermal activity on its seafloor, the same kind of vent chemistry Alvin stumbled onto. Jupiter’s moon Europa holds a larger hidden ocean still.

None of this means anything is living in those places. It is an argument about possibility, not a discovery of life, and that distinction matters. But the reason scientists take ocean worlds seriously at all traces directly back to what turned up around those vents in 1977.

What to watch

Two efforts are worth following. On Earth, Seabed 2030 is trying to finish the map by the end of the decade, which would finally give us a complete picture of the shape of our own planet. Off it, NASA’s Europa Clipper is on its way to survey that moon’s icy shell and the ocean beneath.

There is something fitting in the pairing. We are still learning the floor of our own sea, and at the same time reaching for oceans hundreds of millions of kilometres away, chasing a possibility that a submersible first raised at the bottom of ours.

Written by

Lachlan Brown

Lachlan Brown is a co-founder of Brown Brothers Media and one of Space Daily's two publishers. He is the author of several books on Buddhism, mindfulness, and relationships, and oversees content operations and publishing strategy across the network. At Space Daily, Lachlan focuses on the Mind & Meaning pillar — the psychology of ambition, isolation, and meaning under extremes.