What lives where sunlight never reaches?

At hydrothermal vents, microbes oxidise hydrogen sulphide and other reduced compounds coming out of the crust, and use the released energy to fix carbon. Larger animals either graze these microorganisms or house them inside their own tissues. The giant tube worm has no mouth and no gut; it is a container for symbiotic bacteria.

The same principle operates at cold seeps and on the skeletons of dead whales. Most of the deep sea still lives on food that sinks from the sunlit surface, and these are patches within it. But they are a second way of running a food web, and not a wholly separate one: the animals at a vent breathe oxygen that photosynthesis made.

By volume, most of the living space on Earth is dark, cold, and under pressure. Bioluminescence is not a curiosity in this environment; it is the normal way of producing light, and a survey that classified more than 350,000 animals seen in seventeen years of video off California, from the surface to 3,900 metres, found that 76 per cent of those it could assign belonged to kinds probably able to make light.

Amphipods, holothurians and active microbial communities have been found in hadal trenches, the Mariana Trench among them, and snailfish, the dominant fish of the trenches, have been filmed as deep as 8,336 metres. In 2025 a team using the submersible Fendouzhe reported communities of tube worms and clams living on chemosynthesis along 2,500 kilometres of two northwest Pacific trenches, as deep as 9,533 metres. Amphipods caught in two trenches deeper than 10,000 metres carried persistent organic pollutants at levels matching or exceeding those recorded near regions of heavy industry: the deepest parts of the ocean are isolated, but not separate.

A substantial microbial biosphere lives in the sediment under the sea floor and in continental rock, much of it so slow that its biomass turns over in anything from months to thousands of years. Estimates of its mass have been revised downward as more places were sampled, and they still disagree: two published in 2018 put all the bacteria and archaea on Earth at 23 to 31 billion tonnes of carbon and at about 77, most of it underground in the second.

Vent fields open, run and close, and how long one lasts depends on the ridge it sits on. Fields on fast-spreading ridges are under magmatic control and the shortest-lived, and eruptions there can wipe out their communities within decades. Fields on slow-spreading ridges are held open by tectonics and can last thousands of years. Measured by age, they run from about a century at 13 degrees north on the fast-spreading East Pacific Rise to about twenty thousand years at the TAG field on the Mid-Atlantic Ridge. How long one stays active, as against how long ago it started, is the part that is not well measured.

How larvae reach new fields, which can be a few kilometres apart or many hundreds, is only partly understood, and it matters, because it helps decide how vulnerable these communities are to disturbance.

People have seen less than a thousandth of one per cent of the deep seafloor. Expeditions to water nobody has surveyed routinely return with species that have never been described: one two-week survey of the midwater off Brazil confirmed 31.

This is not a claim about monsters. It is a claim about sampling: the ordinary result of looking somewhere new down there is finding something new.

If chemosynthetic ecosystems are common on Earth, the requirement for a planet to host life may be chemical rather than solar, which would substantially widen where it is worth looking elsewhere. Whether that generalisation holds is genuinely open.

Europa and Enceladus appear to have oceans of liquid water under their ice, and on Enceladus that water appears to be reacting with rock, which is the condition that makes vent chemistry possible. Enceladus vents material into space, and that plume contains molecular hydrogen, consistent with water-rock reactions of the kind that feed vent ecosystems here. This is a reason to look, not evidence of life.

Some estimates place most of Earth’s bacteria and archaea below the surface, in sediment and rock. Even then the deep biosphere would not outweigh the sunlit one, which plants dominate: one census puts the deep subsurface at about 15 per cent of the world’s biomass.