Scientists long thought life in the deepest ocean trenches depended mostly on scraps of sunlight-fed life drifting down from above — but at 9,500 metres beneath the Pacific, researchers found whole communities of tubeworms and clams powered instead by methane and hydrogen sulfide seeping from cracks in the seafloor.

For much of modern ocean science, the deepest trenches were treated as places where life survived on leftovers.

Sunlight does not reach the hadal zone, the region of ocean below about 6,000 metres. No plants grow there. Photosynthesis cannot run. The familiar explanation was that animals living on or near the deepest seafloor depended largely on what NOAA calls marine snow: dead organisms, faecal material, and other organic particles falling slowly from the productive waters above.

That model is still important. But a 2025 trench expedition in the northwest Pacific has made it harder to treat it as the whole story.

What the submersible saw

In research reported by Nature in July 2025, an international team using the crewed Chinese submersible Fendouzhe explored the Kuril-Kamchatka and Aleutian trenches, working across depths from about 5,800 metres to 9,533 metres. Nature’s own coverage described the result as Earth’s deepest ecosystem discovered six miles below the sea.

The striking part was not simply that animals were present. Deep trenches are not sterile. The striking part was the kind of community the team documented: dense fields of tubeworms, bivalves, snails, anemones, sea cucumbers, and microbial mats associated with chemical seepage from the seafloor.

Live Science, summarising the Nature paper and Springer Nature’s statement, reported that the communities were dominated by siboglinid polychaetes and bivalves, with animals using hydrogen sulphide and methane from tectonic faults as part of a chemosynthetic system. The dives covered a geologically active region where cracks and faults can provide pathways for reduced chemicals to enter the ocean.

The careful version is that the researchers found chemosynthesis-based communities at multiple hadal sites, including depths below 9 kilometres. The discovery does not mean every deep trench is built this way. It means the deepest parts of the ocean can host more local energy systems than the old picture allowed.

How life works without sunlight

Chemosynthesis is not new to ocean science. Since the discovery of hydrothermal vent communities in the late 1970s, researchers have known that microbes can use chemical energy instead of sunlight to build organic matter. Animals such as tubeworms and clams can then live with symbiotic microbes that convert chemicals into food for the host.

What is different here is the setting. Hydrothermal vents are often associated with hot fluids at spreading centres. Cold seeps are cooler places where methane, hydrogen sulphide, and other chemicals leak from sediments or crustal pathways. The northwest Pacific trench communities appear to belong closer to that cold seep logic, but at depths far below most well-studied examples.

At 9,500 metres, pressure is not just a background condition. It is a physical constraint on proteins, membranes, shells, movement, reproduction, and every chemical exchange the animals and their microbial partners depend on. The water is dark, cold, and difficult to reach. Instruments have to survive conditions that make routine observation almost impossible.

That is why direct observation matters. A chemical signal can suggest a seep. A sediment sample can suggest microbial activity. But seeing fields of animals living around these sources changes the scale of the question.

The methane source matters

One of the more interesting interpretations concerns where the methane comes from. The Washington Post’s report on the Nature paper noted that analysis of gases seeping from the seafloor suggested microbes in trench sediments may be producing methane by consuming organic matter accumulated there. Symbiotic bacteria inside tubeworms and molluscs could then use methane and hydrogen sulphide from those seeps to produce organic matter for their hosts.

That gives the system a layered quality. Sunlight-fed material may still matter, because organic particles from above can collect in trenches. But the animals are not simply eating fallen debris directly. In the proposed pathway, buried microbial processes transform accumulated material into chemical fuel, and other microbes then use that fuel to sustain larger animals.

This is not a clean replacement of one model by another. It is a more complicated energy map.

The trench becomes less like a passive collection basin and more like a reactor: organic matter, sediment, microbes, faulting, seep chemistry, and animal symbiosis interacting under pressure.

Why this belongs in a space publication

Deep ocean discoveries often enter space writing too quickly. It is easy to point at an extreme Earth environment and turn it into a loose claim about life elsewhere. That is not what this finding shows.

It does, however, sharpen one question that astrobiology already asks: what can life do when sunlight is absent but liquid water, rock, chemistry, and time are present?

NASA’s Europa Clipper material frames habitability around three ingredients: water, chemistry, and energy. On its Europa ingredients for life page, NASA notes that any life in Europa’s subsurface ocean would likely have to be powered by chemical reactions rather than photosynthesis, because it would exist beneath ice where sunlight cannot reach.

The Pacific trench finding does not make Europa inhabited. It does not make Enceladus inhabited. It does not even tell us that animals could exist in extraterrestrial oceans. Earth has a long biological history, oxygenated oceans, and evolutionary pathways that ocean worlds elsewhere may not share.

What it does show is narrower and useful: on Earth, chemical seepage in dark, high-pressure marine environments can support more organised animal communities than scientists had documented at those depths.

What the discovery does not settle

The obvious next questions are practical and biological. How widespread are these hadal seep communities? Are the animals new species, known relatives at unexpected depths, or some mixture of both? How do their symbiotic microbes work under such pressure? How stable are the seeps over time?

The expedition found communities across multiple dives and locations, which argues against a one-off anomaly. But the deep seafloor is still scarcely observed. A few dozen dives can change the map without completing it.

There is also a carbon question. Trenches are places where organic matter can accumulate, be buried, be transformed, and sometimes re-enter local ecosystems through methane and other reduced compounds. Understanding that circulation matters for ocean biogeochemistry as well as for biology.

The old image of the deepest trench as a place waiting for scraps from above was not wrong. It was incomplete.

At least in parts of the northwest Pacific, the seafloor itself is feeding life back.