With 30 minutes left in her submersible dive, geochemist Mengran Du steered toward one last stretch of the deep trenches between Russia and Alaska — and found life where almost no one expected it. Published in 2025, the discovery revealed a 2,500-kilometre ecosystem of clams and tubeworms as deep as 9,533 metres, powered not by sunlight but by methane and hydrogen sulfide seeping from the seafloor.

With about 30 minutes left in a submersible dive, geochemist Mengran Du and her crewmates pushed toward one more stretch of the deep trenches between Russia and Alaska. At that depth, sunlight had been gone for kilometres. The pressure was crushing. The seafloor should have been sparse, dark and dependent on whatever scraps of organic matter fell from the surface ocean.

Instead, the lights of the submersible picked out an ecosystem.

There were dense fields of tubeworms, their red tentacles extending into the black water. There were clams in the sediment, snails and other animals living among them, and microbial mats marking places where chemistry was leaking from the seafloor. The discovery, published in Nature in 2025, revealed the deepest and most extensive chemosynthesis-based animal communities yet documented on Earth.

The sites lie in the Kuril-Kamchatka Trench and the western Aleutian Trench, a vast hadal system in the northwest Pacific, between the Russian Far East and Alaska. Using the full-ocean-depth Chinese crewed submersible Fendouzhe, an international team found communities spread across a 2,500-kilometre zone at depths from 5,800 metres to 9,533 metres.

They are not powered by sunlight. They are powered by methane and hydrogen sulfide rising through faults and sediments at the bottom of the trenches.

Life below the usual food chain

Most life on Earth ultimately depends on photosynthesis. Plants, algae and photosynthetic microbes turn sunlight into chemical energy, and that energy moves through food webs. Even in much of the deep sea, where sunlight never reaches, animals often depend on what oceanographers call marine snow: dead plankton, faecal pellets, carcasses and organic debris sinking from the sunlit layers above.

Hadal trenches were long expected to work largely that way. They are the deepest parts of the ocean, generally below 6,000 metres, carved where tectonic plates bend and descend into the mantle. They can trap organic matter, and they host strange animals adapted to enormous pressure. But large, thriving animal communities based on seafloor chemistry had rarely been documented at those depths.

The new study changes that. The communities reported by Xiaotong Peng, Mengran Du and colleagues are dominated by siboglinid polychaetes, a group that includes tubeworms, and by bivalves, including clams. These animals are not simply grazing on falling debris. Many of them rely on symbiotic bacteria that can use reduced chemicals such as methane and hydrogen sulfide to produce organic matter.

That process is chemosynthesis. Around hydrothermal vents and cold seeps at shallower depths, chemosynthesis is already known to support dense life. The surprise here is scale and depth. The Nature paper reports these communities across 19 of 23 dives, in geologically similar settings along the base of the accretionary prism, where faults can provide pathways for chemically rich fluids.

The deepest seep field

The first dense community was encountered during a dive in the Kuril-Kamchatka Trench at 9,533 metres. The researchers designated the site “The Deepest”, because it marked the deepest known seepage location discovered so far. At that depth, the pressure is roughly 950 times atmospheric pressure at sea level.

That site was not an isolated oddity. Subsequent dives found more seep communities in both the Kuril-Kamchatka and western Aleutian trenches. Some were dominated by frenulate siboglinid tubeworms, their thin tubes forming dense patches on black muds. Others were dominated by clams. One site called Clam Bed extended roughly 2 kilometres along a steep fault at about 5,800 metres. Other named fields included Wintersweet Valley, Cotton Field, Blue Marsh and Icy River.

The numbers are striking because they turn what might have been a single strange patch into a regional ecological feature. The communities were not continuous like a forest, but they appeared again and again along the trench system. The paper describes a 2,500-kilometre zone in which cold-seep communities occur in distinct patches tied to the geology of the trenches.

That patchiness matters. Chemosynthetic ecosystems need the right chemical flow. Too little methane or sulfide, and the symbiotic food source is absent. Too much disturbance, and communities may not persist. The animals cluster where the seafloor plumbing works.

Methane from buried biology

The source of the methane is one of the study’s most important details. It was not simply gas rising from deep geological reservoirs. Isotopic analysis suggested that the methane was produced microbially from organic matter buried in trench sediments.

In that version of the system, surface life still matters, but indirectly. Organic material sinks into the trenches and is buried. Microbes in the sediment break it down and produce methane. Faults and permeable pathways move methane-rich and hydrogen-sulfide-rich fluids toward the seafloor. There, animals with chemosynthetic symbionts convert that chemistry into biomass.

The result is a deep carbon loop that is more complex than the older picture of trench animals simply waiting for leftovers from above. The trench does not just receive carbon. It transforms it, stores it and leaks some of its chemical energy back into ecosystems at the seabed.

That has implications for the deep-ocean carbon cycle. Hadal trenches cover only a small fraction of the seafloor, but they can be intense sites of sediment accumulation and microbial processing. If chemosynthetic communities are more common in trenches than previously expected, then the biological and geochemical role of these regions may have been underestimated.

Why no one saw it sooner

The absence of earlier evidence was not proof that such ecosystems did not exist. It was partly a problem of access. The hadal zone is one of the least observed environments on Earth. A camera or submersible must survive darkness, cold, navigation challenges and pressures that can crush ordinary equipment.

The Fendouzhe expedition changed the observational scale. The team investigated the trench bottoms from July 8 to August 17, 2024, aboard the research vessel Tansuoyihao, using a crewed submersible capable of reaching nearly 11,000 metres. That allowed scientists not only to see the communities but to record video, collect samples and take geochemical measurements from the same extreme setting.

Previous hadal chemosynthetic communities had been known, including clam communities in the Japan Trench at more than 7,000 metres. But they were small compared with the new northwest Pacific discovery. The 2025 Nature paper argues that the Kuril-Kamchatka and western Aleutian communities are both the deepest and the most extensive chemosynthesis-based communities known.

The finding also hints at a broader map. The same kinds of geological pathways may occur in other hadal trenches, especially where subduction, faults, organic-rich sediments and fluid flow intersect. The authors suggest such ecosystems may be more widespread than scientists had assumed.

A different kind of habitability

The discovery has an obvious pull for astrobiology, though it should not be stretched too far. Clams and tubeworms are animals with long evolutionary histories on Earth. Finding them in a trench does not mean complex life waits in every dark ocean world.

But it does reinforce a more basic point: sunlight is not the only way to run an ecosystem. If liquid water, chemical gradients and microbial metabolisms coexist, biology can build food webs in places that look hostile from the surface.

That is why deep-sea seeps and vents keep appearing in discussions of ocean worlds such as Europa and Enceladus. Those moons may have dark oceans beneath ice shells, and the key question is whether chemistry from rock and water could provide usable energy. Earth’s hadal trenches are not direct analogues, but they show how life can exploit energy from fluids, faults and microbial chemistry in darkness.

For Earth science, the message is more immediate. The deepest seafloor is not a blank end point. It is active, connected and capable of supporting dense life where the geology allows it. The trenches are not merely graveyards for sinking debris. They can be chemical engines.

That is what made the final stretch of Du’s dive so startling. At the edge of the expedition’s time, in a place almost no human eyes had seen, the seafloor was not empty. It was threaded with chemistry, crowded with animals and bright with the old lesson of the deep ocean: life does not always wait for sunlight. Sometimes it waits for a crack in the Earth.

Sources

Peng, Du et al., Flourishing chemosynthetic life at the greatest depths of hadal trenches, Nature, 2025
Nature article page
The Washington Post report on the hadal trench ecosystem discovery
Live Science report on the Fendouzhe trench findings
The Week summary of the discovery