What Lives at the Bottom of the Mariana Trench?

Background

The Mariana Trench, a crescent-shaped scar in the Pacific Ocean floor east of the Mariana Islands, contains Challenger Deep, the deepest known point in any ocean on Earth. NOAA's most precise measurement, published in 2021 from nearly 60 days of pressure-sensor data, puts its depth at roughly 10,935 metres (35,876 feet), deep enough that if Mount Everest were placed inside it, its peak would still sit more than two kilometres underwater. Only a handful of crewed descents have ever reached the bottom: Jacques Piccard and Don Walsh in the bathyscaphe Trieste in 1960, filmmaker James Cameron solo in 2012, and financier and explorer Victor Vescovo, who made the first of several solo dives there in 2019 and reported seeing what looked like a plastic bag resting on the seafloor.

Despite pressure exceeding 1,000 atmospheres and total darkness, the trench is not lifeless. Life at these depths, the hadal zone, is sparse and specialised rather than absent, dependent almost entirely on organic material that sinks down from the sunlit ocean above, since no sunlight reaches deep enough to support photosynthesis.

What Actually Lives There

The deepest confirmed fish

On 15 August 2022, a joint expedition from the University of Western Australia's Minderoo-UWA Deep Sea Research Centre and the Tokyo University of Marine Science and Technology filmed a juvenile snailfish of the genus Pseudoliparis at 8,336 metres in the Izu-Ogasawara Trench, a related trench near Japan rather than the Mariana Trench itself, setting the current record for the deepest fish ever recorded on camera, 158 metres deeper than the same team's previous 2017 record. Researchers consider this close to the practical depth limit for fish: below roughly 8,200 metres, the biochemistry that keeps cell membranes flexible under extreme pressure appears to stop working reliably, which is why no fish has ever been found in the Mariana Trench's own deepest reaches, even though invertebrates continue much further down.

Amphipods, xenophyophores, and microbial life

Below the depth fish can tolerate, life continues in other forms. Shrimp-like amphipod crustaceans, some species growing unusually large for their family in a phenomenon called "abyssal gigantism," scavenge organic debris across the trench floor. Xenophyophores, single-celled organisms that can grow up to 10 centimetres wide, among the largest single cells known, form fragile structures on the sediment and were first photographed at Challenger Deep depths by Cameron's 2012 expedition. At the very bottom, bacterial mats survive on chemosynthesis, extracting energy from chemical reactions rather than sunlight, in a manner related to the ecosystems found at deep-sea hydrothermal vents, the environment central to the hydrothermal vent hypothesis for the origin of life.

Pollution has already reached the trench

A 2017 study in Nature Ecology & Evolution by Alan Jamieson and colleagues found that amphipods collected from the Mariana and Kermadec trenches carried extraordinarily high levels of persistent organic pollutants, including PCBs, in their fatty tissue, in one case up to 50 times the concentration found in crabs from one of China's most polluted rivers. Because these pollutants bind to the organic "marine snow" that hadal scavengers depend on for food, the trench's isolation has not protected it from decades of industrial contamination, a finding that surprised researchers who had expected the deepest, most remote parts of the ocean to be comparatively pristine.

The Trench's Solved Mysteries

The Western Pacific Biotwang

In October 2014, autonomous hydrophone gliders recorded a strange, metallic, multi-part call near the Mariana Islands, quickly nicknamed the "Western Pacific Biotwang" for its unsettling, almost mechanical sound. Unlike the Bloop, the earlier ultra-low-frequency sound identified in 2005 as a large Antarctic icequake, the Biotwang remained formally unattributed for a decade. A 2024 study in Frontiers in Marine Science by Ann Allen and colleagues combined acoustic recordings with visual survey data to confirm the call as a vocalisation produced by Bryde's whales, a call that recurs seasonally across the Mariana Archipelago and Wake Island, typically peaking between August and November.

What the "plastic bag" sighting actually showed

Vescovo's 2019 report of a plastic bag and candy wrappers at the literal bottom of the ocean became widely repeated as proof that human litter has reached everywhere, though the exact identity of the object he observed was never retrieved or forensically confirmed. Independently, the Jamieson pollution study above establishes the same underlying point through direct laboratory sampling, that anthropogenic pollution reaches the hadal zone, on firmer evidential ground than a single visual sighting alone would provide.

Current Consensus

Marine biologists agree that the Mariana Trench supports a genuine, if sparse and highly specialised, hadal ecosystem, and that snailfish mark the practical depth limit for vertebrate life, with invertebrates and microbes surviving further down through pressure-tolerant adaptations and chemosynthesis. Both of the trench's widely circulated "mysteries," the Biotwang sound and the plastic-bag sighting, have documented, non-speculative explanations: a whale call confirmed through matched acoustic and visual data, and human pollution confirmed through direct chemical sampling of trench-floor animals. What remains genuinely open is not whether anything stranger exists there, but how much of the hadal zone remains unsurveyed: researchers estimate a large share of deep-trench species have never been formally described, so new organisms, not new anomalies, are the likeliest future discoveries. A different kind of open question comes from elsewhere in the Pacific abyss: the disputed 2024 claim that polymetallic nodules produce "dark oxygen" without sunlight, a live scientific dispute rather than a folklore mystery, but a reminder that genuinely unresolved deep-sea science still turns up alongside the debunked ones.

Why This Mystery Endures

The trench endures as a cultural fixation partly because its numbers are so difficult to hold in mind, a pressure equivalent to fifty jumbo jets stacked on a single point, a depth that would fully submerge Everest, that popular imagination fills the gap with something correspondingly dramatic: an undiscovered predator, a portal, a monster. The reality inverts that expectation. Life there is real but modest in scale, small, slow, and built for endurance rather than menace, which is arguably the more genuinely strange fact: complex biology persisting at all in an environment that extreme, sustained on nothing but debris falling from an ocean surface ten kilometres above.

The trench also keeps generating fresh material for the fascination even as science resolves individual claims, because so much of it remains unexplored. Fewer than a handful of crewed dives have ever reached Challenger Deep, a track record thinner than crewed spaceflight to the Moon, so every new expedition has genuine odds of finding a previously undescribed species, keeping the sense of an open frontier alive even as specific past mysteries, like the Biotwang, get closed out one by one. Why the ocean floor as a whole remains less mapped than Mars explains the structural reason expeditions like these stay rare and expensive rather than routine: sonar has to be carried to the seabed one ship-track at a time, and nowhere is that slower, harder process more consequential than at the bottom of the ocean's deepest trench.