Nearly seven miles beneath the surface of the Pacific Ocean lies a place so hostile that only a handful of human beings in all of history have ever seen it — total darkness, near-freezing water, and pressure equivalent to a jet's weight bearing down on a single postage stamp

Somewhere on the way down, a window cracked.

Jacques Piccard and Don Walsh were sealed inside a bathyscaphe called Trieste on 23 January 1960, hours into a descent that Britannica records at 10,916 metres, when the plexiglass on the outer hull gave way. They carried on down anyway, sat there briefly, and came back up. Nobody followed them for 52 years, until James Cameron piloted a lime green submersible to 10,898 metres by himself and set the record for the deepest solo dive.

Fewer than thirty human beings have ever been down there. Esri, reporting on oceanographer Dawn Wright’s 2022 dive, counted 27, which is more than the number who have walked on the Moon.

Cost explains most of that. Reaching the floor requires a pressure hull rated for full ocean depth, a support vessel parked above it, and a budget large enough that the visitor list skews heavily towards people funding their own expeditions.

How much water is sitting on top of you

Pressure at the floor of the Mariana Trench runs to roughly eight tonnes per square inch, about a thousand times the atmospheric pressure at sea level, as documented by NOAA Ocean Exploration. Sunlight gives up at around 200 metres. Everything below that is black except for the light animals make themselves, and the water sits a degree or two above freezing.

Depth keeps getting revised. Guinness World Records currently lists the deepest confirmed point at 10,935 metres, give or take six, in the eastern pool of the Challenger Deep. That figure comes from the fifteen dives Victor Vescovo made in the submersible Limiting Factor between 2019 and 2022.

Who lives at the bottom

Drop a baited trap into the trench and it tends to come back holding one thing: amphipods. Shrimp-like scavengers, about three centimetres long, arriving in numbers that make the abyss look a lot busier than advertised.

Scavenging is the only career available. Nothing photosynthesises seven miles down, so the whole food web survives on leftovers falling from the sunlit water above, a slow drift of dead plankton and sinking debris known as marine snow. The deep-sea biologist Alan Jamieson, writing in The Conversation, calls that drift the primary way energy reaches these depths at all.

Building a shell down there should be close to impossible, because calcium carbonate dissolves under that kind of pressure. A team led by Hideki Kobayashi at the Japan Agency for Marine-Earth Science and Technology worked out the trick in a paper published in PLOS One. Their amphipod, Hirondellea gigas, secretes gluconic acid to strip aluminium out of trench sediment. Alkaline seawater then converts that aluminium into a hydroxide gel, which coats the animal like body armour. One species, one study, but a lovely piece of improvised chemistry.

Bigger things live there too. Researchers at Scripps Institution of Oceanography, using free-falling cameras built with National Geographic engineers, filmed xenophyophores at 10,641 metres in the Sirena Deep. Those are single cells the size of a fist, ten centimetres across, and they hoover up lead, uranium and mercury without any obvious complaint.

A fish that gave up on its own skeleton

Picture a tadpole built out of wet tissue paper, and you have a rough idea what the hadal snailfish looks like. Pseudoliparis swirei is the deepest vertebrate ever recorded, and vertebrates have a much harder time of these depths than the invertebrates covered so far.

Chinese researchers sequenced its genome for a paper in Nature Ecology and Evolution, and the list of compromises is startling. Its skull never fully closes. A frameshift mutation in the bone Gla protein gene appears to shut down calcification early, leaving the skeleton thin and half-finished. Mutations turned up in the enzyme that produces trimethylamine N-oxide, a compound that stops proteins buckling into useless shapes under load.

Even that has a ceiling. In 2023 an expedition led by Jamieson, who founded the Minderoo-UWA Deep Sea Research Centre, filmed a juvenile snailfish at 8,336 metres in the Izu-Ogasawara Trench off Japan, which the Natural History Museum reported as the deepest fish ever caught on camera. Jamieson has said any future record will be broken by metres, not kilometres. That trench runs warmer than Mariana’s, and he considers this depth close to the physiological ceiling for fish anywhere, which leaves roughly the last 2,600 metres of the Challenger Deep to the invertebrates.

What got there first

Jamieson also ran the depressing study.

Sampling amphipods from the Mariana and Kermadec trenches, his team found persistent organic pollutants in every single specimen, at concentrations that in some cases ran fifty times higher than in crabs living in one of China’s most contaminated river systems. Those findings were published in Nature Ecology and Evolution in 2017. PCBs, banned in the 1970s, refuse to break down. They bind to organic matter, and organic matter drifts steadily downward for weeks or months until something hungry down there eats it.

A trench is a funnel with no exit.

Twenty-seven people needed titanium pressure spheres, buoyancy foam and a ship holding station overhead to spend a few hours in that darkness. A flame retardant made in a factory sixty years ago got down there unassisted, and it was already waiting when they arrived.