Mariana Trench: The Deepest Dark on Earth and What Might Be In It

Drop a steel ball into the sea over the Mariana Trench and it would fall for the better part of an hour before touching bottom. Down there the water presses with the weight of roughly a hundred elephants standing on your head, the temperature hovers just above freezing, and no sunlight has ever reached. We have put more bootprints on the Moon than we have put people at the bottom of this trench. It is, in every practical sense, the least-known place on your own planet.

The Mariana Trench is the deepest known point in Earth's oceans, and the dark it holds is doing a good job of keeping its secrets.

Just how deep, and what that means

The trench is a crescent-shaped gash in the western Pacific seafloor, and its lowest known point, the Challenger Deep, plunges to nearly 36,000 feet, deeper than Mount Everest is tall. Only a handful of humans have ever reached the bottom, beginning with Jacques Piccard and Don Walsh in 1960 and, decades later, the filmmaker James Cameron in a solo dive. The pressure at that depth is so extreme that submersibles must be engineering marvels just to avoid being crushed like a can.

And yet it is not lifeless. Cameras and landers have found translucent snailfish drifting near the bottom, giant single-celled organisms, and amphipods that thrive where nothing has any business living, some of them, dispiritingly, with plastic already in their guts. Life has colonized the crushing dark by chemistry and patience, feeding on the slow rain of dead matter from far above.

The sounds and the shapes

The deep sea has a habit of producing noises we cannot immediately explain. Hydrophones in the region have recorded strange calls, including a bizarre metallic-organic sound dubbed the "Western Pacific Biotwang," which researchers eventually attributed to baleen whales, probably Bryde's whales, though the identification took years. That is the pattern down here: an unexplained signal, a long uneasy gap, and usually a natural answer arriving late.

It is worth being plain that no trench monster, no surviving prehistoric giant, has ever been found. The megalodon is extinct; the deep is not hiding it. But the deep is genuinely full of animals we have never seen alive, described from a single specimen or a few seconds of grainy footage, which is a different and more honest kind of mystery.

Where the dark stays open

The science is advancing fast, and still the trench holds its atmosphere of the unknown. Most of the deep ocean has never been mapped in detail, let alone visited; new species turn up on nearly every serious expedition. People who work the deep-sea cameras describe the particular unease of watching something large move at the edge of the light and then leave before you can name it. Suggestion and low resolution explain much of that. A residue does not tidy away, and it is what our readers argue over.

Below, divers, sailors and deep-sea workers have filed their own encounters with the things that live in the dark water, each voted True, Fake or Not sure by the jury. Read them and cast your verdict.