Into the Mariana Trench

What lives at the bottom of the ocean, and what else has arrived there

A The deepest point in the ocean lies at the southern end of a crescent-shaped trench in the western Pacific, roughly eleven kilometres below the surface. The figure is difficult to hold in the mind, so the standard comparison is made: the highest mountain on earth, placed at the bottom, would still be more than two kilometres short of the surface. The first indication that such depths existed came from a British survey expedition in the 1870s, which lowered a weighted rope and recorded a sounding that was widely disbelieved. It was, as it happens, an underestimate, and the modern figure has itself been revised several times as measurement has improved.

B The trench is a product of one plate of the earth's crust descending beneath another. The descending plate bends sharply as it goes down, and the trench is the surface expression of that bend rather than a valley eroded into anything. This explains a feature of the environment that matters for everything living in it: the trench is not a basin of still water but a slot that concentrates whatever falls into it, so material sinking from the productive waters far above accumulates at the bottom in quantities greater than the surrounding sea floor receives. Earthquakes contribute to the same process by shaking loose sediment from the walls, which slides to the deepest point and buries whatever is there under a fresh supply of organic material.

C The pressure at the deepest point is around a thousand times that at the surface, equivalent to the weight of some fifty jumbo jets bearing on a person. The engineering consequences are severe and counter-intuitive: a hull must be spherical, since any other shape concentrates stress at its weak points, and the vessel that first reached the bottom in 1960 was essentially a steel ball slung beneath a tank of petrol, petrol being both lighter than water and, crucially, effectively incompressible. Its two occupants spent about twenty minutes on the sea floor, during which a window cracked, and saw very little, because their descent had raised a cloud of sediment that did not settle while they were there. The descent itself had taken nearly five hours and the ascent more than three, which gives the twenty minutes at the bottom a certain poignancy.

D No one returned for fifty-two years, a gap that says more about funding than about engineering. The second manned descent, in 2012, was made alone in a vessel designed to stand upright rather than lie horizontally, and it produced photographs and samples rather than a claim. Since then, uncrewed vehicles and landers left on the bottom for weeks at a time have supplied the great majority of what is known, which is the usual pattern in deep exploration: the record-breaking descent captures the attention, and the unglamorous instrument left in the dark collects the data. A lander is also a better instrument for the purpose, since it can wait. An animal that will not approach a vehicle full of lights and moving parts will approach a baited frame that has been sitting motionless for six hours, and a substantial part of what is known about the behaviour of these communities has been recorded in exactly that way.

E Life is present throughout, and its existence at such depths ceased to be the surprise it once was. The animals encountered most often are pale scavenging crustaceans, which arrive within minutes at any bait lowered to the bottom and are numerous enough to strip it in hours. There are also single-celled organisms of remarkable size, some the width of a hand, which build shells from grains of sediment and appear to serve as habitat for smaller animals; that a single cell should reach such dimensions at all is among the more surprising findings of deep-sea work. Fish are present but not to the very bottom; below about eight thousand metres, the chemistry that stabilises proteins under pressure appears to reach a limit, and this rather than food supply seems to set the boundary.

F The food itself arrives from above. Nothing photosynthesises in permanent darkness, so the entire community depends on what sinks: dead plankton, the occasional carcass of a large animal, and organic particles that drift down for weeks. The trench's shape concentrates this material, which is why the deepest points support more life than the plains beside them. In addition, chemical reactions between sea water and the rock exposed by the bending plate release compounds that some bacteria can use directly, providing a second and entirely independent basis for a food web.

G The trench has also acquired a less welcome distinction. Plastic has been photographed on the bottom, and the crustaceans collected there carry industrial pollutants at concentrations exceeding those found in animals from some of the most contaminated rivers on earth. The explanation is the same funnel effect that supplies the food: contaminants bind to sinking particles and are delivered downwards, and a trench that concentrates nutrition concentrates everything else as well. The deepest and most remote habitat on the planet turns out to be not a refuge from what happens at the surface but a collecting point for it.