What Is at the Bottom of the Deepest Places? Where the Seafloor Goes Down

A trench marks the line where an oceanic plate begins descending beneath another plate, which is what subduction means, and the depression is produced by the downward bending of the descending plate as it turns from horizontal to inclined. That bend pulls the seafloor down by several kilometres below the surrounding abyssal plain, producing a feature that is narrow, extremely long and asymmetric, with a steeper slope on the overriding side. The deepest points on earth are all in such trenches, and the Pacific holds most of them because it is ringed by subduction zones, which is the same arrangement producing the volcanoes and earthquakes of that margin.

The environment is extreme in several respects at once:

• •Pressure exceeding a thousand times that at the surface at the deepest points
• •Complete darkness below a few hundred metres, so no photosynthesis anywhere
• •Temperature just above freezing, and stable
• •Food arriving only as material sinking from far above, which is scarce and irregular
• •Sediment accumulating in the trench floor, funnelled in from the slopes
• •Isolation, since each trench is separated from the next by shallower seafloor

Life is present throughout and is adapted in specific ways. Amphipods, which are crustaceans, are abundant and include species reaching unusual sizes for the group. Snailfish are the deepest fish recorded and appear to reach a physiological limit somewhat above the deepest trench floors, set by the concentration of a compound that stabilises proteins against pressure, which can only be raised so far. Sea cucumbers, worms and single-celled organisms of considerable size occupy the sediment. Bacterial communities are active throughout. What is absent is any primary production from light, so everything depends on material sinking from above, supplemented in places by chemical energy. Several trench species are found in one trench and nowhere else, which the isolation predicts.

Trenches accumulate material and what arrives is informative and increasingly troubling. Sediment funnels down the slopes and collects on the floor in thicknesses far greater than the surrounding seafloor carries, which makes trenches sinks for anything sinking through the water column. Organic material concentrates there, which is why life is more abundant in trenches than at comparable depths elsewhere. Pollutants concentrate too, and amphipods sampled from the deepest trenches have been found carrying industrial compounds at levels comparable to heavily contaminated coastal industrial areas, which was reported in 2017 and was not expected. Plastic has been recovered from the deepest points reached, including items identifiable enough to be dated. Nothing about the depth provides any isolation from what happens at the surface.

Descending to full ocean depth has been achieved by very few vehicles and the difficulty is instructive. A hull must resist pressure that increases with depth and is enormous at the bottom, which drives designs towards spheres of thick steel, titanium or ceramic, since a sphere distributes the load evenly. Buoyancy must be provided by material that does not compress, which rules out air and led to the use of petrol-filled floats in the earliest vehicle and syntactic foam in modern ones. Communication through kilometres of water is slow and limited. The first crewed descent to the deepest known point was made in 1960, the second in 2012, and repeated visits only became routine in 2019, which means fewer people have been to the bottom of the ocean than have walked on the moon.

Trenches are the depressions produced where a plate bends downward to descend beneath another, which is why the deepest points are all in subduction zones and mostly in the Pacific. Pressure exceeds a thousand atmospheres, there is no light and food arrives only from above. The first crewed descent to the deepest point was in 1960 and the second not until 2012.