The deepest parts of the ocean are long, narrow depressions where one tectonic plate bends and slides beneath another. These ocean trenches can exceed 10,000 meters in depth. The Mariana Trench in the western Pacific holds the record at the Challenger Deep.

Trenches form at convergent plate boundaries and are often paralleled by volcanic island arcs. The water inside is cold, dark, and under extreme pressure, yet specialized animals—amphipods, holothurians, and microbial communities—inhabit even the greatest depths. Sediment and organic matter funneled into trenches create unique biogeochemical environments.

Exploring trenches requires specially designed landers, submersibles, or autonomous vehicles capable of withstanding pressures greater than one thousand atmospheres. Scientific interest includes the limits of life, the fate of carbon in the deepest ocean, and the seismic hazards associated with subduction. Only a handful of humans have visited the deepest points, and much of the trench system remains unsampled.

The pressure at the bottom of a deep trench is equivalent to a large elephant balanced on a postage stamp.