Ocean trenches are long, narrow depressions on the seafloor that are the deepest parts of the ocean. They are typically 50 to 100 kilometers wide and 3 to 4 km below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. Trenches are a result of tectonic activity, which describes the movement of the Earth’s lithosphere. In particular, they are a feature of convergent plate boundaries, where two or more tectonic plates meet. At many convergent plate boundaries, dense lithosphere melts or slides beneath less-dense lithosphere in a process called subduction, creating a trench.

Trenches are geomorphologically distinct from troughs. Troughs are elongated depressions of the sea floor with steep sides and flat bottoms, while trenches are characterized by a V-shaped profile. Oceanic trenches are the deepest places on the Earths solid surface and range down to 11 km below sea-level. The deepest trench in the world is the Mariana Trench located near the Mariana Islands, which is 1,580 miles long and averages just 43 miles wide. It is home to the Challenger Deep, which, at 10,911 meters (35,797 feet), is the deepest part of the ocean.

Despite their scarcity, studying trenches can help scientists better understand the physical process of earthquakes and the existence of previously unknown processes, species, and ecosystems.