What are the deep depressions in the Earth's crust that comprise the deepest part of the ocean called?

The deepest depressions in the Earth's crust that form the deepest parts of the ocean are known as ocean trenches. These trenches are formed by tectonic processes, usually at convergent plate boundaries where one tectonic plate is being forced under another in a process known as subduction. Ocean trenches can reach extraordinary depths, with some of the most notable examples being the Mariana Trench, which is the deepest known point in the Earth's seabed.

The nature and formation of ocean trenches make them critical for a variety of geological and biological processes, including providing habitats for unique marine life adapted to extreme pressure and darkness. Understanding ocean trenches is crucial in marine biology and geology, as they play a significant role in the Earth's geological activity and ecosystem dynamics.