An ocean trench is a geological structure which occurs undersea along the boundary of a tectonic plate. Specifically, ocean trenches form along subduction zones, in areas where one plate is being sucked under another. The largest of these trenches are in the Pacific Ocean, where numerous active subduction zones have been identified, but smaller trenches can also be seen in the Indian and Atlantic Ocean. One could consider an ocean trench a sort of undersea valley.
Oceanic trenches, as ocean trenches are sometimes called, are deep depressions in the Earth's crust, and they comprise the deepest part of the ocean. To think about the way in which ocean trenches form, it may help to have a visual aid; imagine sliding one hand under the other, and think about the trench which forms in the area where the hands meet. This miniature trench is identical to the trenches which are formed in subduction zones. New material to be pulled under is constantly generated at oceanic ridges, where the seafloor is constantly spreading with fresh infusions of rock from beneath the Earth's crust.
The deepest ocean trench is the Mariana Trench, in the Western Pacific Ocean. Numerous other trenches criss-cross the Pacific, and appear along the edge of South America. In some cases, ocean trenches have become filled with sediment, in which case they are not readily identifiable as trenches, although investigation with radar and other imaging tools can reveal the underlying structure of the trench.
Typically, ocean trenches are close to areas of volcanic activity, because volcanoes are very common in subduction zones. The incredible depth of ocean trenches can also create a unique habitat which houses unusual ocean animals which prefer deep water; off the coast of Chile, for example, the fishing is quite good, thanks to a large ocean trench which is present in this area. The depths of an ocean trench pose some very interesting challenges to marine animals, as trenches are extremely dark and very cold, and the pressure of the overlying seawater is intense. Organisms which have adapted to this environment have some rather remarkable traits.
Exploring and mapping ocean trenches has posed some challenges to humans. People are not capable of handling the dark, cold, high-pressure environment of trenches, and the bulk of exploration has been performed with imaging equipment and robotics. People have descended into the Mariana Trench in specially-equipped submarines, but they haven't stayed long.