An ocean trench, or oceanic trench, is a deep, elongated depression in the ocean floor. Trenches form at convergent plate boundaries, where one tectonic plate is subducted beneath another, and they are the deepest parts of the ocean. They are key to understanding subduction zones, the recycling of oceanic crust and the dynamics of plate tectonics, and the volcanic arcs and earthquakes that form above a subducting plate.
Source Stern, R. J. (2002). “Subduction Zones.” Reviews of Geophysics, 40(4), 3-1-3-38; Hilde, T. W. C., Uyeda, S., & Kroenke, L. W. (1977). “Evolution of the Western Pacific and Its Margin.” Tectonophysics, 38(1-2), 145-165.