Narrow, elongate, steep-sided and often rock-walled depression that is 3000 m or more deeper than the adjacent deep-sea floor. Trenches are the deepest parts of the ocean, the greatest known depth being that of the Challenger Deep in the Marianas Trench where a depth of 11 022 m has been recorded. Trenches usually mark the position of a destructive plate margin, where the sea floor is slowly being destroyed by subduction.