This is called subduction. At subduction zones, where the downgoing plate bends and dives beneath the overriding plate, we get oceanic trenches. These are the deepest parts of the ocean. The Mariana Trench, located in the western Pacific, is the deepest trench of all. Scientists still know little about the bottom of the ocean, mostly because it is dark and hard to get to (we can't just get down there using SCUBA gear to look around).