Oceanic trenches are typically 3 to 4 kilometers deeper than the surrounding sea floor.
The average overall depth of the ocean is 4 kilometers.
And the Deepest part of the the ocean is the Marianas Trench in the Pacific which reaches depths of 11,000 meters.
Ocean trenches such as the Marianas Trench are the places where giant slabs of ocean floor actually sink back toward the mantle of the earth and melt.
These trenches and subduction zones are a portion of the process of Plate Tectonics which moves Earth's continents and sea floors and recycles the entire crust of the Earth throughout its history.
Deep Eh?