The ocean floor and its depth vary a lot from place to place. Maybe it is expected that the central parts of the oceans would be the deepest parts of them, while the closer the ocean floor is to the land it will be shallower, but that is not the case.
The central parts of the oceans have mid-ocean ridges. The mid-ocean ridges are volcanic underwater mountain ranges. There is constant propelling of magma, and as the magma cools off quickly it creates new crust. This results in the mid-ocean ridges gaining heights of up to 1.5 km above the surrounding area.
The further away we move from the mid-ocean ridges, or rather being closer to the oceanic margins, the depth actually increases. The reason for this is the erosiove power of the water, but also the subduction zones. The erosion manages to flatten the ocean floor, thus making it deeper, while the subduction zones are the places where the oceanic plate is moving downward toward the mantle, and it create ocean trenches which are very deep, in fact the deepest parts of the oceans.
