Why are ocean trenches the deepest parts of the ocean?

A trench in general results when you dig a hole in the
ground which extends for some distance. Imagine you are on top of the grand canyon
looking down. What you are seeing is a huge trench that is lower than the surrounding
ground.

Now imagine that you are on the bottom of the ocean
and the bottom is all flat. Then you would be at the deepest part of the ocean. Now
imagine that you have a gigantic plow that can start digging into the bottom of the
ocean floor. As you dig you will go lower and lower below the surface of the ocean. In
other words you will be going deeper and deeper below the ocean's
surface.