Ocean trenches are steep depression in the deepest part of an Ocean (Ocean floor), they are natural occurring phenomenon. They are also the deepest part of an ocean. Ocean trench is formed when two tectonic plates carrying oceanic crust meet.
The deepest valley on Earth is an ocean trench, it is known as the Challenger Deep, which is also part of the Mariana Trench in the Pacific near Guam. The deepest point of this trench is 36,070 feet below the ocean surface.