Deep trenches are known as ocean deeps. Some ocean deeps are as deep as 36,000 feet. The deepest part of the ocean in the world is Mariana Trench which is about 11,033 metres deep. Its depth is more than the height of Mount Everest on the land. Another deep trench is the Tonga Trench with a depth which exceeds 10,000 metres.
Also, Read Importance of Oceans