Mariana trench is the known deepest part of the sea. It reaches the maximum depth of 10971 meters at a point called Challenger deep. It is located in the western pacific ocean.
It is believed that only 5% of the ocean bottom is discovered and 95% of the ocean bottom is still not touched. So in this case the deepest part of the ocean could be more than the Mariana trench. But for now, Mariana trench remains the deepest part of the sea known.
If you wish to have a sense of how deep this point is, see below image (Source of image).