Deepest Part of the Ocean

Challenger Deep in the Mariana Trench is the deepest point in Earth's oceans. The bottom there is 10,924 meters (35,840 feet) below sea level. If Mount Everest, the highest mountain on Earth, were placed at this location it would be covered by over one mile of water. The Challenger Deep is named after the British survey ship Challenger II, which discovered this deepest location in 1951. 

The Challenger Deep was first explored by Jacques Piccard and Don Walsh in the Trieste bathyscaphe in 1960. They reached a depth of 10,916 meters (35,814 feet). In 2009 researchers from Woods Hole Oceanographic Institution completed the deepest dive by an unmanned robotic vehicle in the Challenger Deep. Their Nereus robotic vehicle reached a depth of 10,902 meters. 

Why is the ocean so deep here?

The Mariana Trench is located at a convergent plate boundary. Here two converging lithospheric plates collide with one another. At this collision point, one of the plates descends into the mantle. At the line of contact between the two plates the downward flexure forms a trough known as an ocean trench. An example of an ocean trench is shown in the diagram below.