The Mariana Trench is the deepest part of the world’s oceans, and the lowest elevation of the surface of the Earth’s crust. It is located in the western Pacific Ocean, to the east of the Mariana Islands. The trench is about 2,550 kilometres (1,580 mi) long but has a mean width of only 69 kilometres (43 mi). It reaches a maximum-known depth of about 10.91 kilometres (6.78 mi) at the Challenger Deep, a small slot-shaped valley in its floor, at its southern end, although some unrepeated measurements place the deepest portion at 11.03 kilometres (6.85 mi). If Mount Everest, the highest mountain on Earth at 8,848 metres (29,029 ft), was set in the deepest part of the Mariana Trench, there would be 2,076 metres (6,811 ft) of water left above it.
The Mariana Trench is part of the Izu-Bonin-Mariana Arc geological boundary system that forms the boundary between two tectonic plates. In this system, the western edge of one plate, the Pacific Plate, is subducted beneath the smaller Mariana Plate that lies to the west. Because the Pacific plate is the largest of all the tectonic plates on Earth, crustal material at its western edge has had a long time since formation (up to 170 million years) to compact and become very dense; hence its great height-difference relative to the higher-riding Mariana Plate, at the point where the Pacific Plate crust is subducted. This deep area is the Mariana Trench proper. The movement of these plates is also responsible for the formation of the Mariana Islands.
At the bottom of the trench, where the plates meet, the water column above exerts a pressure of 1,086 bars (15,750 psi), over one thousand times the standard atmospheric pressure at sea level.
Because the Earth is not a perfect sphere, the trench is not the part of the seafloor closest to the center of the Earth – parts of the Arctic Ocean seabed are at least 13,000 metres (43,000 ft) closer to the Earth’s center than the Challenger Deep seafloor.