Mariana Trench, depression in the floor of the Pacific Ocean, the deepest seafloor depression in the world. It is located just east of the Mariana Islands in the western part of the ocean basin. The Mariana Trench is an arc-shaped valley extending generally northeast to southwest for 2,550 km (1,580 mi); its average width is 70 km (40 mi). The Mariana is one of many deepwater ocean trenches formed by the geologic process of subduction (see Plate Tectonics). Near its southwestern extremity, 340 km (210 mi) southwest of the island of Guam, is the deepest point on earth. This point, the Challenger Deep, is estimated to be 11,033 m (36,198 ft) deep. The Challenger Deep was named after HMS Challenger II, the vessel of those who discovered the point in 1948.
In January 1960 Swiss ocean engineer Jacques Piccard and United States Navy Lieutenant Donald Walsh descended into the Challenger Deep in the French-built, U.S. Navy-operated bathyscaphe Trieste. Piccard—whose father, Auguste Piccard, invented the bathyscaphe—and Walsh took the Trieste to a depth of 10,915 m (35,810 ft), the deepest descent in history.