submarine trench in the floor of the western North Pacific Ocean, situated east of the Mariana Islands. It is the deepest such trench known, part of the western Pacific system of oceanic trenches coinciding with subduction zones—points where two adjacent plates collide, one descending below the other. An arcing depression, the Mariana Trench stretches for more than 1,580 miles (2,550 km) with a mean width of 43 miles (69 km). There is a smaller steep-walled valley on the floor of the main trench. In 1899 Nero Deep (31,693 feet 【9,660 metres】) was discovered southeast of Guam. This sounding was not exceeded until a 32,197-foot (9,813-metre) hole was found in the vicinity 30 years later. In 1957, during the International Geophysical Year, the Soviet research ship Vityaz sounded a new world record depth of 36,056 feet (10,990 metres). This was later increased to 36,201 feet (11,034 metres). On Jan. 23, 1960, the French-built U.S. Navy-operated bathyscaphe Trieste, with the inventor's son, Jacques Piccard, aboard, made a record dive to 35,800 feet (10,911 metres) in the trench. In 2009 the Mariana Trench was designated a U.S. national monument.