Marianas trench, Marianas trough, or Marianas deep mâr˝ēăn´əz [key], elongated depression on the Pacific Ocean floor, 210 mi (338 km) SW of Guam. It is the deepest known depression on the earth's surface, having been measured by various means at 35,760–36,089 ft (10,900–11,000 m) below sea level at Challenger Deep. The Trieste, a U.S. navy bathyscape crewed by Swiss oceanographyer Jacques Piccard and U.S. Navy Lieutenant Donald Walsh, reached its bottom in 1960. A 1995 Japanese probe made what is probably the most accurate measurement of its depth (35,798.6 ft/10,911.5 m), and in 2009 a U.S. robotic vessel explored the seafloor of Challenger Deep. In 2012 James Cameron became the third person to descend to the Deep, in the submersible Deepsea Challenger. The
Marianas Trench Marine National Monument, 95,216 sq mi (246,608 sq km), was established in 2009 to protect the submerged lands and waters around the trench and around the three northernmost Mariana Islands and 21 sites with undersea mud volcanoes and hydrothermal vents.
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