Oceanic trench the level of the surrounding oceanic floor. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of below sea level. Oceanic lithosphere moves into trenches at a global rate of about 3 km/yr. There are approximately of convergent plate margins, mostly around the Pacific Ocean—the reason for the reference “Pacific-type” margin—but they are also in the eastern Indian Ocean, with relatively short convergent margin segments in the Atlantic Ocean and in the Mediterranean Sea. Globally, there are over 50 major ocean trenches covering an area of 1.9 million km or about 0.5%
Mariana Trench during the "Challenger" expedition in 1875, using a weighted rope, which recorded a depth of . In 1877, a map was published called "Tiefenkarte des Grossen Ozeans" ("Deep map of the Great Ocean") by Petermann, which showed a "Challenger Tief" ("Challenger deep") at the location of that sounding. In 1899, USS Nero, a converted collier, recorded a depth of . In 1951, "Challenger II" surveyed the trench using echo sounding, a much more precise and vastly easier way to measure depth than the sounding equipment and drag lines used in the original expedition. During this survey, the deepest part of