Depths

Ocean depths are expressed in fathoms (1 fathom = 6 feet or 1.8 m) as well as in meters or feet. Large sections of the sea are 5,000 to 15,000 feet (1,524 to 4,572 m) deep, with the average near 12,500 feet (3,810 m). The floor of some ocean basins may lie 18,000 to 20,000 feet (5,486 to 6,096 m) or more beneath the surface. The greatest depths, however, occur not in the central portion of the ocean but in trenches—long, narrow, deep cracks in the ocean bottom that are usually found near continents and on the seaward side of island chains.

The greatest known depth of any ocean is in the Challenger Deep of the Mariana Trench in the Pacific Ocean, about 250 miles (400 km) southwest of the island of Guam. It was discovered in 1951 by the British survey ship Challenger, which reported a depth of 35,760 feet (10,900 m). Later expeditions from various countries have reported even greater maximum depths. Recorded echo soundings indicate a maximum depth of about 36,000 feet (10,976 m). It was in these same waters that the Trieste made its record descent in 1960.