The deepest point in the sea is the Challenger Deep, located beneath the Pacific Ocean about 300 km (186 mi) southwest of Guam. According to measurements made in October 2010 by the US Navy survey ship USNS Sumner, the bottom of the Challenger Deep lies 10,994 m (36,070 ft) below sea level. Other surveys have measured roughly similar depths, with figures varying from 10,900 m (35,760 ft) (measured as part of the British Challenger II expedition in 1951) to 11,034 (36,201 ft) (measured by the Soviet research ship Vityaz in 1957).The Challenger Deep forms part of a large fault in the ocean floor called the Mariana Trench and was first discovered by the HMS Challenger – for which the feature is named – on 23 March 1875. The original depth soundings – done by lowering a weighted line into the sea – resulted in an estimated depth of 8,184 m (26,850 ft / 4,475 fathoms), which has been consistently revised down as measuring methods improve. Confusingly, the Royal Navy has had 8 ships called Challenger over the last 200 years, three of which were survey ships.