The Bathyscaphe Trieste is a deep sea diving boat. It is the only manned underwater craft to visit the Challenger Deep, the deepest part of the Mariana trench. This Swiss built vessel was purchased in 1958 by the U.S. Navy for deep sea exploration. The upper part of the craft is the ballast tank, a combination of water, gasoline and iron shot were used to control the buoyancy. The smaller round chamber underneath the ballast tank is the pressure sphere from where the crew of two controlled and observed. On January 23, 1960 the Trieste reached the deepest part of the ocean, the Challenger Deep, at a depth of 35,761 feet.