Is it ensure, insure, or assure?
A free-diving vessel used to explore the ocean at great depths. The original bathyscaphe, constructed in 1948, was made of a cylindrical metal float and a suspended steel ball that could hold two people. The float contained gasoline used to lift the vessel, and heavy iron material used for ballast. Design improvements allowed the second bathyscaphe in 1960 to descend to a record 10,912 m (35,791 ft) in the Marianas Trench in the western Pacific Ocean, almost to the deepest level ever sounded on Earth.