bathyscaphe

batyskooppi, syvänmeren tutkimusalus, batyskafi A self-propelled deep-sea diving submersible for exploring the ocean depths, consisting of a crew cabin similar to a bathysphere suspended below a float filled with a buoyant liquid such as petrol.
1972, Oceanology, (w), volume 12, issues 4–6, https://books.google.com.au/books?id=BCNPAQAAIAAJ&q=%22bathyscaphe%22%7C%22bathyscaphes%22&dq=%22bathyscaphe%22%7C%22bathyscaphes%22&hl=en&sa=X&redir_esc=y page 931:

The fact that the bathyscaphes are self-propelled makes it possible to use them to study the ocean microstructure in the horizontal, although the experiment should include temporal changes, which can be achieved either by using associated buoy stations or using a second suspended bathyscaphe.

It's ironic that when Piccard built his first bathyscaphe in the 1940s people assumed the revolutionary design had evolved from his famous stratospheric balloon. In fact, the reverse was true. In fact, Piccard's prototype bathyscaphe FNRS-2 (named for the Belgian national research foundation), which underwent sea trials in the Atlantic off Senegal in 1948, did possess all the attributes of a clumsy underwater balloon.

After World War II, engineers built specialized submarines called bathyscaphes to survive the intense pressures of the deep ocean. A pioneer in bathyscaphes was the Swiss-born Belgian physicist Auguste Piccard (1884–1962), who had set the world altitude record in a balloon in 1932. Having turned his attention to the sea, Piccard and his son, Jacques Piccard (1922– ), built a bathyscaphe called the Trieste after World War II.