A bathyscaph (or bathyscaphe or bathyscape) is a special kind of submarine capable of diving to great depths.
It's operating principle is like an underwater dirigible. It consists of a large tank filled with oil which, being lighter than water, provides lift, like the gas in a dirigible bag. A thick-walled steel sphere is attached to the bottom of the hull, protecting the crew and equipment. Ballast is attached magnetically to the hull of the ship. This provides for a fail-safe mechanism: the ballast would be released were the power to fail, causing the ship to ascend to the surface. The craft can go up by dropping ballast, and down by releasing oil. It has two small battery-driven propellers for horizontal movement.
The term "bathyscaph" is a composite of "bathy" = deep (Greek: bathos) and "scaph" = ship (Greek: skaphos).
The Swiss scientist Auguste Piccard built the first bathyscaph, the Trieste, in 1953. She lacked manipulators or samplers and was painfully slow. However, she proved excellent for underwater observation. She was purchased by the US Navy in 1958, under the sponsorship of the Office of Naval Research.
On the 27th January 1960 the Trieste dived down into the Marianas Trench, to a depth of 10'917 metres, setting the still unbroken record for the deepest dive ever. A record unlikely ever to be broken, considering that the Marianas Trench is the deepest spot in the ocean. Nowadays, there are no submarines left capable of diving that deep. The deepest the US Navy can go now is 6000 metres, using the DSV Sea Cliff.
The US Navy built two more bathyscaphs. One of them was the Trieste II. The other, also called "Trieste", was built by a highly classified program to develop a sophisticated bathyscaph capable of clandestine operations in the deep ocean. Unfortunately, this operation still is classified, so I could not find coherent information about the program.
If you want to know more about this murky story, I suggest reading "Blind Man's Bluff: The Untold Story of American Submarine Espionage", by Christopher Drew and Sherry Sontag and "The Universe Below", by William J. Broad. Some information can also be found at http://pw1.netcom.com/~sjplaw/.