A bathyscape, bathyscaphe, or bathyscaph is a free-diving self-propelled deep-sea diving submersible, consisting of a crew cabin similar to a bathysphere suspended below a float (rather than from a surface cable, as in the classic bathysphere design).
The float is filled with gasoline because this is readily available, buoyant, and for all practical purposes, incompressible. The incompressibility of the gasoline means the tanks can be very lightly constructed as the pressure inside and outside of the tanks equalises and they are not required to withstand any pressure differential at all. By contrast the crew cabin must withstand a huge pressure differential and is massively built. Buoyancy can be trimmed easily by replacing gasoline with water, which is denser.
To descend, a bathyscaphe floods air tanks with sea water but unlike a submarine the water in the flooded tanks cannot be displaced with compressed air to ascend because the water pressures at the depths for which the craft was designed to operate are too great. For example, the pressure at the bottom of the Challenger Deep is more than seven times that in a standard "H-type" compressed gas cylinder. Instead, ballast in the form of iron shot is released to ascend, the shot being lost to the ocean floor. The iron shot containers are in the form of one or more hoppers which are open at the bottom throughout the dive, the iron shot being held in place by an electromagnet at the neck. This is a fail-safe device as it requires no power to ascend; in fact, in the event of a power failure, shot runs out by gravity and ascent is automatic.
The first bathyscaphe was dubbed FNRS-2, named after the Fonds National de la Recherche Scientifique, and built in Belgium between 1946-48 by Piccard. Propulsion was provided by battery-driven electric motors.
Piccard's second bathyscaphe was Trieste, which was purchased by the United States Navy in 1957. It had two water ballast tanks and eleven buoyancy tanks holding 120,000 litres of gasoline.In 1960 Trieste, carrying Piccard's son Jacques Piccard and Lt. Don Walsh, reached the deepest point on the earth's surface, the Challenger Deep, in the Mariana Trench. As of 2008 the two remain the only people to reach this extreme depth. No manned vessel has ever repeated this feat. In 1995, the Japanese sent an unmanned submersible to this depth, Kaikō, but it was later lost at sea. Today there are no submersibles that can reach this depth.
The onboard systems indicated a depth of 37,800 ft (11,521 m) but this was later corrected to 35,813 ft (10,916 m) by taking into account variations arising from salinity and temperature. Later and more accurate measurements made in 1995 have found the Challenger Deep to be shallower at 35,798 ft (10,911 m).
The bathyscaphe was equipped with a powerful light, which illuminated a small flounder-like fish, putting to rest the question of whether or not there was life at such a depth in the complete absence of light.