A deep-submergence vehicle (DSV) is a deep-diving crewed submersible that is self-propelled. Several navies operate vehicles that can be accurately described as DSVs. DSVs are commonly divided into two types: research DSVs, which are used for exploration and surveying, and DSRVs (Deep Submergence Rescue Vehicle), which are intended to be used for rescuing the crew of a sunken navy submarine, clandestine (espionage) missions (primarily installing wiretaps on undersea communications cables), or both. DSRVs are equipped with docking chambers to allow personnel ingress and egress via a manhole. Strictly speaking, bathyscaphes are not submarines because they have minimal mobility and are built like a balloon, using a habitable spherical pressure vessel hung under a liquid hydrocarbon filled float drum. In a DSV/DSRV, the passenger compartment and the ballast tank functionality is incorporated into a single structure to afford more habitable space (up to 24 people in the case of a DSRV). Most DSV/DSRV vehicles are powered by traditional electric battery propulsion and have very limited endurance, while few (like NR-1 or AS-12/31) are/were nuclear-powered, and could sustain much longer missions. Plans have been made to equip DSVs with LOX Stirling engines but none have been realized so far due to cost and maintenance considerations. All DSVs to date (2023) are dependent upon a surface support ship or a mother submarine, that can piggyback or tow them (in case of the NR-1) to the scene of operations. Some DSRV vessels are air transportable in very large military cargo planes to speed up deployment in case of emergency rescue missions. List of deep submergence vehicles Trieste-class bathyscaphes FNRS-2 The first bathyscaphe, developed by Swiss engineer Auguste Piccard and named after the Belgian Fonds National de la Recherche Scientifique (FNRS), the funding organization for the venture. FNRS also funded the FNRS-1, a balloon that set a world altitude record, also built by Piccard. This bathyscaphe consisted of a gasoline filled float, 22 feet long and 10 feet wide, and oval shaped. Gasoline being less dense than water, would provide buoyancy for when the bathyscaphe needed to rise to the surface. The ovular float was divided into six tanks for holding gasoline, having a combined total of 6,600 gallons. FNRS-3 After damage to the FNRS-2 during its sea trials in 1948, the FNRS ran out of funding and the submersible was sold to the French Navy in 1950. It was subsequently substantially rebuilt and improved