There are many types of submersible craft, and each are designed for different uses. Wet submersibles and tourist submarines give amateurs underwater access, while pressurized submersibles and remotely operated vehicles are used by professionals for research and exploration.
Wet submersibles are relatively simple vehicles that are mostly propelled by battery-powered electric motors. They are used at shallow depths to carry scuba divers. Divers using wet submersibles can cover area more easily than by swimming. Types of wet submersibles vary; some are shaped like sleds, while others are steered much like underwater bicycles or jet skis. All wet submersibles are open to the sea, which limits their operating depth to 76m (250 ft), the depth to which scuba divers can descend with compressed air tanks. Some wet submersibles contain a small Plexiglas dome, filled with air, that covers just the userís head, so extra breathing tanks are not needed. Other styles of wet submersibles require the user to carry an oxygen supply, such as a scuba tank. See also Diving (underwater).
Pressurized submersibles are piloted mini submarines designed to withstand the crushing pressure of deep dives. They are used for research and salvage. The earliest submersible was the bathysphere, a simple spherical chamber lowered by a cable from a ship and used for deep-ocean exploration. Contemporary pressurized submersibles have ballast tanks for diving and surfacing, small propellers called thrusters for maneuvering, and mounts that can be fitted with cameras and other research equipment. These craft carry their own supplies of breathing air, and the hulls of some can withstand pressure at undersea depths as great as 6,000 m (20,000 ft). The U.S. Navy operates mini-subs such as the nuclear-powered NR-1 mini-sub, which was used to locate wreckage from the U.S. space shuttle Challenger disaster. The Navy also maintains the deep submergence rescue vehicle (DSRV) Mystic, which is used for submarine rescues. It can descend to a depth of several hundred meters (its exact capability is classified).
Scientists use deep-sea submersibles, or deep submergence vehicles (DSVs) for a wide variety of scientific and search and recovery operations. The U.S. Navy built Sea Cliff in the late 1960s for deep-water search-and-retrieval tasks. The Sea Cliff weighed 26 metric tons, could hold three people, and could dive to 6,000 m (20,000 ft). The Navy decommissioned Sea Cliff in 1998.
The U.S. Navy and the Central Intelligence Agency (CIA) have also developed systems for retrieving large objects from the ocean floor. In 1968 the American submarine USS Halibut explored a sunken submarine that belonged to the Union of Soviet Socialist Republics (USSR). Several years later the research ship Glomar Explorer attempted to raise the entire hull of the sunken sub using a massive clawlike device deployed from within a vast compartment in the center of the 188 m (618 ft) ship.
Towed submersibles have been a mainstay of deep-ocean exploration and salvage for many years. They are large, pilotless platforms loaded with instruments. Towed submersibles do not have thrusters. They are submerged to a few feet above the ocean floor, and simply pulled behind a surface ship. The U.S. Navy located the wrecks of the submarines USS Thresher and USS Scorpion during the 1960s by trolling a submersible mounted with cameras, sonar, and magnetic detectors over the general area in which the subs sank. The wreck of the Titanic was located in 1985 by a towed submersible. The Titanic was later explored in Alvin by oceanographer Robert Ballard, with the use of a prototype remote camera called Jason Jr., which was tethered to Alvin.
Remotely operated vehicles, or ROVs, are small, maneuverable craft that are piloted by remote control. Most ROVs are tethered to a surface ship by a cable that is used to hoist and lower the vehicle, but some can be tethered to piloted submersibles as well. The cable connecting the ROV to the mother ship also transmits electrical power, instructions from the pilot, and research information. These highly maneuverable remote craft are fitted with thrusters, video and still cameras, lights, and sensors. Some ROVs have small remote arms that can grasp and retrieve objects.