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A Japan Maritime Self-Defense Force Oyashio-class submarine in 2006 A submarine is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term submarine most commonly refers to a large crewed autonomous vessel. However, historically or colloquially, submarine can also refer to medium-sized or smaller vessels (midget submarines, wet subs), remotely operated vehicles or robots.

The adjective submarine, in terms such as submarine cable, means "under the sea". The noun submarine evolved as a shortened form of submarine boat (and is often further shortened to sub).[citation needed] Submarines are usually referred to as "boats" rather than as "ships", regardless of their size.

Although experimental submarines had been built before, submarine design took off during the 19th century, and they were adopted by several different navies. Submarines were first widely used during World War I (1914–1918) and now figure in many large navies. Military usage includes attacking enemy surface ships or submarines, aircraft carrier protection, blockade running, ballistic missile submarines as part of a nuclear strike force, reconnaissance, conventional land attack (for example using a cruise missile), and covert insertion of special forces. Civilian uses for submarines include marine science, salvage, exploration and facility inspection/maintenance. Submarines can also be modified to perform more specialized functions such as search-and-rescue missions or undersea cable repair. Submarines are also used in tourism, and for undersea archaeology.

Most large submarines consist of a cylindrical body with hemispherical (and/or conical) ends and a vertical structure, usually located amidships, which houses communications and sensing devices as well as periscopes. In modern submarines this structure is the "sail" in American usage, and "fin" in European usage. A "conning tower" was a feature of earlier designs: a separate pressure hull above the main body of the boat that allowed the use of shorter periscopes. There is a propeller (or pump jet) at the rear and various hydrodynamic control fins as well as ballast tanks. Smaller, deep diving and specialty submarines may deviate significantly from this traditional layout.

Submarines have one of the largest ranges of capabilities in any vessel, ranging from small autonomous examples to one- or two-person vessels operating for a few hours, to vessels which can remain submerged for 6 months such as the Russian Typhoon class - the biggest submarines ever built and in use. Submarines can work at greater depths than are survivable or practical for human divers. Modern deep diving submarines are derived from the bathyscaphe, which in turn was an evolution of the diving bell.