Beneath the surface of the sea; SYN. undersea.

A submersible warship usually armed with torpedos; SYN. pigboat, sub, U-boat.
Vessel capable of traveling and functioning under water, used in research and military operations. The first underwater boat was constructed for James I of England by the Dutch scientist Cornelius van Drebbel (1572–1633) in 1620. In the 1760s, the American David Bushnell (1742–1824) designed a submarine called Turtle for attacking British ships, and in 1800, Robert Fulton designed a submarine called Nautilus for Napoleon for the same purpose. John P Holland, an Irish emigrant to the US, designed a submarine about 1875, which was used by both the US and the British navies at the turn of the century. A naval submarine, or submersible torpedo boat, the Gymnote, was launched by France 1888. The conventional submarine of World War I was driven by diesel engine on the surface and by battery-powered electric motors underwater. The diesel engine also drove a generator that produced electricity to charge the batteries. Submarine warfare was really established as a distinct form of naval tactics during World War I and
submarines, from the oceangoing to the midget type, played a vital role in both world wars. In particular, German U-boats caused great difficulty to Allied merchant shipping. The first nuclear-powered submarine, the Nautilus, was launched by the US 1954. In oceanography, salvage, and pipe-laying, smaller submarines called submersibles are used.