It is an extreme water vehicle that generally runs underwater is seas. A submarine is a warcraft most commonly used by the military that is used below the surface of water for operation. It is a large, cylinder shaped vehicle that is exclusively used for military puposes. They were used in WWI and WWII. Submarines also need electric power to operate the equipment on board. To supply this power, submarines are equipped with diesel engines that burn fuel and nuclear reactors that use nuclear fission.