A nuclear submarine is an underwater vessel powered by a nuclear reactor. The great advantage of nuclear submarines is that they use little fuel and may remain submerged for many weeks, even months. Heat produced by the reactor turns water into steam. The steam turns turbines, which are linked to electricity generators that charge the submarine's batteries. The vessel's propeller may be driven by an electric motor, or by linking its shaft to a turbine. The first nuclear submarine, the USS Nautilus, was launched by the US Navy in 1954. Nautilus travelled 100,000 kilometers (62,000 miles) in its first two years of service, and in 1958 was the first vessel to sail under the ice cap at the North Pole. Britain's first nuclear submarine was HMS Dreadnaught (launched 1963). Nuclear submarines carrying missiles have enormous firepower and are difficult to detect and destroy.