Diesel-powered submarines are sometimes referred to as "diving ships", since they spent most of their time on the surface and submerged only just prior to approaching their targets, or in an emergency. On top of that, their undarwater engines ran on batteries and so were required to be very economic, which meant they were unable of generating high speeds. Nuclear energy promised to break out of those limitations. By 1955, the US built the world's first nuclear submarine, "USS Nautilus", named after the submarine from the 19th century Verne's fiction, "20,000 Leagues Under The Sea". It soon proved its worth by sailing under the North Pole. Nuclear reactor powering this kind of submarine not only allowed it to attain high speeds underwater, but also made it independent from air; the oxygen for the crew could be recycled from carbon dioxide, using vast energy surplus. There was no need to refuel either - uranium stored in the reactor was more than enough to last through entire lifetime of a ship. The time that could be spent submerged was only limited by food supplies and psychological endurance of the crew. With those qualities it was only natural that the concept of a submarine armed with nuclear missiles apperared. It could sail in the depths of the ocean, unknown to the world and thus indestructible, and either attack enemy territory at a short range giving no time to react, or retaliate even afer its home country was completely destroyed. First submarine of this type, "USS George Washington", was commissioned in 1959. The other superpower of the time, the USSR, soon followed with their own nuclear-powered and nuclear-armed submarines.