Nuclear-powered submarines can stay submerged for long periods of time. They are designed and manned to stay underwater long enough to support a wide variety of missions, which can last for several months. Submarines have equipment to make oxygen and keep the air safe. Food and supplies are the only limitations on submergence time for a nuclear submarine. Normally, submarines carry a 90-day supply of food. Historically, diesel-powered submarines operated internal-combustion, air-breathing engines on the surface or just below the surface by using a snorkel mast (snorkeling). When completely submerged, a diesel-powered submarine uses its battery power and electric motors for propulsion. Depending on speed and other battery use, the submarine could stay underwater for up to several days before recharging batteries and exchanging stale air for fresh air.