Diesel submarines use a snorkel while running on the surface or at periscope depth, which allows them to take in air without exposing the entire boat. Once submerged beyond snorkel depth, diesel submarines draw in air that has been compressed into diesel fuel tanks, a system known as 'Diesel Electric Propulsion' or 'Electric Drive". During this submerged process, they rely entirely on battery-powered electric motors for propulsion and other operations onboard.