Scuba diving is a mode of underwater diving in which a diver uses a self-contained underwater breathing apparatus (scuba) to breathe underwater. Unlike other modes of diving, which rely either on breath-hold or on breathing gas pumped from the surface, scuba divers carry their own source of breathing gas, usually compressed air, allowing them greater freedom of movement than with an air line or diver's umbilical and longer underwater endurance than breath-hold.