Unlike other modes of diving, which rely either on breath-hold or on breathing supplied under pressure from the surface, scuba divers carry their own source of breathing gas, usually filtered compressed air, allowing them greater freedom of movement than with an air line or diver's umbilical and longer underwater endurance than breath-hold. Scuba diving may be done recreationally or professionally in a number of applications, including scientific, military and public safety roles, but most commercial diving uses surface supplied diving equipment for main gas supply when this is practicable. Surface supplied divers may be required to carry scuba as an emergency breathing gas supply to get them to safety in the event of a failure of surface gas supply.