An underground hangar is a type of hangar for military aircraft, usually dug into the side of a mountain for protection. It is bigger and more protected than a hardened aircraft shelter (HAS).
An underground hangar complex may include tunnels containing the normal elements of a military air base—fuel storage, weapon storage, rooms for maintaining the aircraft systems, a communications centre, briefing rooms, kitchen, dining rooms, sleeping areas and generators for electrical power.
Countries that have used underground hangars include Albania, China, Italy, North Korea, Norway, Sweden, Switzerland, Taiwan, Yugoslavia, Iran, Vietnam, South Africa and Pakistan.