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).
A Mirage IIIRS in front of an aircraft cavern in Buochs Airport, Switzerland
Sweden
A Mirage IIIRS being towed out of a cavern on Buochs Airport, Switzerland (1999)
A hangar is a building or structure designed to hold aircraft or spacecraft. Hangars are built of metal, wood, or concrete. The word hangar comes from Middle French hanghart, of Germanic origin, from Frankish *haimgard, from *haim and gard ("yard"). The term, gard, comes from the Old Norse garðr.
An outside view of a hangar in a Royal Air Force base
The Wright Flyer outside the aircraft's makeshift hangar
Carl Richard Nyberg's hangar for his Flugan (fly) from 1908, Täcka udden in Lidingö, Sweden
Six helium-filled blimps stored in one of the two hangars at the former US Marine Corps Air Station Tustin