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, Taiwan, Vietnam, South Africa and Pakistan. In 2023, Iran unveiled a large underground base called "Eagle 44" in this base, Iran's upgraded F-4 Phantom fighters were present under Shahid Doran. Also, pictures and replicas of Su 35 were seen in that base. This base, where Iranian drones are also present, is probably somewhere in the south of Iran. While broadcasting a video of another base, on the other side of which 24 Fenser planes equipped with +2000 km cruises, Iran announced that it has more underground bases that will be unveiled in the future. Eight Israeli air force F-16I Sufa fighter aircraft were damaged in the winter of early 2020 by flooding when they were improperly left inside underground hangars in a Negev desert airbase during severe flooding. The Washington Post reported in 2012 based on the results FOIA requests to the US government that the US Army Corps of Engineers brought in US construction contractors to build installations in the south of Israel including underground hangars for fighter-bombers as a part of US military aid to Israel. Built in World War II on the Island of Pantelleria. In 1941 the Swedish Air Force began building its first underground hangar at Göta Wing (F 9), located near Gothenburg in south-west Sweden, it was commissioned in 1944. After World War II plans were made up for building underground hangars at every air force base that had suitable rock conditions.