If you want to hide and protect something, it is best to bury it in the mountain. Of course, if you have enough money for that.
On the border between Croatia and Bosnia and Herzegovina, deep below Mount Plješivica, the Yugoslav People’s Army built an air base one of the largest in Europe. Codenamed O-505, Željava was completed in 1968 after 20 years of construction and $ 6 billion invested.
Contrary to today’s appearance, when it was built, the base was an impressive building. It could withstand blast from an atomic bomb like one that hit Nagasaki. It had four entrances, each protected by a steel door weighing 100 tons. In 3.5 kilometers of tunnels and rooms, it could accommodate 58 planes and 1,000 people. The soldiers who were housed in it had everything they needed to survive for 30 days. Its own diesel power plant with two auxiliary generators provided the electricity needed to operate the machines at the base, and was supplied from a warehouse on the hill via a 10-kilometer-long pipeline.
Electric tugs deployed planes around the base. There were also dormitories, a hospital, warehouses for jet fuel and ammunition, video surveillance, 13 evacuation exits to the surface, everything protected by 56 armored doors.
There was a communication station, 5 airfields and a large number of well-camouflaged antennas for communication and early warning on the surface.
Unfortunately, during the Homeland War, the base was desrtroyed by explosives, and what was not destroyed by the Yugoslav army was looted afterwards.