Zeljava Underground Air Base. The Yugoslavian underground airbase.
One of the most expensive military project in post-war Europe, it took 20 years to build. The base could house 3 full squadron of MiG-21 and was surrounded on the surface by 5 runways.
Up to 1000 people could operate and shelter inside the base for up to 30 days in a row if needed. Zeljava was designed to withstand a direct impact from a 40 kiloton nuclear warhead. Nowadays, the base straddles the border between Croatia and Bosnia. Visiting is extremely dangerous as thousands of anti-personal mines are still present throughout the site on the surface.