Željava Air Base
The world's largest underground airport was built in 1965 in present-day Bosnia-Herzegovina. The Yugoslavian government constructed this massively expensive facility to house 1000 military officials and to withstand a nuclear strike on par with Nagasaki. The facility was destroyed in conflicts stemming from the dissolution of Yugoslavia in the early 1990s and is currently a precarious place to visit given the amount of unexploded mines in its environs.