ŽELJAVA AIRBASE, CROATIA
1948-1968 construction of the largest underground airport & military airbase, code-named Objekat 505 under Mountain Plješevica approximately costing $6 billion (x3 current annual military budgets of Croatia & Serbia together.
4 x 100 ton pressurised doored entrances. The bunker was built to withstand a direct hit of a 20-kilotonnuclear bomb.
1992 56 tons of explosives were set off to prevent further opposing forces possible use of the airbase