Zeljava Airbase was the largest underground airport in Yugoslavia

Located on the border of Croatia and Bosnia and Herzegovina, Željava Airbase was the largest underground airport and military airbase in the former Yugoslavia, and one of the largest in Europe. 

Željava Airbase

Was Objekat 505 the largest underground airport in the Balkans?

Today extreme caution must be used when visiting the Željava Airfield Complex, in view of the extensive number of landmines on and around the former air base. Situated in the west of Bosnia on the border with Croatia, Objekat 505, as it was officially known, was the largest underground airport in the Balkans. The primary purpose of the Objekat 505 was to house a long-range radar early warning system, akin to NORAD, as well as to provide a strategic command center for the country's defense. It could house two hunter squadrons and one thousand people, including military commanders.

When was Željava Airbase built?

Construction of the base was conducted in utmost secrecy between 1957 and 1965, and the cost of its construction was a whooping 6 billion dollars, three times more than the combined yearly military budgets of Yugoslavia's two biggest successor states, Serbia and Croatia. The secret airbase was positioned at the center of a dense sprawling network of military installations, with five auxiliary airfields nearby as well as numerous radar and air defense outposts. The airbase was used intensively in 1991, during the Yugoslav war. A rotting Douglas C-47 stands at the entrance, a curious image in itself. 

Is the access to Željava Airbase dangerous?

Situated under the Plješevica Mountain, access to this base that today remains as an eerily desolate place, is extremely dangerous. Local police and the CPA use the area to train dogs with the use of actual landmines due to the enormous amounts of mines that exist within the complex area. Extreme caution needs to be used when approaching this site, due to the large number of unexploded landmines and other munitions there. Though technically accessible, visiting may require a permit from local police to avoid suspicion. Entering the field is still at your own risk (mines!) and entering the underground facility is possible, but downright life threatening. Any attempt to enter into the underground complex carries 5 basic risks: radiation, mines, unexploded munitions, gases and landslides.
Pic: http://static.panoramio.com/photos/large/104470734.jpg