Zeljava Air Base

Željava Air Base, situated on the border between Croatia and Bosnia and Herzegovina under the Gola Plješevica mountain, near the city of Bihać, was the largest underground hangar and military air base in the Socialist Federal Republic of Yugoslavia, and one of the largest in Europe. The facilities are shared by the local governments of Bihać and Lika-Senj County of Croatia.

Source: Wikipedia