Željava Air Base, situated on the border between Croatia and Bosnia and Herzegovina under the mountain, near the city of Bihać, was the largest underground airport and military air base in the Socialist Federal Republic of Yugoslavia (SFRY), and one of the largest in Europe. The facilities are shared by the local governments of Bihać and Lika-Senj County of Croatia. History The "Objekat" series of military installations can be found in secluded but strategically important areas within the former Yugoslavia, and the construction of these military bases were initialized by the SFRY's defence ministry. During the Yugoslav Wars, most of these bases were used by the Serbs in certain operations, but due to extenuating circumstances imposed by the Croat forces, they were destroyed and later rendered useless for military use. Nowadays, they are popular for urban exploration, although it is risky due to the possibility of anti-personnel landmines being located in unexplored areas. However, some were spared the destruction due to other circumstances where the Serbs did not control these installations. An example of some that are still in use is the D-0 Armijska Ratna Komanda nuclear bunker in Konjic, however the latter was turned into an art complex, but is still owned by the Ministry of Defence of Bosnia and Herzegovina. The construction of the Željava or Bihać Air Base (code named "Objekat 505") was inspired by mountain hangars used by the Swedish Göta Wing (F 9), began in 1948 and was completed in 1968. During those two decades, SFRY spent approximately $6 billion on its construction, three times the combined current annual military budgets of Serbia and Croatia. It was one of the largest and most expensive military construction projects in Europe. The airfield was documented by American CORONA satellite reconnaissance in 1968. Description The role of the facility was to establish, integrate and coordinate a nationwide early warning radar network in SFRY akin to NORAD in the US. The complex was designed and built to sustain a direct hit from a 20-kiloton nuclear bomb, equivalent to that dropped on Nagasaki. Strategic role The main advantage of the base was the strategic location of its "Celopek" intercept and surveillance radar on Mount , at the nerve center of an advanced integrated air defence network covering the airspace and territory of Yugoslavia, and possibly further. In addition to its main roles as a protected radar installation, control centre, and