Željava Air Base Željava Airbase , situated on the border between Croatia and Bosnia and Herzegovina under Plješevica Mountain , near the city of Bihać , Bosnia , was the largest underground airport and military airbase in the former Yugoslavia , and one of the largest in Europe . The facilities are shared by the local governments of Bihać and Lika - Senj County in Croatia . History . Construction of the Željava or Bihać Airbase , code - name d " Objekat 505 " , began in 1948 and was completed in 1968 . During those two decades , SFRJ 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 . Description . The role of the facility was to establish , integrate , and coordinate a nationwide early warning radar network in SFRJ akin to NORAD . The complex was designed and built to sustain a direct hit from a 20 - kiloton nuclear bomb , equivalent to the one dropped on Nagasaki . Strategic role . The main advantage of the base was the strategic location of its " " Celopek " " intercept and surveillance radar on Mount Pljesevica , at the nerve center of an advanced integrated air defense network covering the airspace and territory of Yugoslavia , and possibly further . In addition to its main roles as a protected radar installation , control center , and secure communications facility , the airbase contained underground tunnels housing two full fighter squadrons , one reconnaissance squadron , and associated maintenance facilities . The units based there were the 124 . LAE ( Fighter Aviation Squadron ) and 125 . LAE , both equipped with MiG - 21bis fighter aircraft , and the 352 . IAE ( Reconnaissance Aviation Squadron ) , equipped with MiG - 21R reconnaissance - fighter aircraft . The underground tunnels ran a total length of 3 . 5 kilometers , and the bunker had four entrances protected by 100 - ton pressurized doors , three of which were customized for use by fixed - wing aircraft . Eventually , it was hoped that the base would be re - equipped with the indigenously developed Yu Supersonik aircraft . Underground " KLEK " complex . The underground facility was lined with semicircular concrete shields , arranged every ten meters , to cushion the impact of incoming munitions . The complex included an underground water source , power generators , crew quarters , and other strategic military facilities . It also housed a mess hall that could feed 1 , 000 people simultaneously , along with enough food , fuel , and arms to last 30 days without resupply . Fuel was supplied by a 20 - kilometer underground pipe network that ran from a military warehouse on Pokoj Hill near Bihać . Surface . Topside , the facility had five runways . In the immediate vicinity of the base , there were numerous short - range mobile tracking and targeting radars , missile - equipped sites , 2K12 " Kub " ( NATO : SA - 6 ) mobile surface - to - air missile interceptor systems , motorized infantry bases , military police stations , and a hunting lodge used by civilian and military leaders on occasional leisure trips . Access points were heavily monitored and guards authorized to fire on anyone attempting to enter without authorization . In practice , however , only special permits were required and unauthorized visitors usually turned away . Destruction . The airbase was used intensively in 1991 , during the Yugoslav Wars . During its withdrawal , the Yugoslav People ' s Army destroyed the runway by filling pre - built spaces ( explicitly designed for the purpose ) with explosives and detonating them . To prevent any possible further use of the complex by opposing forces , the Military of Serbian Krajina completed the destruction in 1992 by setting off an additional 56 tons of explosives there . The ensuing explosion was so powerful that it shook the nearby city of Bihać . Villagers claimed that smoke continued to rise from the tunnels for six months after the explosion . Current status . Local police forces and the CPA currently use the area to train canines with actual land mines , given the extensive number of mines still in the vicinity . Because of the mines , extreme caution must be used when visiting the Željava complex . In November 2000 , a Bosnian Air Force Major died from his injuries after setting off a PROM - 2 anti - personnel mine while searching for mushrooms . The toll of the destruction on base buildings and equipment is incalculable and caused great environmental damage . Potential reconstruction endeavors are limited by a lack of financial resources . An international border cuts the base area in two , and the entire area is heavily mined . The barracks in the nearby village of Ličko Petrovo Selo are operated by the Croatian Army . Today , the base often serves as a waypoint for illegal migrants . A facility for asylum seekers was scheduled to open there in 2004 or 2005 , but the idea was abandoned , and new plans were developed for it to become part of the Slunj military training grounds , and barracks from the nearby Udbina complex . This idea was dropped , however , in line with the agreement between the countries of former Yugoslavia which bans any military facility up to 15 km inside the borders . The Bihać Municipality launched an initiative to open a local airport using the runway .