Željava Air Base, situated on the border between Croatia and Bosnia and Herzegovina under the Plješevica mountain, near the city of Bihać, Bosnia, was the largest underground airport and a military airbase in Yugoslavia, and one of the largest in Europe.
Situated in the east of Croatia on the border with Bosnia, 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.
Although it would enjoy the benefits of relative peace and free trade as part of the Non-Aligned Movement, Yugoslavia nevertheless made significant military-defensive investment after WWII. In 1945 the US had unleashed nuclear weapons against Japan, and three years later, in 1948, construction began on Željava Airbase. Protected by the mountain above, it was claimed this new facility could withstand a direct hit from a 20-kiloton nuclear bomb – the same force as the ‘Fat Man’ bomb that fell on Nagasaki. Tito’s government spent the equivalent of $6 billion on the project. It was one of the largest – and most expensive – military construction projects Europe had ever seen (its cost equal to roughly three times the contemporary annual military budgets of Croatia and Serbia combined). At least some of that money is said to have come from the World Bank, who believed they were investing in the construction of new motorways in Yugoslavia.
Željava Airbase was completed and operational from 1968, and the facility was to play a key role in Yugoslavia’s early warning radar network. Much like NORAD (the North American Aerospace Defense Command), or NATO’s CAOCs (Combined Air Operations Centres) in Western Europe, it would serve as a central hub for Yugoslavia’s integrated air defence programme. To this end the facility featured short-range tracking and targeting radars, and was armed with Soviet-made 2K12 ‘Kub’ mobile surface-to-air missile systems.
The airbase’s underground complex – known as the ‘KLEK’ facility – included a network totalling 3.5 kilometres of tunnels inside the mountain which could house as many as 60 aircraft. The main galleries, arranged in an ‘M’ shape, measured as much as 16 metres wide, and up to 12 metres in height. The mountain housed two fighter squadrons and one reconnaissance squadron – all originally outfitted with Soviet-built MiG-21s – in additional to their associated maintenance and refuelling facilities. The base was designed, as much as possible, to be self-sufficient. It had generators and an independent underground water source. There were crew quarters and a mess hall able to feed a thousand people, while stores contained enough food, fuel and ammunition to last 30 days should the mountain need to be sealed against the outside world. Aircraft fuel could be resupplied via a 20 kilometre underground pipeline, connected to a military warehouse near Bihać (now Bosnia & Herzegovina). The complex incorporated communication and operations centres, missile and bomb stores, weapons testing facilities and an advanced air conditioning system.
The ‘KLEK’ complex had four entrances, three of them large enough to drive aircraft through, and each protected by a 100-ton pressurised door. These opened onto five runways for take-off and landing. Additionally, the overground territory of the airbase featured 34 external buildings, including nearby barracks (located 3 kilometres from the entrance to the tunnels), as well as vehicle garages, workshops, and a radar station situated at the top of Mount Plješevica.
Around 25 buildings stand decaying and dilapidated with crumbling roofs and hazardous conditions. This base stands a forgotten mess very close to the Bosnian border. Part of the airbase also goes underground, this section has had dirt piled in front of it sloppily. However, it is in a pretty remote and overlooked area. The grounds offered 5 run ways, a hunting lodge and military police stations plus other air base buildings used for efficiently running an operation.
The impressive facility was one of the most expensive in Europe and was constructed to withstand a direct hit from a 20-kiloton nuclear bomb (comparable to the size that hit Nagasaki). The underground section housed 2 full fighter squadrons and 1 reconnaissance squadron. 100 ton doors protected each of the 4 entries to the bunker. 3 of the entrances had specialized doors for fixed wing aircraft! The underground tunnels stretched a little over 2 miles (3.5 kilometers). The underground KLEK complex was so large it had maintenance facilities and parts for planes, its own underground water source, crew quarters, power generators, a mess hall that could feed 1,000 people and other rooms of bunker and military needs.
This base was one of the largest bases in Yugoslavia and at the time of its creation, the largest in Europe. At the time of its inception it was code named Objekat 505. Styled after the mountain hangers used by the Swedish Air Force, this base began construction in 1948. It took 20 years to complete and was finished in 1968 at a cost of $6 billion. It was set to be the Yugoslav version of the North American Aerospace Defense Command (NORAD).
The bunker was built and originally stocked to be able to provide for 1,000 people complete with food, water, fuel and arms to last 30 days without resupply. During the Yugoslav wars in 1991, the base was heavily used. At the retreat of the Yugoslav People’s Army the runways were set with explosives as they did not want the base used by opposing forces. In 1992, the Serbian military detonated 56 tons of explosives there as well. The villagers of Bihac state that the explosion was so massive it shook their town and smoke billowed from the tunnels for 6 months.
As of 2019, the area is used to train canines with actual land mines by local police forces. There are still active mines in the vicinity making trips here dangerous. In 2000, a Bosnian Major died from his wounds after setting off a PROM-2 anti-personnel mine when searching for mushrooms on the base. However, this location has become a spot for illegal immigrants to use as a waypoint.
NB. Entrance 1 to the underground ‘KLEK’ complex is currently sealed. The police in recent years have recommended that curious visitors enter through Entrance 2, take a brief walk through the main galleries, and then exit the same way they went in. Entrance 4 is strictly off-limits. While on the map above this appears to still be within the borders of Croatia, the border police describe exiting the complex through Entrance 4 as an illegal border crossing – and it’s probably not worth arguing the point with them.
What happened to and where is the Zeljava Air Base of Croatia located? You can find it with these coordinates. 44.857313, 15.735133.