The ghosts of Yugoslavia still rise tall over the various nations that now take its place. These landscapes – some of the most scenic in Europe – lay scattered today with monuments, tower blocks, bunkers and tunnels, all built for the needs and design of a completely different era. Some have found new purpose, but others, as orphans, have since fallen into grievous states of disrepair. Though of all the Yugoslav ghosts that haunt the Balkans, there is perhaps nothing else so grand, so ambitious (and ultimately then, so wasteful), as the ruins of the subterranean Željava Airbase.

Located now on the border between Croatia and Bosnia & Herzegovina, Željava Airbase was a Yugoslav military installation incorporating hangars, barracks, and an advanced communications centre designed to serve as a Cold War-era ballistic missile early warning system. Most contemporary first- and second-world countries had something similar… but what made Željava unique, was its location. In an effort to render the base virtually indestructible, it was built inside a hollowed-out mountain.

What follows is a brief history of Yugoslavia’s Željava Airbase – from creation to destruction – as well as my own observations from six trips into these cavernous mountain tunnels, and some advice on how you might go about visiting for yourself.

Background

The Cold War period was marked not only by visible, external competition – the arms race, the space race – but also, hidden beneath the earth’s surface, it heralded a new mania for tunnel building. In Greenland, the Americans developed a secret plan (codenamed ‘Project Iceworm’) to build a huge network of tunnels, with missile launch facilities hidden under the ice and located within striking range of the Soviet Union. Meanwhile, in the east, the Soviets constructed some of the largest underground complexes ever built. In the 1950s they hollowed out Mount Tavros, on the Crimean coast, to create a series of aquatic tunnels known as Object 825: the Balaklava Nuclear Submarine Base. A little further inland, in the 1970s, they would build an even larger complex inside Mount Mishen. This installation was called Object 221, and though never completed, it was designed as a self-sustainable military headquarters for the Soviet Red Army, featuring a satellite communications centre and powered by its own nuclear reactor inside the heart of the mountain. Sheltered beneath 180 metres of solid rock, this bomb-proof back-up command would have guaranteed the Soviets second-strike capabilities, should Moscow ever have been targeted by nuclear weapons.

Putting a military base inside a mountain made it practically indestructible… and it wasn’t a new idea. Around the turn of the 20th century, the Germans who occupied Qingdao (then ‘Tsingtao’) in China, fearing naval assault on the city from the British fleet harboured at Hong Kong, built a defensive complex inside a mountain that overlooked the sea. The German army blasted tunnels into Fushan (or Mount Fu) to create a warren of concrete-lined passages linking heavy gun placements, ammo stores, mess halls and dormitories, and culminating in pillbox-style lookout points dotted along the top of Dragonback Ridge. Mountains could also be used to prevent things from getting out; in the 1980s, the Americans would build a maximum security nuclear waste repository inside Yucca Mountain, Nevada.

However the Americans and the Soviets were not the only Cold War-era powers to construct maximum-security bomb-proof facilities inside mountains. Positioned between the two, both geographically and politically, the Socialist Federal Republic of Yugoslavia was no less enthusiastic for tunnels, bunkers and underground bases; with perhaps their most ambitious military project being built inside Mount Plješevica, on the eastern edge of the Socialist Republic of Croatia. Officially they called this place Object 505, though it is better known today as the Željava Airbase.

Željava Airbase in Use

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.

Destruction

The airbase was effectively destroyed during the Croatian War of Independence (1991-95). Croatia, under Franjo Tuđman, had decided to follow Slovenia’s lead and break away from Yugoslavia. However it was not an amicable split. Enclaves of ethnic Serbs lived scattered throughout Croatia, making it impossible to draw any kind of mutually-satisfactory border between the two countries; and Slobodan Milošević, Yugoslavia’s new Serb leader in Belgrade, quickly mobilised the JNA (Yugoslav People’s Army) against Croatia in an attempt to maintain his power. JNA warships sailed along the Dalmatian Coast in 1991 and laid siege to Dubrovnik, as well as utterly destroying the nearby exclusive resort at Kupari. From Željava Airbase, the JNA sent fighter jets against their former compatriots.

In April 1992 the tide of war turned however, and the JNA was forced to pull back from Croatia. Rather than abandon a fully operational airbase to the Croatian Army, they destroyed the runways with explosive charges as they left. The airbase fell under the control of the short-lived state of Serbian Krajina after that, through which Milošević was still able to exert some power in the region. But when their grip subsequently began to weaken, it was the Serbian Krajina military who, on 16 May 1992, resolved to completely destroy Željava Airbase. More than 56 tons of explosives were used and the six-billion-dollar airbase was devastated by the blasts, consumed with internal fires. It was reported that locals living in the nearby city of Bihać felt the earth shake, and could still see plumes of smoke coming out of the tunnels six months later.

Inside Object 505

The first time I visited Željava Airbase was in the spring of 2016. Following coordinates I had found online, we drove close to the runways before steering our hire car off the road and parking it behind bushes. We walked the rest of the way to the entrance. The base is located right on the border between Croatia and Bosnia & Herzegovina (a border which also marks the outer perimeter of the European Union), and with tunnels that exit the mountain into two different countries, the object is now subject to regular patrols by border police. Even as we watched the road ahead for police cars though, signs at the roadside warned of unexploded mines in the undergrowth beyond. This felt like a real, high-stakes infiltration mission… though later I would learn that we needn’t have gone to so much effort.

On the Croatian side of the border, police have become so accustomed to tourists that they have given up trying to keep people out of Željava. Instead they simply ask that visitors announce themselves first to police at the border crossing. Would-be visitors are instructed to keep to the main gallery inside the base, and stay out of the smaller tunnels and halls, many of which are in the process of collapsing; to avoid rubble, bushes, or any other place where leftover landmines might lurk; and on no account to exit the tunnels into a different country. Since 2016 I have been back to Željava five more times, on numerous occasions leading tour groups around the abandoned military complex. We did so with police approval every time, and only on one occasion did an older border guard attempt to extract a bribe from me for the privilege. Mostly, the police just want to know how many people are inside the tunnels at any given time… in case they ever need to go looking for them.

But we didn’t know any of that back in 2016. So we needlessly snuck past the patrols, and reaching one of the entrances to the ‘KLEK’ complex, scrambled quickly up and over the mound of dirt that had been bulldozed to block the mouth of the tunnel. What waited beyond was a yawning expanse of blackness.

Željava eats light. The three of us were armed with headlamps, strong enough to light the rubble-scattered floor around us; but they barely touched the side walls that arched invisibly up and over us, let alone illuminating the passage ahead. Most of the time we could sense – but not see – the enormity of this space. The first interior blast door, shaped for the profile of a plane, had been badly damaged by explosives, and twisted rebar flayed outwards from obliterated concrete. When I tried to photograph it though, setting my camera on a tripod with a sixty-second exposure, our three torches combined were still only able to give a suggestion of the shape. Over the following years I would go back better equipped. I took stronger lights, and more people too; twelve headlamps walking in procession, instead of three. (Though with tour groups I would always be slightly paranoid: turning every 90 seconds to make a headcount of the bobbing lamps behind me.)

The layout of the complex is actually fairly simple… but it doesn’t feel it, when you’re inside. The main passages are long and straight, but they seem to meet and feed into one another at irregular angles. In the darkness, you can walk straight past turnings or other landmarks (graffiti tags, car tyres, a burnt-out truck) without noticing them; then, when you run straight into these same objects on your way back, it makes you think you’re in an altogether different tunnel.

At one point I think we accidentally crossed into Bosnia & Herzegovina. At the far end of one of the main galleries we turned a corner to see a welcoming semi-circle of daylight – Entrance 4 – and almost immediately, our phones began to chirp with incoming text messages, welcoming us to the Bosnian Federation. We stayed to enjoy the fresh air a few moments longer, though we didn’t want to step outside, and risk running into Bosnian border police.

On that first trip, I hadn’t yet seen a map of the complex – so I made my own, sketching out lines for passageways under the mountain. We spent the entire afternoon in there. While most features of the ‘KLEK’ facility were thoroughly destroyed in 1992, some details remained. The security offices inside the main Croatian entrance rose as a cube of concrete and rust-red metal, a building inside the tunnel. The mess hall was almost completely lost to a cave-in, but one side passage, near Entrance 3, led to a series of generator rooms where rusted winches dangled on chains from multi-level gantries. But perhaps the most dramatic sight was finding the remains of Željava’s old kerosene tanks. The kerosene store is accessed via a small low-ceilinged passage, that opens to reveal a towering mess of mangled metal. The fires in 1992 presumably caused the liquids to combust, detonating one tank after another in a series of fuel explosions. I stood in the middle of an old tank, its metal walls burst outwards into a formation like the petals of a rose, and tried to imagine the sound it must have made.

Visiting Željava Airbase

Željava Airbase is both straightforward and legal to visit. Twice a year I lead groups there as part of a two-week Yugoslav Modernist heritage tour I run for Atlas Obscura. But if you’re feeling plucky, you can easily get there on your own too. Just be sure to stop at the border crossing first, and let the police know your intentions. They’ll appreciate it, and it’s better for everyone that way.

The underground complex is relatively safe, so long as you pay attention to your surroundings. It helps that the main complex was built across one single level. Along the main galleries there are occasional inspection pits, which would have allowed technicians to work on the underside of aircraft – at maybe a metre-or-so deep, you certainly wouldn’t want to walk into one, but it’s not quite as terrifying as the sudden four-storey drops that punctuate the passageways of Object 221 in Crimea.

Landmines can still be found in the area, dating from the Croatian War of Independence in the early 1990s. Local police train dogs to locate them, and in 2000, a Bosnian Air Force Major suffered fatal injury after stepping on one while he was picking mushrooms. Mines have been found inside the tunnels too, apparently… though if there are any still left, they’ll be found in the piles of dirt and rubble that clog the side passages – and not on the mostly smooth, flat surface of the gallery floor. This is one of the reasons local police will tell you to stick to the main corridors.

Additionally, the air quality inside the Željava tunnels isn’t great. A toxicology report in 2008 (currently no longer available online) showed signs of airborne chlorine compounds (Polychlorinated biphenyls) released from coolant fluids, as well as highlighting the potential presence of radioactive americium-241 that was used in the complex’s ionisation smoke alarm system. A gentle breeze blows all the way through the spacious main gallery though, from one country to the other. A quarter-century of air movement means these larger spaces are much less likely to harbour dangerous pollutants than the smaller stores and chambers where the air is still and stagnant; nevertheless, the police advise that if any member of your party begins to feel light-headed at any point, you get out immediately. (On the plus side, the stink of soot and old diesel tends to keep the bears away from Željava.)

It is also worth keeping in mind that Željava Airbase represents an illegal backdoor into the EU. While many migrants and refugees wait patiently to enter Croatia at regular border crossings nearby, local smugglers and people traffickers will likely be well aware of the existence of these tunnels. In six trips, the only other people I saw in Željava was a pair of Dutch backpackers on bicycles… but you never know who you might run into, so if you ever see lights ahead, it’s probably best to give them a wide berth.

There were plans proposed, sometime around 2005, to turn Željava Airbase into an official waypoint for asylum seekers entering the EU. Nothing came of it though. A later proposal to turn the airbase into a military training ground was scrapped too, as the peace agreements between the former Yugoslav republics bans the creation of military facilities within 15 kilometres of the borders. On the Bosnian side, there has been talk of restoring some runways for use, but nothing has yet come of that either. The problem is, no one in this region has the kind of money it takes to maintain a subterranean mountain hideout. Željava cost Yugoslavia a fortune to build, and it would cost another fortune to now restore the place to any useable standard. Yet it was built to be indestructible, and so here it remains: a six billion dollar tunnel complex, impossible to use, and impossible to destroy.

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.

Sources

Both these websites have been a huge help as I researched this article, and if you’re curious, you can visit them for additional details, history, photo galleries and even archive video from inside Željava Airbase.

www.zeljava-lybi.com
www.forgottenairfields.com

Archive Photographs