Beneath a European mountain sits a vast, very unusual and now abandoned airport.

If you’re on or interested in the dark tourism scene, then there’s a good chance you’ve heard of Željava Air Base.

The abandoned Cold War military complex has no official gates, ticket booths or opening hours, making it a dream for those who like to poke around in the forgotten and strange corners of civilisation.

And visitors can do more than just poke around. They can drive on the runways and explore the extensive network of underground tunnels.

Željava Air Base, historically codenamed Objekt 505 or Klek, was the largest underground military airport in former Yugoslavia and one of the most expensive defensive engineering projects in European history.

It was built in complete secrecy between 1948 and 1968, and cost a huge amount of money. The complex cost the Socialist Federal Republic of Yugoslavia an estimated $6 billion USD (equivalent to tens of billions today), according to Atlas Obscura.

President Josip Broz Tito ordered its construction during the height of the Cold War, fearing a full-scale nuclear assault or invasion from either Western forces or the Soviet Union.

The base was carved deep into the core of Plješevica Mountain, with the rock used as a natural armor.

The underground segment featured 3.5 kilometres of interconnected tunnels carved into a semi-circular layout. The corridors were massive, measuring roughly 16 metres wide and up to 12 metres high, so heavy fighter aircraft stored inside could be easily maneuvered.

The entrance portals were shielded by 100-ton reinforced concrete blast doors. The entire complex was engineered to withstand a direct hit from a 20-kiloton nuclear warhead, which is the scale of the atomic bomb dropped on Nagasaki.

The bunker acted as a self-sustaining underground city. It contained its own water supply, power generators, air filtration systems, and barracks to house and feed up to 1,000 soldiers and pilots for 30 days in total isolation.

Inside, two full squadrons of MiG-21 jet fighters (roughly 60 planes) could be housed, according to Journal.

Outside the bunker stood five runways cutting directly across the flat landscape. Because of how the base was built, jets could roll straight out of the mountain tunnels directly onto the runways to scramble instantly during an alert.

Despite its current dilapidated and abandoned state, the base was never actually deactivated or captured in a foreign war. Instead, it was systematically destroyed by the very military that spent decades defending it.

In May 1992, during the collapse of Yugoslavia and the escalating Yugoslav Wars, retreating forces from the Yugoslav People’s Army and Serbian military didn’t want the strategic asset falling into Croatian or Bosnian hands. They lined the tunnels with 56 tons of heavy explosives and triggered a blast so massive that it shook the nearby city of Bihać.

The explosions collapsed key structural arches, shattered the blast doors, and filled the base with highly toxic chemical smoke that burned for days.

The Airbase, which sits near the town of Bihać in Bosnia and Herzegovina and the Plitvice Lakes in Croatia, is now open to the public and free for everyone to visit. Tour guides can be hired, but if you do choose to go by yourself, make sure you check the official map.

It sits directly on the border between Croatia and Bosnia, and Herzegovina, which means extensive minefields from the 1990s Yugoslav Wars still surround the area. It is crucial that if you do visit, you don’t step off the concrete runways or paved surfaces. Doing so could cost you your life.

The underground blast tunnels are pitch black, structurally compromised, and filled with toxic dust, loose metal debris, and deep holes.

“Visiting Željava airbase is generally safe. However, some areas may still have unexploded ordnance, so it’s crucial to stay within designated areas. As long as you don’t leave paved surfaces, you should be fine. Be careful of holes and metal debris on the ground in the tunnels. Bring a strong flashlight! Thrill Quest warns.

Because the site spans an active EU external border, Croatian and Bosnian border police regularly patrol the runways. Which is why you should carry your passport with you if you’re exploring, and avoid crossing into the Bosnian side through the woods or unpaved paths, as this constitutes an illegal border crossing.

The tours are hosted year-round, with two daily sessions at 10am and 4pm.