Like something out of a Bond film, the giant Zeljava airbase was carved into a mountain between Bosnia and Croatia and designed to withstand a nuclear strike.
But for decades it sat idle, with only the occasional intrepid tourist daring to venture into its crumbling cavernous core.
Built in secret in the 1960s to hide a fleet of Soviet fighter jets in what was then Yugoslavia - a socialist federation that sought a middle ground between Moscow and Washington during the Cold War - it had its own power, water purification and ventilation systems and could operate autonomously.
In its heyday, the underground base could hold nearly 60 MiG-21 aircraft, with its 3.5km (2.2 miles) or so of tunnels also home to command centres, offices and dormitories.
The remains of the enormous 100-tonne retractable concrete doors at its four entrances are still visible with metal reinforcements protruding from the structures.
Beyond its cavernous interior, the base had five runaways that straddle the border between Croatia and Bosnia.
"All the systems were state-of-the-art at that time," said Mirsad Fazlic, a former pilot who worked at the base for nearly a decade in the 1980s.
"It was the then best military and civilian technology."
'Everything was burned'
During the wars that followed the breakup of Yugoslavia in 1990s, the facility was destroyed by the remnants of the Yugoslav army, using powerful explosives.
"All that was inside, all that equipment, everything was burned," said Fazlic.
"Only the tunnels and the walls remained."
After its destruction, the base sat largely vacant and in disrepair, attracting adventurous tourists looking to explore old relics from the communist era.
That all changed in 2016 with the release of a Slovenian mockumentary called Houston, We Have A Problem! featuring the base.
Since then, locals estimate that the state-owned complex has been drawing more than 150,000 people a year.
Authorities in the area have high hopes that with the right marketing, the base could attract many more, notably some of the 1.7 million tourists that visit the nearby Plitvice Lakes national park every year.
"By revitalising Zeljava, we would create additional content for the national park enabling tourists to stay a day longer," said Ante Kovac, the mayor of the area.
Car races have already been staged at the base, and officials believe its extraordinary size means it could house data centres, or host parties or a Cold War museum.
'Frozen in time'
At the moment, visitors walk with flashlights through its humid, pitch-black tunnels, carefully avoiding holes in the ground, while some drive through portions of the base.
"It's crazy that it has been frozen in time," said Angelo Virag, a photographer visiting from the Croatian capital Zagreb, who was in awe of the "absolute ingenuity of engineering".
His cousin Mario Garbin - from Perth, Australia - gushed over the "raw, authentic nature of the infrastructure that has been left untouched for the last 30 years."
Aviation fanatic Hamdija Mesic from the nearby Bosnian town of Bihac said he hopes that the two runaways located in Bosnia would be reopened to fellow pilots soon.
"Such a huge facility abandoned to the ravages of time cannot be found anywhere else in the world," he told AFP.
However, others hoped the site would remain as it is.
"You don't have signs where you have to go and what to see, it's more like a discovery place," Maria Moreno, a 33-year-old interior designer from Spain, told AFP. "This is why I liked it.
"Turning it into a tourist attraction would lose its charm." - AFP
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