CHILLING new photos show Europe's largest underground airport and military base that was built to survive a direct hit from a nuclear bomb.
Constructed in the 1960s under a shroud of secrecy, the Zeljava Underground Airbase has been left decaying within a hollowed-out Balkan mountainside for over two decades.
Also known as Objekat 505, the ambitious military project is nestled inside Mount Pljesevica, situated on the border of Croatia and Bosnia-Herzegovina.
It burrows two miles beneath the mountainside with a maze of twisting tunnels like an underground city that has been since reclaimed by nature.
Zeljava Airbase was completed and operational in 1968, and the facility was destined to play a vital role in Yugoslavia’s early warning radar network.
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Similar to today's NORAD [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.
The £4.7billion Zeljava Airbase was designed and fortified to withstand a direct hit from a 20-kilotonne nuclear bomb, which is bigger than the one dropped on Hiroshima by the US in 1945.
With its self-reliant power sources, water purification, and ventilation systems, the airbase was a marvel of military engineering at the time of its creation.
But now, all that is left is the skeleton of its remains, with abandoned warplanes and deep underground tunnels filled with the spectacular mystery of what was once one of the largest and most expensive military complexes in Europe.
Chilling photos show the remains of mangled metal reinforcements and cavernous ceilings deep underground - a stark relic of the Cold War.
Although the site is far from its prime, the colossal base often sees the odd visitor within its crumbling walls.
Kamila Napora, a Polish travel writer and urban explorer, visited the derelict airbase in September 2021 and said she felt a sense of unease in the abandoned military installation.
"Since it was so dark inside, I felt slightly uneasy there - the place itself was safe to visit," she told The Sun.
"It was just the complete darkness that was playing tricks with my mind.
"I'm a big fan of exploring abandoned places and Zeljava airbase was especially interesting considering its history and secrecy in the past."
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In its prime, the Zeljava Airbase could house almost 60 MiG-21 aircraft.
Its winding 2.2-mile labyrinth network of tunnels led to several command centres, military offices, and dormitories.
The main galleries, arranged in an M shape, measured as much as 16 metres wide, and up to 12 metres in height.
The 100-tonne concrete doors at its entrances, adorned with heavy visible reinforcements designed to protect against nuclear, biological, and chemical attacks, serve as an everlasting reminder of its daunting past.
But during the wars that followed the breakup of Yugoslavia in the 1990s, the grand facility was severely damaged by powerful explosives.
The Yugoslav Army retreated from the base on May 16, 1992, and carried out a scorched earth policy to prevent Zeljava from falling into enemy hands.
Around 56 tons of explosives were used to destroy the underground complex, ultimately rendering it unusable.
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.
Now, mounting piles of rubble and shattered materials lay scattered across the grounds, making the site incredibly difficult - and dangerous - to navigate if not appropriately equipped.
"It was very dark inside the tunnels, and sadly, I was not properly prepared to explore them," Kamila recalled.
"When going inside, at first you can still get a bit of daylight but the further you go in, the darker the place gets.
"I only had a flashlight in my mobile phone, which was not enough for such an enormous place, especially since there were plenty of destroyed parts and rubble on the ground".
The security offices inside the main entrance can be seen in haunting photos as a cube of concrete and rust-red metal.
Exposed and frayed piping can be seen sprawled over the concrete as phone torch lighting casts nightmarish shadows across the walls.
Located outside the base, an entrance through which jets could exit the complex leads straight onto the runway.
The runways were constructed to be thick and resilient, capable of handling the takeoff and landing of heavy military aircraft.
The site had five runways in total and could hold 60 fighter jets, but these were all destroyed in the blast.
"What was even more alluring was the fact that today, the state border runs directly through the airbase and runway, making the place even more quirky," Kamila said.
This has left the base a common waypoint for illegal migrants, and a facility for asylum seekers was scheduled to open there in 2004 but the idea was quickly abandoned.
Also situated on the grounds outside the former barracks, an abandoned Douglas C-47 collects rust under the Croatian sun.
The battered and dented American aircraft can be explored by visitors like Kamila who climb on board the derelict plane.
"The whole airbase was interesting to explore, but there were two places that were the highlights: the abandoned Douglas C-47 plane that you can climb into and explore inside and the tunnels inside the mountain - it was surprising how huge they were!" she said.
As late as 2012 there were reportedly also two American Republic F-84 Thunderjets left abandoned in the vicinity, though those have since disappeared.
"When I saw pictures of Zeljava Airbase from a few years back there were more abandoned planes around, but we could only find this one," Kamila said.
But dotted around the base are live landmines, dating from the Croatian War of Independence in the early 1990s.
They are now feared to be able to detonate at any moment after a Bosnian Air Force Major reportedly suffered a fatal injury after stepping on one while he was picking mushrooms in 2000.
Although fatalities linked to the landmines are few and far between, local police have now trained dogs to locate them.
"Since I was aware of the landmines that still might have been placed around it, I made sure to stick only to the paved parts of the airbase," Kamila said.
"For years, there have been no accidents there, but nevertheless, it's better to use common sense in a place like this".
Not only do the landmines pose a threat to guests, but the radioactive dust that hangs in the atmosphere following dozens of fires sparked by the bombings is also enough to make visitors light-headed.
The explosions that destroyed the base caused widespread environmental contamination, with the leakage of aircraft fuel and other toxic materials.
Local police have advised that if any explorers begin to feel dizzy at any point throughout their adventure, they get out immediately.
The overground territory of the airbase also features 34 external buildings, including nearby barracks, as well as vehicle garages, workshops, and a radar station situated at the top of Mount Pljesevica.
Vehicles can be seen toppled over and surrounded by debris in the devastated airbase after it was consumed by internal fires following the hit from the explosives.
In 2016, decades after the once-invincible military stronghold was laid to waste, public interest began to rise in the mysterious location.
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.
Locals have estimated more than 150,000 people already make the trek to see the base each year - and soon, official tours could begin.
The mayor of the area said: "By revitalising Zeljava, we would create additional content for the national park enabling tourists to stay a day longer."
But others believe the mysterious Soviet base should remain in its current abandoned state.
One visitor said: "You don't have signs where you have to go and what to see, it's more like a discovery place. This is why I liked it.
"Turning it into a tourist attraction would lose its charm."
