There is an underground airbase between Croatia and Bosnia. Its situated in the mountain of Pljesevica, the mountain starts in Croatia and then goes on the border of Bosnia and Herzegovina.
Also known as “Objekat 505”; was built between 1948-1968, and it was supposed to act like an early warning radar?? The entire thing can withstand a 20 kiloton Nuclear bomb.
The airbase was used a lot in the 90s, mainly for the Yugoslav People’s Army. And apparently so no one against them can use it they themselves destroyed it. They also spent 6 billion dollars, one of the most expensive projects during that era.
As of now it’s abandoned, I do not know if people can visit the area but if you do, be very weary of all the bombs that are still there.
The first picture says “New mystery Bihac Airbase”