13 Abandoned Cold War Bunkers Hiding Incredible Secrets

11. Željava Air Base, Croatia/Bosnia: The Mountain's Aircraft Carrier

Built into the base of Mount Plješevica, Željava was one of Europe's largest underground airbases, designed to protect Yugoslavian fighter jets from nuclear attack. It housed multiple squadrons within extensive tunnels and featured five runways emerging directly from the mountain. Heavily damaged during the Yugoslav Wars, its entrances are now sealed or dangerous to explore. Its secret is the sheer ambition of constructing hangers and operational facilities for jet fighters inside a mountain – an incredible feat now left as a haunting, ruined testament to Cold War strategy.