Up until the 1990s vast stretches of Eastern Europe had barely been seen by those in the West.
Political differences had kept the two spheres apart for decades, with some areas deemed so sensitive and top secret in the Eastern Bloc that not even their own citizens had access.
The eerie Željava Air Base is one of these fixtures: a nondescript wall etched into the side of a mountain where all sorts of military secrets were held.
Back then, the airbase sat in Yugoslavia, the huge country that made up much of the central and southwestern Balkans.
Today, it sits on the border between Croatia and Bosnia and Herzegovina, and was the largest such base found anywhere in Europe, and is today open to visitors.
Its entrance is like something straight out of a spy film or science-fiction flick: a giant plane-shaped opening where aircraft would arrive and disappear into the darkness.
Officially known as Obiekat 505, the base is located under Plješevica Mountain, and once housed two hunter squadrons and 1,000 people, including military commanders.
Fixtures were installed that would seal the base from the outside world in a matter of moments in case of emergency. Its interior was stocked to the brim with foodstuffs and had access to independent water and energy sources.
It was even built to be capable of resisting a nuclear attack similar to that inflicted on Nagasaki.
But why did Yugoslavia need to build such a behemoth? Well, the goal was to house a long-range early warning radar system and prevent any danger that came with a warning.
It never actually had to fulfil this purpose, but at its time of building between 1957 and 1965, the Cold War was entering an unpredictable and new phase.
While parts of Yugoslavia, like Slovenia, were at the time wealthy, the socialist republic was generally poor, but this didn’t stop the power that was splashing $6billion (£4.8billion) on the airbase.
Things were made trickier and more expensive by the fact that it was built in complete secrecy.
Currently, the airbase stands on the edge of the EU in the Balkans, with Bosnia and Herzegovina yet to ascend to the bloc.
Technically, the base is accessible and open to the public, but online forums suggest any prospective visitor gain a permit from local police to avoid suspicion.
Whether or not that permit may be granted is another story altogether.
Željava is filled with dangerous hangovers from the Cold War, including radiation, mines and unexploded munitions.
Croatian police are now said to use the facility to train K-9 dogs in finding explosives.
