Looking like an alien spaceship docked on a remote Bulgarian mountainside, the Buzludzha Monument was built in the 1970s and served as the seat of the Bulgarian Socialist Party until Communism fell in 1990. It sits near the Shipka Pass, on the spot where Bulgarian revolutionaries bravely attempted to hold off Ottoman forces in 1868.

With its brutalist form, Marxist mosaics and red-star ceiling, it is the epitome of Soviet-era architecture and a popular destination for urban explorers – if they can get in. A guard is on duty 24/7.

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