Still standing today, a fierce-looking prison and an innocuous looking office building in Berlin were actually the headquarters of a feared and ruthless organisation - the Stasi. Also, a covert Communist base hid Cold War submarines for Tito, the former leader of Yugoslavia on the island of Vis. A seemingly innocuous sports centre hides a sinister underground bunker. And the former HQ of the Soviet Army in East Germany was a forbidden Soviet city within a city.
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