Teufelsberg

The Teufelsberg (German for Devil's Mountain) is a hill in Berlin, Germany, in former West Berlin. It rises about 80 meters above the surrounding Brandenburg plain, more precisely the north of Berlin's Grunewald forest.
It is an artificial hill with a curious history: it was built by the Allies after the Second World War from the rubble of Berlin during the following twenty years as the city was rebuilt. One estimate for the amount of rubble is about 12 million cubic meters, or about 400,000 buildings.
Teufelsberg's origin does not in itself make Teufelsberg unique, as there are many similar man-made rubble mounds in Germany (see Schuttberg) and other war-torn cities of Europe. The curiousness begins with what is buried underneath the hill: a Nazi military-technical college designed by Albert Speer. The Allies tried using explosives to demolish the school, but it was so sturdy that covering it with debris turned out to be easier.

The US National Security Agency (NSA) built one of its largest listening stations on top of the hill, rumoured to be part of the global ECHELON intelligence gathering network...
Opened: around 1950 / Closed: 1992

wikipedia
a late post mortem
More info (spanish)

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