Teufelsberg, literally "Devil's Mountain", an 80-metre hill made from an estimated 12 million cubic metres of war rubble (apparently about 400,000 bombed houses) pushed together in the north of the Grünewald forest in West Berlin. Buried deep beneath is what's left of a planned Nazi-military training school. So sturdy was it that attempts by the Allies to destroy it failed, so they covered it with rubble instead. Perched atop this man-made mound now sits the old abandoned intelligence-gathering station used during the Cold War by the Americans and British to learn what was going on in Russian-controlled East Germany.
(txt source, abandonedberlin)