Teufelsberg is a man-made hill, created in the 20 years following the Second World War by moving approximately 75 million m3 of debris from Berlin.
It was named after the Teufelssee (Devil's Lake) in its southerly vicinity. The hill is made of rubble, and covers an unfinished Nazi military-technical college (Wehrtechnische Fakultät). During the Cold War, there was a U.S. listening station on the hill, Field Station Berlin. The site of the former field station is now fenced off and is currently being managed by an organisation which charges 5 to 10 euros for public access. https://en.wikipedia.org/wiki/Teufelsberg