Deep in Berlin's Grunewald Forest, Teufelsberg (literally, Devil’s Mountain; the name comes from the nearby Teufelssee, or Devil’s Lake) was once the site of a Nazi technology college. After the war, it was decided that the school would be buried beneath rubble and debris left in the city after the war, and the resulting man-made hill rose to more than 300 feet; during the cold war, American troops built a base on top from which to spy on East Germany. Since the wall fell, the base’s radar domes have become graffiti-spattered ruins and a haunting pilgrimage for urban explorers, street-art fans, and those seeking great views over the Grunewald and the Havel River. The hill is also used as a ski slope and sled run in winter; paragliding aficionados and picnickers enjoy it in summer. The website has official tour options.