Under the wooded hill of Teufelsberg in the Grunewald forest in Berlin-West lies a building that was never completed. It is the Wehrtechnische Fakultät, a military technology faculty designed by Albert Speer, a prominent architect during the Nazi era. The construction began in 1937 as part of the Nazi project to restructure Berlin, but it was abandoned before completion due to World War II. The structure was too solid to be destroyed by explosives, so the Allies covered it with 26 million cubic meters of ruins between 1950 and 1972. This amount is equivalent to about 10,400 Olympic-sized swimming pools, creating the largest rubble hill in Berlin, which rises about 80 meters above the Teltow plateau. The Wehrtechnische Fakultät was intended to be the heart of a Hochschulstadt, a university city dedicated to technical and military education. It was part of Hitler and Speer's plan to reshape the capital into "Welthauptstadt Germania," a grand vision that included a giant Olympic stadium, a triumphal avenue, and a triumphal arch. The first stone was laid in the presence of Hitler himself, near the Heerstraße. However, the priorities of the war halted the construction, which never resumed. In April 1945, the neighboring forest became the scene of battles against the 55th armored guard brigade of the Soviet army. After the war, the Allies tried several times to demolish the structure with explosives, but nothing worked. The structure was too solid, so the Allies opted for a cheaper solution: covering it with the ruins of the city. Starting in 1950, trucks dumped the rubble of bombed buildings on the site. The filling lasted until 1972. The Berlin blockade even interrupted the convoys for a time, due to a lack of fuel in the western part of the city. Since the 1960s, Teufelsberg has hosted an American spy station, later converted into a street art gallery and a venue for cultural events. The height of 120 meters in the British sector made it an excellent listening post, and the NSA understood this as early as the 1960s. It built one of its largest stations there in 1963 in close cooperation with British intelligence. The Field Station Berlin intercepted military and civilian communications from the Eastern bloc. Three white spheres, the radomes that housed the antennas, still crown the summit, surrounded by concrete buildings. The Americans left the site in 1992 after the fall of the Wall, and Berlin used the installation starting in 1994 for the monitoring of civilian air traffic. The largest dome has retained a unique acoustics, now used for musical and artistic events. What one can see today is an abandoned station that has become an open-air street art gallery, whose walls are covered with frescoes by Berlin and international artists. Entry costs 12 euros for an adult, and the hours in October run from 11 am to 7 pm, with the last entry one hour before closing. Elsewhere on the hill, the Berlin state marks a climbing rock and a sled track. The name "devil's mountain" comes from the nearby Teufelssee lake. The Teufelsberg has been protected as a monument since October 30, 2018. The autumn calendar remains busy: an exhibition titled Orte der Demokratie (places of democracy) is announced on the site from October 3 to 18, 2026. In November, closing time advances to 4:30 pm.