The Berlin Underworlds Association (Verein Berliner Unterwelten) was founded in 1997 and is devoted to exploring a unique chapter in the history of Berlin: it performs research into the various structures found in the city's underground and makes them available for guided tours. Among many other things, guests are invited to visit WWII bunkers and so-called "ghost train stations" ("Geisterbahnhöfe") and also to find out about Cold War escape attempts to the West via tunnels and canals.
One of the tours – called "Dunkle Welten" or "Dark Worlds" – takes place in a former civilian bunker in the Gesundbrunnen U-Bahn station, which the association has rented since 1998. This bunker, which was built into the U-Bahn tunnel in 1941/42, was designed to give shelter to roughly 1,300 Berliners during bombing raids: however, sometimes over three times that number found shelter here. The Berlin Underworlds Association maintains the complex in its original state, and the various steel doors and ventilation systems give visitors an idea of the intense environment in these narrow, stuffy rooms.
The Berlin Underworlds Association also has four floors of exhibits showcasing other underground facilities, such as the pipe mail network, brewery cellars and sewers. For example, a number of photographs and items focus on the "Fahrerbunker," which belonged to the Reich Chancellery and was discovered in 1990. The exhibition also showcases a very special find in a Tempelhof bunker: a list of forced labourers who worked at an electric company during WWII.