Founded in 1997, Underworlds is a not-for-profit organization dedicated to the documentation and preservation of Berlin’s vast network of subterranean spaces. It funds its projects by giving tours of bunkers, sewers, air raid shelters and catacombs. The tours are offered in a variety of languages, including English.
From the website Berliner Unterwelten:
Experience the history of Berlin from an unconventional perspective! Since 1997, the Berlin Underworlds Association has been offering regular tours into some of the most important underground structures in the city. Although the majority of our tours are in or near the Gesundbrunnen station in the north of Berlin, we also offer tours in several other subterranean complexes that are otherwise not publicly accessible.
I am thinking of adding a tour to my to-do list for 2010.