Berlin offers a plethora of guided tours to visitors and locals, both paid and for free. One of my favourite tour operators are Berliner Unterwelten (Berlin underworlds), who take you beneath the surface of Berlin, quite literally. From their headquarter near Humboldthain, they offer tours of either the interior of nearby World War Two-flak bunker or take you on a “Cold War”-tour through two atom bunkers from the 70s located in two subway station; and also multiple other specialised tours around Berlin.
What’s interesting about these tours is that Berliner Unterwelten is also dedicated to the preservation of the sights – all proceeds from the tours go to renovation and preservation of the places visited by the tours, which had been painstakingly cleared of rubble and renovated by the association members, who all work as volunteers and are not paid. So with taking a tour with Unterwelten, you not only learn about the history of Berlin but also help to preserve that memory for others.