Post Corona, you must visit Berlin and go to the privately run Museum Berliner Unterwelten. Berlin Underground next to it is aWW2 Flak site on a hill with a great view of Berlin. Stefan.
Steve, forgot to mention: Until I visited the place, I always thought most tunnels were short and started to be dug from East Berlin. Not true,many were pretty long and started with help from West Berlin. Another WW2talk member (British) went together with me and he ilves in Berlin in these days. Stefan.