Is there any index of people exploring the underground in different cities? I have found one crew from Sweden that shows people around under Stockholm (interactingarts.org/under/). Do you have any tips on explorers from other countries?
[ Edit: Link made unclickable as posters own site ... ]
Good question! I know that in London you can, although this tends to focus old arms/stations of the tube systems and the old post office railway, and i'd guess enthusiasts in most major cities would be able to offer similar tours. I've been on one in Berlin a few years ago which was really good, but sorry, can't remember any details about who ran it. Other than that, some of the medievel cities probably have labyrinths of old tunnels/servants/animal/escape passages etc which may or may not be open to the public periodically.
Hi, during the Brighton festival (Brighton, Sussex, UK) you can go down in the sewers - given to understand they are huge!