I recently read an article in February 2011's National Geographic about the Underworld of Paris and the people that frequent it - the Cataphiles. A place of secret raves, hushed book meetings, stunning art and where non conformity is a rule.
My mind runs wild with imagination.
I can only imagine it is super difficult to get an "invite" into the depths of this Paris belly but perhaps something to seriously to investigate.
For full article: http://ngm.nationalgeographic.com/2011/02/paris-underground/shea-text