Going underground

Here's something I don't remember reading about in the Rough Guide To Paris: exploring the labyrinthine catacombs beneath the city. I quite fancy myself as a potential cataphile, though membership of the secretive organisation UX does seem to make you talk like someone out of The Matrix or Fight Club: "As we grew up we pursued our different careers on the surface - everything that's outside UX we call 'the surface'. We have two important founding principles. Firstly we never ask permission - we never consult or inform the authorities about our work - and, of course, we never ask them for subsidies."