Ever since I read The Phantom of the Opera as a teenager, I've wanted to visit the Opera Ghost's underground lair and see where he kept Christine in captivity. I've heard rumours that it's possible to see the lake from some of the underground vaults of BNP Paribas, the bank which owns most of the property around the theatre, but I don't think you can just march up to the customer service desk and ask for a tour.
There are other parts of the world under the city's surface, however, that are apparently more accessible (if not legally so), as these guys proved by running the tunnels of the Paris metro. Their blog explains in fascinating detail the layers of history and contemporary life that lie beneath the streets - unused metro trains in pristine conditions, stations that were never open, and even people who hold parties in the sub-surface caverns.
I read their blog from start to finish and found it really interesting. I'd even go so far as to say I wouldn't be averse to a little visit to an abandoned metro station or a little stroll down one of the tunnels.
Except that I'd be terrified of what I might find down there. I've seen more rats in Paris's stations than anywhere else in my entire life, so imagine what it must be like when you get far away from the crowds. And perhaps even more scary is the thought of the people you might meet - apart from the train workers, what kind of folk spend their lives hanging out in tunnels ?
Maybe I'll just have to invest in some gold ingots and put them for safekeeping in a BNP Paribas vault instead.