If I ever need a new hobby, I think it will be urban spelunking.  It reminds me of semi-terrified explorations of the underground tunnels below Van Tech when I was just a punk trying to find the fabled subterranean swimming pool and archery range that were supposed to be down there somewhere, before they got locked away because of some unspeakable accident involving the death of one student or another.  Most of the terror came from virtual certainty that if the boiler room custodian (a giant, balding man who bore an uncanny resemblance to a medieval gaoler or perhaps a torturer) ever caught us, we would be torn limb from limb, brained with his giant crescent wrench, or perhaps even suspended from class.  Terrific article here about the drains of Toronto, but it appears we have a nice little community in Vancouver as well.  No stories about giant medieval gaolers, but apparent the sounds of a subway train passing close by has an uncanny resemblance to the sound of a giant wall of water bearing down the sewer toward you.  The Vancouver site had a fairly creepy anecdote about descending into one sewer in winter, but finding that the manhole cover had frozen securely in place when they tried to get back out.

I have no idea why this sounds fun, but it does.