Damn, I love Urbex! In Adelaide, Australia the whole city is build atop a winding system of storm drains. Me and my friends often explore them, and I bring along my dSLR to snap pictures.
Not sure if you've heard of this phrase in America, but here in Australia all the drainers have a common catch phrase:
When it rains, don't do drains!
There's also a bunch of abandoned cement works and quarries near my area, it's amazing. It's summer soon, too, so I'll be able to go check out storm drains again!
I'll scrounge around and see if I can dig some pictures up (: