You'll find lots of good stuff under the "Draining" heading here:
http://www.uer.ca/...=1&threadid=118368 I'd say storm dreams are a good place to start. Sewers aren't as bad as people tend to assume, but definitely not a place for new explorers! And of course storm drains are much safer when it comes to air quality. Just don't go near them if there's any chance of rain.
I'm going to move this over to the rookie forum.