"New Yorkers go about unaware of what is happening just beneath their feet: Power pulses, information flies, and steam flows. The city’s infrastructure starts just below street level, but it doesn’t stop there. "

This is a really cool presentation of what goes on and where underground in New York City:


The drawing on that intro page is not to scale. Here's the link to see everything in scale:


I found this extremely interesting, and not just because I live in NYC. It's a great resource for anyone doing urban mapping, especially of the subterranean variety.