My grandfather was a pipe fitter in downtown Houston. I'd bet there are several 100 miles of tunnels there as well. Consisting of many levels. Obviously some people know about the public ones with shopping malls but there are unimaginable other tunnels as well. As a kid I'd go to work with him (in the 80's) and I saw all sorts of interesting things. With a google search you can checkout some of the NYC tunnels that people actually live in.