I too am pretty fascinated by the mysteries that lie underfoot. Here's a pretty cool website that has lots of information on subterranean facilities and whatnot, both military and civillian.
http://www.subbrit.org.uk/There are lots of pictures and information about various underground and other interesting sites, in the UK and some in Europe. They also sometimes have special events for members, to tour some sites. I've been looking to see if there is a similar organization here in the US, but haven't found one yet.
