I have always had an obsession with the world of the underground. I don't know what it is, but half the scripts I write have a subterranean location. When I was a kid I conceived of digging tunnels to all my friends houses and creating a tram system that would operate in these tunnels. In my mind, it was not unlike something you might find in a Myst game although I think this all predates my playing that game. I convinces friends that there was a tunnel made out of gold under my parents yard to convince them to help me start the digging process. This all had to do with the alien neighbor who ate dog shit... but that's another story.
I could go on with my self psycho-analysis of what it is I like about this realm and why but I'll spare you and just direct you to this cool gallery of underground enviroments in The Atlantic. There are lots of cool photos and interesting back stories.