This video was sent to me by a friend and I thought it was pretty interesting.
I used to live in Hiyoshi on the Toyoko Line. I guess the Japanese military used to have bases in Hiyoshi and there are tunnels all over many of the mountainous areas near the station. Of course they are all boarded up with a "Absolutely No Entrance!" sign, but, being a tad bit crazy, I've always wanted to go exploring in those caves (probably get yourself killed doing it though!)
Here's a video of some cave explorers in Japan that might give you a vicarious thrill!