I first learned about the Louisville underground cavern system with bike park on the show Kentucky Life via the Kentucky PBS channel. There is also a member here that is building a bike to ride it but I can't remember where his post is nor find it now. Anyhow I found this article on Yahoo the other day that has some detail about the park with pictures and video. Thought it might interest a few members.
http://www.grindtv.com/outdoor/excursions/post/gigantic-bike-park-open-abandoned-mine/
http://www.grindtv.com/outdoor/excursions/post/gigantic-bike-park-open-abandoned-mine/