18 mile loop that includes singletrack, access road, old hiking trails, special sections (such as the ROCK GARDEN, and BLOOD ROCK). Kept in very good condition by the BUMP club. For extra information you can call Crestline Cycle in Birmingham and someone their could probably help you out. The USA MTB team trained here a month before the Olympics which says something about the quality of the trails.
Take I-65 south from Birmingham, look for Oak Mtn State Park signs. Very easy to find. Opens 6:00 most days til dusk.
http://www.singletracks.com/item.php?c=1&i=48