I just watched a cool race at the Adventure Mine in Greenland MI called the Miners Revenge. The race course included a half a mile of mine shaft (lights mandatory). Killer "hike a bike" uphills and long, steep, rocky downhills. The mine shaft section was only open for race day but the rest of the trails are open to the public. There's only about 4 miles of trails, but they're working on more. You can pay for a walking tour of the mine, it's just dark, wet, chilly and with dark gray rock all around you. The part of the shaft in the race had just one spot where the ceiling was too low to ride for about five feet.
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