Northern Michigan has over 200 sinkholes with roughly 100 clustered around Alpena. That’s the second largest concentration of sinkholes in the world, behind Yugoslavia. No kidding...
We listed that along with several other Best-est, Biggest, Most-ests to which Northern Michigan claims...
Another Michigan spot that has some sink holes is located in a very intersting spot in the U.P. called the "Fiborn Quarry". It's owned and governed by the Michigan Karst Conservancy. Kiosks tell the story of an early 1900's development just outside of Trout Lake that mined limestone. Several online sites tell the story and a visit to the site invites guests to take some trail systems that traverse the cave systems and quarry banks. It was a walk through time and the bones of a former village exist in the quarry. I've lived here since my teens and always heard of it but never knew what it really was. The story is a great one and further defines Michigan history and the need to discover it. The best information I've found is http://www.caves.org/conservancy/mkc/