Northeastern Michigan is an active karst area. We have numerous sink holes through out the area but there is no higher concentration of sinks than the Rock Port Sinks. I have personally found 5 or 6 holes where their bottom can not be decerned from shore. The rest are either mostly dry or look no more than 5 feet deep. Only further exploration will reveal their secrets.
These sinks MUST be approached with extreme caution as they are still active and many are surrounded with vernicle cliffs over 20' in height. There has been mentioned that there may exist small holes not much bigger than a man that open up to a cavern of unknown depths. Please explore the area with great caution. At present this author has identified 7 wet sink holes and has the bruises to prove how difficult they are to access. Caution should be taken entering and exiting the water as some shores are covered by many feet of soft muck and vegetation and could suck one in. Treat the shores as if they were quick sand!
The area is profuse with Petoskey Stone and other Devonion fossils.
Rock Port Quarry deserves a visit by any serious Rock Hound, although the area is owned by the State of Michigan who lays claim to any and all items found thereon.
I hope you enjoy and are intrigued by these images and plan some time to explore the Rock Port Sinks.