Underwater archeologists from Northwestern Michigan University stumbled across a Stonehenge-like structure 40-feet underwater in Lake Michigan's Grand Traverse Bay while they were sniffing out old boatwrecks using something called a "sector scan sonar" device.
Oh, and one of the rocks? Has a carved picture of a mastadon on it, which potentially dates the site as being 10,000 years-old, or right after the last glacial ice age. They still have to authenticate that they're seeing what they're actually seeing, but apparently Michigan has its share of ancient artifacts.
So, while the find is surprising due to its location, no one will die of shock if it turns out to be real.
Here's a nifty sonar photo of the site that I snurched off the Intrawebs:
You gotta admit that looks coooooool.
More about the Lake Michigan find from MetaFilter.
More detailed information from the NowPowered site.
Bldgblog has more details, more links, and more pictures than any other site.
The Traverse City Record-Eagle 2007 article when the discover was first announced.