Another Stonehenge Discovered Under Lake Michigan?

A group of researchers using sonar to find shipwrecks on the bottom of Lake Michigan have found something far older than crashed cargo ships. They believe they've found a 10-thousand-year-old stone structure like Stonehenge, including a rock carved with the image of a mastodon. io9 pal Geoff Manaugh reports over at… » 1/13/09 1:56pm 1/13/09 1:56pm