Just saw this link on Jason Kottke's site. The article regards the possible discovery of a structure similar to Stonehenge deep underwater in Lake Michigan.
This is a top-down view of the structure, taken by an underwater camera. Apparently one of the stones has a carving of a mastodon on it, although I'm not sure how they picked that one out. If genuine, the stones could be up to 10,000 years old. Here's a link to the original post. I'm always fascinated by this sort of stuff. I guess it goes back to the fact that Indiana Jones and the Last Crusade was my favourite movie as a kid.