13 May 2009 · 01:16 · 12 of the World's Most Mysterious Monuments & Ruins | WebUrbanistr

A group of researchers using sonar to look for shipwrecks at the bottom of Lake Michigan got quite a surprise when they found what appears to be an ancient Stonehenge-like structure 40 feet beneath the surface of the water. Some of the stones are arranged in a circle and one appears to show carvings of a mastodon. The formation could be as much as 10,000 years old, which is coincident with the post-Ice Age presence of both humans and mastodons in the area. Michigan already has petroglyph sites and standing stones.