In a surprisingly under-reported story from 2007, Mark Holley, a professor of underwater archaeology at Northwestern Michigan University, discovered a series of stones – some of them arranged in a circle and one of which seemed to show carvings of a mastodon – 40-feet beneath the surface waters of Lake Michigan. If verified, the carvings would be roughly 10,000 years old – coincident with the post-Ice Age presence of both humans and mastodons in the upper midwest.
Nobody's sure what to make of the find. Is it all real, or are we having a partially apophenic vision? Either way, I'm with Jeff on this one: the pictures are cool enough on their own, and the data are interesting enough to look further.