A strange stone formation with more than a passing resemblance to Great Britain's Stonehenge lies at the bottom of Lake Michigan. The monoliths' origins are unknown, but they are thought to be thousands of years old.
The site was uncovered by Mark Holley, a professor of underwater archaeology at Northwestern Michigan University College, and his colleague Brian Abbot in 2007 while they crossed the lake in a ship loaded up with sonar equipment, but the discovery has thrown up more questions than answers over the years.
The plot thickened when a carving of a mastodon, a distant relative of the elephants that's been extinct for over 10,000 years, was discovered on one of the pillars, suggesting its origins could stretch all the way back to the last ice age.
A similar monolith has also been discovered in Europe, this time in salt water off the coast of Sicily, although experts believe these stones once served as a primitive form of lighthouse technology, illuminating the way for seafarers in a bygone age.