When it comes to mysterious prehistoric sites, North America’s rich treasure trove of ancient puzzles tend to get overshadowed by more famous examples from Africa, Eurasia, and South America. Earthworks like the Serpent Mound in Ohio, and numerous medicine wheel sites such as Wyoming’s Bighorn Medicine Wheel are but two types of these archaeological enigmas, who’s ages can be disputed due to the difficulty in determining how old the constructions are.

There is, however, a more modest site that lies 40 feet under the waters of Grand Traverse Bay on Lake Michigan, that, if it proves to be an authentic site, dates back at least to the end of the last ice age. This site was discovered in 2007 by Northwestern Michigan University College archaeology professor Mark Holley, and his colleague Brian Abbot, who were searching for undiscovered shipwreck sites. During the course of their sonar scans, they found a series of five stones, each the same size, that were arranged in a circular pattern, with another series of similar stones laid out in a line leading to the circle. Intrigued, they decided to dive the site.

What they found only deepened the mystery: one of the stones had what appeared to be an image depicting a mastodon carved into it — a species that went extinct at the end of the Pleistocene 11,000 years ago. At that time, the area that the bay now occupies would have been above water, and would only have been flooded after the melting of the massive glaciers that covered the northern end of the continent.

Further investigation by paleontologists and archaeologists will help determine if the site was indeed shaped by humans, or if the stones’ arrangement and apparent image is just a fluke. One problem hampering investigation of the site is that there are few experts on petroglyphs that are also scuba divers, making accessibility difficult. The location of the site is also currently being kept secret by Holley and Abbot, at the behest of Grand Traverse Bay’s American Indian community.