In 2022, a team of researchers mapping the floor of Lake Michigan with sonar detected a series of large, circular depressions at a depth of roughly 500 feet. The features, which number in the dozens, range from 300 to 600 feet in width and reach depths of about 40 feet. Their near-perfect circular shape and alignment initially puzzled the scientists, who could not identify a natural process that would produce such formations.

A closer examination in August 2024, using a remotely operated vehicle, confirmed the presence of about 40 of these structures. No similar formations had previously been documented in the Great Lakes. The researchers began considering possible explanations, including sinkholes, which form when water dissolves underlying limestone and the surface collapses. Lake Michigan does sit atop a limestone foundation, and similar features have been found in Lake Huron.

However, the uniformity of the sizes and their unusual pattern raised questions about that theory. A second hypothesis involved glacial activity from the last ice age. The movement of massive ice sheets across the region could have carved deep depressions into softer bedrock. While this explanation is plausible given the region’s geological history, researchers noted that glacial action typically produces crescent-shaped grooves rather than the large, circular craters observed here.

The craters may also hold scientific value beyond their origin. Deep, low-oxygen zones in the lake resemble certain ancient ocean environments. Scientists believe that studying the microorganisms and other life in these areas could offer clues about how early life forms thrived on Earth before the atmosphere became oxygen-rich. The area is also home to various species, including opossum shrimp and deepwater sculpin fish, that are adapted to cold, dark conditions.

The Great Lakes have long been associated with various mysteries. Around 6,000 shipwrecks are known to lie in their waters, and in 2007, researchers discovered an ancient stone structure in Grand Traverse Bay, estimated to be about 9,000 years old. A stone slab at the site bears a carving that some believe depicts a mastodon.

Only about 15 percent of the Great Lakes floor has been explored, according to researchers. Projects like Lakebed 2030 aim to map the entire underwater terrain, which could lead to further discoveries about the region’s geological past and the history preserved beneath its waters.