Explorers mapping the bottom of Lake Michigan have identified dozens of large circular depressions in the lakebed. The discovery was made using sonar equipment, and researchers are still determining what created the formations.

In 2022, a team of researchers was using sonar to map the lake floor and search for shipwrecks. They expected to see standard rock formations and sand deposits. Instead, the scans revealed a series of large, nearly perfect circular holes.

The depressions are located at a depth of about 500 feet, where the water is dark. They range from 300 to 600 feet in width and reach depths of about 40 feet. The team counted roughly 40 of the features, which appear to be arranged in patterns that some observers have compared to footprints.

A closer inspection was conducted in August 2024 using a remotely operated vehicle. No one has previously documented similar formations in the Great Lakes. Researchers have proposed several theories to explain their origin.

One possibility is that the holes are sinkholes. Lake Michigan sits on a limestone foundation. Limestone erodes relatively easily when water flows through it, which can create underground voids. If the surface above a void collapses, it forms a hole. Similar sinkholes have been found in Lake Huron.

There are openings in this theory. It is unclear why all the Lake Michigan formations are so large, and sinkholes are rarely found beneath lakes. The holes also appear to align in a loose pattern heading south, which is unusual for a natural collapse.

A second theory involves glaciers. During the last ice age, glaciers covered much of the northern United States. The heavy ice moved slowly across the landscape, grinding and scraping the ground beneath it. This process created deep depressions called glacial basins. Varied underlying rock hardness might explain why some holes are deeper than others. However, glacial erosion typically produces crescent-shaped grooves rather than large circular holes.

The formations may also hold scientific value beyond solving a geological puzzle. Some parts of Lake Michigan are low in oxygen, similar to deep ocean environments. The dark, low-oxygen conditions within the holes could support specialized organisms, such as deep-water crustaceans, bottom-dwelling fish, and filter-feeding mussels. They might even host microbes adapted to harsh conditions, similar to those found in Lake Huron.

Studying these environments could provide insight into how life thrived in the low-oxygen oceans of early Earth.

Large portions of the Great Lakes remain unmapped, and scientists are planning further studies of the Lake Michigan formations. The “Lakebed 2030” initiative aims to map the entirety of the lakebed. Researchers hope that future analysis of the surrounding sediment may explain the origin of the holes.