Scientists confirm there are 40 huge craters at the bottom of Lake Michigan

Two years ago, sonar images revealed strange circles at the bottom of Lake Michigan that scientists couldn't explain. Now, a survey has determined the shapes are giant holes — but there are many more secrets left to unravel, researchers say.

The holes were first discovered in 2022, when researchers embarked on a mission to map the lakebed inside the Wisconsin Shipwreck Coast National Marine Sanctuary, a protected area of Lake Michigan that contains 36 known shipwrecks, and may contain many more. Weird circles appeared on the map that looked natural rather than human-made, Russ Green, a maritime archaeologist and superintendent of the sanctuary who took part in the mapping project, told Live Science in an email. The shapes were likely depressions in the lakebed, but the researchers couldn't be sure.

"Any kind of new discovery in the Great Lakes is exciting," Green said. "But these features really stand out — they are in deeper water (500 feet [150 meters] ish) and weren't known before, as far as we can tell."

Brendon Baillod, a local shipwreck hunter, spotted the Mysterious circles around the same time as Green and his colleagues while searching for a sunken freighter. To his eyes, the circles were clearly depressions, or craters, measuring between 20 and 40 feet (6 to 12 m) deep, Baillod told Live Science in an email. "There were dozens of them in our search grid," he said. "Most were 500 to 1,000 feet [150 to 300 m] in diameter and of irregular shapes."

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There was a lag in processing data from the initial mapping expedition, but Green, Baillod and their colleagues eventually contacted scientists at the National Oceanic and Atmospheric Administration's (NOAA) Great Lakes Environmental Research Laboratory (GLERL), Green said.

This year, the research teams conducted a joint survey to examine the circles more closely. On Aug. 21, they used a remotely operated vehicle to confirm that the shapes are enormous, naturally-occurring craters. Scientists counted roughly 40 of them, but there are likely more to be found, Steve Ruberg, a researcher at GLERL, told the Milwaukee Journal Sentinel.