At Bottom of Lake Michigan: Dozens of Massive Craters

NOAA researchers were conducting a sonar survey of the lakebed in the Wisconsin Shipwreck Coast National Marine Sanctuary (WSCNMS) in 2022 when they noticed strange, circular blobs. Around the same time, shipwreck hunter Brendon Baillod spotted dozens of "irregular shapes" at the bottom of Lake Michigan some 14 miles southeast of Sheboygan and running in a line southward toward Port Washington, per Live Science. So began a mystery that is slowly being unraveled. According to Baillod and other experts who performed a new survey last month, the lakebed is dotted with at least 40 massive craters, some up to 600 feet across, per IFL Science.

More are likely to be found, scientist Steve Ruberg of NOAA's Great Lakes Environmental Research Laboratory (GLERL) tells the Milwaukee Journal Sentinel. Just 15% to 20% of the lakebed has been explored, per WTMJ. Ruberg believes the craters—inhabited by shrimp, fish, and invasive quagga mussels—are sinkholes, like those found at the bottom of Lake Huron, formed by groundwater slowly dissolving the limestone, dolomite, and gypsum bedrock. But little is known for sure about what's happening some 500 feet below the lake's surface. "I think [the holes] might be more accurately called craters, which have formed in the deep bottom sediment due either to water upwelling from below or trapped hydrocarbon offgassing," Baillod tells Live Science.

Researchers, who explored the craters with a remotely operated vehicle on Aug. 21, have yet to find evidence of water escaping the holes, which would suggest groundwater is circulating beneath the lakebed, but Ruberg believes it's only a matter of time. He says researchers could find something never seen in the Great Lakes before, per the Journal Sentinel. "We'll be exploring [the craters] for years to come to learn more, and sort out how they got there and what role they play in Lake Michigan's ecosystem," says Russ Green, a maritime archaeologist and WSCNMS superintendent. "Any kind of new discovery in the Great Lakes is exciting," he tells Live Science. "But these features really stand out—they are in deeper water (500 feet-ish) and weren't known before." (More Lake Michigan stories.)