The sinkholes likely formed through typical geological processes.
The National Oceanic and Atmospheric Administration's (NOAA) Great Lakes Environmental Research Laboratory (GLERL) has identified approximately 40 circular structures at the bottom of Lake Michigan, Live Science reported.
In 2022, NOAA researchers surveying the Great Lakes used sonar to detect unusual circular depressions on the lakebed. These depressions, measuring between 91 meters (300 feet) and 183 meters (600 feet) across, also caught the attention of a shipwreck hunter at the time, although scientists have only recently been able to examine them more closely.
During an August expedition, the team discovered around 40 of these features, which are believed to be sinkholes. These formations were found approximately 22.5 kilometres (14 miles) southeast of Sheboygan, aligned in a line extending toward Port Washington.
"There were dozens of them in our search grid," said Brendon Baillod, the shipwreck hunter who first noticed them in 2022, in an interview with Live Science. "Most ranged between 500 and 1,000 feet [152 to 309 meters] in diameter and were irregular in shape."
The sinkholes likely formed through typical geological processes.
"Sinkholes are most common in what geologists refer to as 'karst terrain'," the US Geological Survey notes. "In these regions, certain types of rock beneath the surface can naturally dissolve due to groundwater circulation. Soluble rocks include salt beds, gypsum, limestone, and other carbonate formations."
Lake Michigan's limestone bedrock helps explain the creation of these sinkholes. While their formation is straightforward, these structures are far from ordinary. Similar sinkholes in Lake Huron host unique microbial communities, thriving in oxygen-poor, sulfur-rich groundwater, resembling life on Earth about 2.5 billion years ago.
"Much like early Earth, microorganisms dominate the Middle Island Sinkhole in Lake Huron. Scientists recognize three major domains of life: Bacteria, known for their diversity and adaptability; Archaea, noted for their ability to survive in extreme conditions and produce methane; and Eukarya, which includes plants, animals, fungi, and protists," states a study on Lake Huron sinkholes.
On the lake floor, bacteria form a purple mat.
"DNA analysis revealed that the majority of the purple mat consists of Phormidium autumnale, a photosynthetic cyanobacterium. These filamentous organisms use light energy to convert dissolved carbon dioxide (CO2) into organic carbon."
These microorganisms are found in diverse environments, from Yellowstone National Park to Antarctica's ice-covered lakes.
"We also discovered methane-producing Archaea in the sediment and several Eukaryotic species within the mat. While many DNA sequences resemble known organisms, some represent novel and potentially undiscovered microbial life forms."
Further research at Lake Michigan is necessary to determine if it harbours similar life forms, though GLERL scientists told the Milwaukee Journal Sentinel they believe the sinkholes are similar to those in Lake Huron and may provide insights into early Earth conditions.