The National Oceanic and Atmospheric Administration’s (NOAA) Great Lakes Environmental Research Laboratory (GLERL) has confirmed the existence of around 40 circular structures at the bottom of Lake Michigan.
In 2022, researchers at NOAA were surveying the Great Lake when they spotted strange circular depressions in the lakebed using sonar. The depressions, ranging from 91 meters (300 feet) to 183 meters (600 feet) across, caught the attention of a shipwreck hunter at about the same time, though scientists only recently had a chance to get a better look at them.
Undertaking an expedition in August, the team found around 40 of the marks, presumed to be sinkholes, around 22.5 kilometers (14 miles) southeast of Sheboygan stretching in a line towards Port Washington.
“There were dozens of them in our search grid,” Brendon Baillod, the shipwreck hunter who saw them in 2022, told Live Science. “Most were 500 to 1,000 feet [152 to 309 meters] in diameter and of irregular shapes.”
The sinkholes likely formed through usual geological processes.
“Sinkholes are most common in what geologists call, ‘karst terrain’,” the US Geological Survey explains. “These are regions where the types of rock below the land surface can naturally be dissolved by groundwater circulating through them. Soluble rocks include salt beds and domes, gypsum, limestone and other carbonate rock.”
Beneath Lake Michigan is limestone, explaining how they were created. Though their formation is fairly simple to explain, these sinkholes are far from boring. Similar sinkholes have been found in Lake Huron, where the oxygen-poor and sulfur-rich groundwater sustains microbial communities resembling life as it would have been around 2.5 billion years ago on Earth.
“Much like early Earth, microorganisms dominate the Middle Island Sinkhole in Lake Huron. Scientists now recognize three domains, or superkingdoms, on the tree of life: the Bacteria, physiologically versatile, highly successful, and enormously diverse; the Archaea, renowned for their ability to thrive in extreme environments and perform unique physiologies such as methane generation; and Eukarya, the plants and animals (like us) as well as fungi and protists,” a paper on the Lake Huron sinkholes explains.
Here, the bacteria create a purple mat on the lake floor.
“DNA analysis determined that most of the purple mat microorganisms are of a strain of Phormidium autumnale, a photosynthetic cyanobacterium. These long, filamentous organisms harvest light energy and create organic carbon from carbon dioxide (CO2) dissolved in the groundwater.”
These organisms are found from Yellowstone National Park to ice lakes in Antarctica permanently covered by ice.
“We also found methane-producing Archaea deep within the sediment, and several Eukaryotic species that grow in the mat. While most of their DNA sequences are similar to known organisms, some are new sequences that may represent novel and undiscovered microbial life forms.”
More study is needed at Lake Michigan to see what kind of life it too could harbor, though scientists at GLERL told the Milwaukee Journal Sentinel that they suspect the sinkholes are similar in nature, and could help us learn more about the conditions on early Earth.
