In 2002, divers in Lake Huron were swimming among the shipwrecks in the Thunder Bay National Marine Sanctuary. The sanctuary protects the remains of almost 100 ships that met their demise when trying to navigate through “shipwreck alley.”The divers saw the usual sunken schooner. The overturned steamer. The remains of the tugboat. But there was something else. There appeared to be a massive sinkhole near Middle Island, about a mile off the coast of Michigan.Scientists sent down a remotely operated vehicle (ROV) to get closer to the crater. They soon realized they were looking at an ancient sinkhole, and in the coming years, they would learn there were many others in the Great Lakes.As technology has enabled researchers to get deeper into the sinkholes, scientists have discovered they are time capsules that offer a glimpse into what life was like billions of years ago.Studying Underwater SinkholesUnderneath Lake Huron is an ancient basin that formed 400 million years ago with dolomite, gypsum, and limestone. Over time, these bedrocks have shifted, cracked, and formed corrosion that created caves. Eventually, the cave ceilings collapsed and left behind sinkholes.Depending on the depth, the sinkholes may be light-penetrating. These are typically found closer to shore. Light cannot penetrate some sinkholes further away from shore, like the Middle Island Sinkhole.“The way I would describe them is unique, otherworldly,” says Steven A. Ruberg, a research physical scientist with the National Oceanic and Atmospheric Administration (NOAA) in the Great Lakes Environmental Research Lab in Ann Arbor, Michigan.If a person looked at the camera footage from a ROV, Ruberg says they would first notice the water becoming a little cloudy as the vehicle descended deeper into the lake. This is because the water in the sinkhole is heavier than the freshwater, and the interaction between the two makes it appear like a hazy cloud.As the ROV went lower, the camera might see craters or raised ridges. About 70 feet down, the rover would see purple clumps along the lakebed formed by cyanobacteria, a microorganism that thrives in habitats like sinkholes that are high in sulfate and low in oxygen. The cyanobacteria live alongside chemosynthetic bacteria, which are white in appearance.The two microorganisms have developed a day-shift, night-shift type of swap. Whereas the cyanobacteria photosynthesize and seek light during the day, the chemosynthetic bacteria are into sulfur and mobilize after dark. “On a daily basis, the purple cyanos will rise up to get the light. At night, the white chemosynthetic ones will rise up,” Ruberg says.These microorganisms have fascinated scientists because of what Ruberg calls the “symbiotic relationship.” But they have also intrigued researchers by serving as a living time capsule and offering a glimpse into what life was like billions of years ago.Life Without OxygenSinkholes like the Middle Island one don’t have oxygen. Neither did Earth 2.5 billion years ago at the start of the Proterozoic period.“The early Earth would have had sulfur and salt, of course, but zero oxygen in the water because the atmosphere didn’t have oxygen in it,” Ruberg says.Scholars studying sinkholes suggest that oxygenation may have occurred as Earth’s rotation slowed. In a 2021 study in Nature Geoscience, researchers considered the purple cyanobacteria in the Middle Island sinkhole and suggested that as Earth began spinning more slowly, photosynthesizing cyanobacteria released more oxygen.Billions of years ago, a typical day may have been only six hours long. That was less time for the purple cyanobacteria to commute to the surface for photosynthesis. Over time, Earth’s rotation slowed, days grew longer, and the cyanobacteria gave off more oxygen. The increased oxygen changed the atmosphere and set the stage for Earth to evolve into life as we know it.The unique terrain and chemistry of sinkholes may give us clues to early Earth conditions. (Image Courtesy of Tane Casserley/NOAA)Sinkholes Help Us Understand EarthSinkholes are changing how scientists understand Earth, but there is a lot more they would like to know about the sinkholes themselves. In order to do so, scientists need to have a better sense of where they are located in the Great Lakes. A seafloor mapping initiative began in 2019, and scientists are using multibeam sonars to map the U.S. coastlines and the Great Lakes.So far, only 15 percent of the lake beds have been mapped, which means there could be sinkholes in Lake Michigan or Lake Superior that are unknown to scientists. Further studying the sinkholes could help scientists better understand Earth’s evolution, and Ruberg says they can also help researchers better predict the lakes.Predicting the lakes involves understanding circulation, which is helpful for search and rescue crews trying to locate missing vessels or people. In the event of an oil spill or other man-made disaster, understanding circulation can help determine how the pollutants might spread. Predicting the lakes can also involve seasonal changes in water levels, which tell scientists how much water is coming into the system and what to expect.“Getting a handle on that is helpful in understanding the ecology and the biology. It helps you better understand the food web and make better decisions about the ecosystem,” Ruberg says. Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:The Nature Education Knowledge Project. Rock, Water, Microbes: Underwater Sinkholes in Lake Huron are Habitats for Ancient Microbial LifeNature Geoscience. Possible link between Earth’s rotation rate and oxygenationEmilie Lucchesi has written for some of the country's largest newspapers, including The New York Times, Chicago Tribune and Los Angeles Times. She holds a bachelor's degree in journalism from the University of Missouri and an MA from DePaul University. She also holds a Ph.D. in communication from the University of Illinois-Chicago with an emphasis on media framing, message construction and stigma communication. Emilie has authored three nonfiction books. Her third, A Light in the Dark: Surviving More Than Ted Bundy, releases October 3, 2023, from Chicago Review Press and is co-authored with survivor Kathy Kleiner Rubin.