Something strange lurks beneath Lake Michigan. Five hundred feet below the waves, hidden in absolute darkness, a mystery waited for years to be found. And when researchers finally spotted it, they had no idea what they were looking at.

Picture yourself standing at the edge of one of America’s Great Lakes. Waves lap against the shore. Sailboats drift across the horizon. Everything seems ordinary, peaceful, familiar. But deep beneath that surface, scattered across the lakebed like fingerprints left by an ancient giant, sit roughly 40 massive circular structures that scientists are only beginning to understand.

How did they get there? What created them? And what secrets might they hold about our planet’s distant past?

A Discovery Nobody Expected

Back in 2022, a team of researchers set out on what seemed like a routine mission. Their goal was simple enough. Map the lakebed inside Wisconsin Shipwreck Coast National Marine Sanctuary, a protected stretch of Lake Michigan that contains at least 36 known shipwrecks and possibly many more waiting to be found.

Maritime archaeologist Russ Green led the mapping project. His team deployed sonar equipment and watched as images of the lakebed appeared on their screens. Shipwrecks were expected. What appeared instead caught everyone off guard. Circles. Dozens of them. Perfect, strange, unexplained circles dotting the lake floor.

Green and his colleagues studied the images. Were these human-made structures? Ancient formations? Something else entirely? At first glance, the shapes looked natural. But in waters this deep, nothing could be certain without a closer look.

“Any kind of new discovery in the Great Lakes is exciting,” Green said. “But these features really stand out. They are in deeper water and weren’t known before, as far as we can tell.”

Around that same time, a shipwreck hunter named Brendon Baillod was scanning the lakebed for an entirely different reason. He was searching for a sunken freighter when his equipment picked up the same strange formations. To his trained eye, these circles looked like depressions, massive craters carved into the lake floor.

Baillod counted them as they appeared on his screen. “There were dozens of them in our search grid,” he recalled. “Most were 500 to 1,000 feet in diameter and of irregular shapes.” Two separate teams. Two separate missions. One shared mystery.

Getting Eyes on the Unknown

Processing data takes time. For two years, scientists analyzed information from that initial expedition while curiosity mounted. Green and Baillod eventually connected with researchers at NOAA’s Great Lakes Environmental Research Laboratory to plan a joint investigation.

In August 2024, the teams finally got their chance. Armed with a remotely operated vehicle, they descended into the cold, dark waters about 14 miles southeast of Sheboygan, Wisconsin. What they confirmed would soon make headlines. Those strange circles were real. And they were enormous.

Researcher Steve Ruberg described them as “perfect, little circles” scattered across the lakebed. Scientists counted roughly 40 of these formations, each one measuring between 300 and 600 feet across. Some plunged 20 to 40 feet deep into the sediment. And they stretched in a line running south toward Port Washington.

But here’s what made the discovery even more intriguing. Ruberg believes there are likely more craters beyond the survey area, still waiting in the darkness.

What Created These Massive Holes?

Now comes the question everyone wants answered. What carved these craters into the bottom of Lake Michigan? One theory points to sinkholes. Similar formations were discovered in Lake Huron back in 2001, and scientists eventually confirmed that those were sinkholes. Here’s how they form. Groundwater flows beneath the lake and slowly dissolves the bedrock over time. Limestone is especially vulnerable to this process. Eventually, the rock weakens so much that the surface layer above it collapses, creating a hole.

Lake Michigan sits on a layer of limestone bedrock. Given this geology, many researchers suspect sinkholes are the most likely explanation.

But not everyone agrees. Baillod prefers a different term until more evidence surfaces. “I think they 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,” he explained.

During their August expedition, scientists found no evidence of groundwater escaping from the holes. If these were active sinkholes, you might expect to detect water circulating through them. Ruberg, however, believes groundwater will eventually be found once researchers explore deeper sections.

For now, the debate continues. Sinkholes or craters? Ancient formations or ongoing geological processes? Each answer leads to more questions.

Life in a Cold, Dark World

Imagine living 500 feet below the surface of Lake Michigan. No sunlight reaches you. Water temperature hovers around 38 degrees Fahrenheit year-round. Oxygen levels are low. Most creatures would find survival impossible here. Yet life persists.

When the remotely operated vehicle descended into the craters, cameras captured organisms going about their business in the blackness. Freshwater shrimp, also known as opossum shrimp, scuttled along the sediment. Small fish called deepwater sculpin darted through the water. And of course, invasive quagga mussels clung to every available surface, surviving where many native species cannot.

Scientists expect bacteria adapted to extreme conditions also thrive in these depths. But biological surveys have only just begun. What else might call these craters home remains unknown.

Clues from Lake Huron’s Ancient Ecosystem

Want a glimpse of what scientists might find in Lake Michigan? Look east to Lake Huron.

When researchers explored sinkholes in Lake Huron’s Thunder Bay National Marine Sanctuary, they discovered something remarkable. Mats of bacteria carpeted the sinkhole floors, creating an ecosystem unlike anything else in the Great Lakes.

Purple cyanobacteria and white sulfur-eating bacteria live together on these mats. And twice each day, they perform a kind of dance. In the evening hours, white bacteria rise to the surface, blocking the purple cyanobacteria from any remaining light. When morning arrives, they switch positions. Purple bacteria reclaim the top layer while white bacteria sink below. Scientists call it microbial migration. Everyone else might just call it incredible.

But here’s what makes these bacteria even more significant. Low-oxygen, sulfur-rich conditions in the sinkholes mirror what Earth looked like roughly 2.5 billion years ago, long before oxygen filled our atmosphere. By studying these environments, researchers can peer into our planet’s ancient past without leaving the Midwest.

Greg Dick, a professor at the University of Michigan and director of the Cooperative Institute of Great Lakes Research, put it best. “It’s an extreme environment,” he said. “We typically have to go to Antarctica or Yellowstone National Park or some exotic location to get these extreme ecosystems, but this is in our backyard in the Great Lakes.”

Could Lake Michigan’s craters harbor similar bacterial communities? Could they offer their own window into Earth’s early history? Scientists believe the potential is real. Answers will come with time and continued exploration.

What Happens Next

Discovery marks only the beginning. Now comes the hard work of understanding what these craters mean for Lake Michigan and the millions of people who depend on it.

Researchers plan to measure salt and sulfur levels in the water near the craters. In Lake Huron, groundwater seeping through sinkholes carries high concentrations of both. If similar chemistry exists in Lake Michigan, it could affect the lake’s ecosystem in ways scientists have never considered.

Water levels are another concern. Groundwater flowing into the lake, even in small amounts, contributes to the complex equation that determines whether water levels rise or fall. Understanding how much water moves through these craters could improve predictions for communities along the shoreline.

And then there’s biology. What bacteria live in these craters? What other organisms might have adapted to such extreme conditions? Could something entirely new to science be waiting in the darkness? Ruberg believes there’s a real chance researchers will find life forms never before seen in the Great Lakes.

Green shares that sense of anticipation. His team plans to return to these craters for years to come, driven by curiosity and the promise of discovery.

A Reminder of How Much We Don’t Know

Here’s a number that should stop you in your tracks. Only 15 percent of the Great Lakes floor has been mapped in high resolution. Let that sink in for a moment.

Scientists have mapped the surface of Mars with greater detail than the bottom of the largest freshwater system on Earth. We send rovers to distant planets while mysteries wait just a few hundred feet below the waves of lakes we drive past every day.

But change may be coming. An effort called the Lakebed 2030 Initiative aims to map and explore the bottom of all five Great Lakes. Earlier this year, two Michigan representatives introduced a bipartisan bill that would authorize $200 million to complete this work. With proper funding, scientists estimate they could finish mapping within eight years. Until then, discoveries like these Lake Michigan craters remind us how much remains hidden in our own backyard.

What Does It Mean for You?

Maybe you’ve never visited Lake Michigan. Maybe you’ve swum in its waters your whole life. Either way, these 40 craters carry a message worth hearing.

Our planet still holds secrets. Mysteries wait in places we think we know. And every discovery, every question answered, every new puzzle found, reminds us that curiosity remains one of humanity’s greatest gifts.

So the next time you stand at the edge of a lake, an ocean, or any body of water, remember this. You’re looking at a surface. Beneath it lies a world we’ve barely begun to explore. And sometimes, the most amazing discoveries are hiding right beneath our feet.

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