One of the most amazing discoveries about the Great Lakes is that there are so many discoveries to be made.

The latest: sinkholes at the bottom of Lake Michigan.

On Aug. 21, a team of scientists confirmed there are more than 40 sinkholes on the lakebed in the Wisconsin Shipwreck Coast National Marine Sanctuary.

This isn’t the first time sinkholes have been found in the Great Lakes. In 2001, scientists found sinkholes at the bottom of Lake Huron in the Thunder Bay National Marine Sanctuary. 

Lake Huron’s sinkholes have attracted the attention of scientists ever since, especially once they found dancing bacteria in them. 

You read that right. It’s kind of like a tango.

So, how did scientists find the Lake Michigan sinkholes? How big are they? And do they know what’s hiding in them?

We answer 10 questions. 

More: We know more about the surface of Mars than about the floor of Lake Michigan. But what we do know is remarkable.

When were Lake Michigan’s sinkholes discovered?

In 2022, researchers with the National Oceanic and Atmospheric Administration were out surveying the lakebed in the Wisconsin’s marine sanctuary when circular depressions showed up on sonar scans. 

The depressions stretched for miles, piquing everyone’s curiosity, said Russ Green, superintendent with the marine sanctuary. A shipwreck hunter who was out looking for a wreck also noted them at about the same time, Green said. 

The first time scientists were able to take a closer look was on Aug. 21. Using a remotely operated vehicle, they confirmed they are in fact sinkholes. 

They were “perfect, little circles,” said Steve Ruberg, a scientist at the National Oceanic and Atmospheric Administration’s Great Lakes Environmental Research Laboratory. 

More: Wisconsin’s national marine sanctuary is a museum beneath the water. Here’s what to know.

Where are the sinkholes?

The sinkholes are about 14 miles southeast of Sheboygan, roughly 450 feet below the surface. They extend south in a line towards Port Washington. 

How big are they?

The sinkholes range in size from 300 to 600 feet across. 

The scientists found roughly 40, although Ruberg said there are likely more. 

How do sinkholes in the Great Lakes form?

Whether it’s on land or on the lake bottom, sinkholes form when water dissolves rock, causing the surface layer to collapse and form a hole. 

Lake Michigan sits on a layer of limestone bedrock. Over time, groundwater flowing under the lake erodes the bedrock, forming caverns, Ruberg said. Eventually, parts of the ceilings collapsed, forming sinkholes. 

Does anything live in the sinkholes?

Not many organisms can withstand the cold, dark and oxygen-deprived conditions.

However, the temperature holds at a fairly constant 38 degrees Fahrenheit at the bottom, Ruberg said, and there are a few critters that “doing their thing in the dark down there.” There are freshwater shrimp, known as opossum shrimp as well as deepwater sculpin, which is a small fish. And of course, invasive quagga mussels survive. 

Scientists expect there are also bacteria that can handle extreme environments, but they have yet to explore what kinds.

Can the sinkholes cause problems in the lake?

It’s not yet clear how the sinkholes contribute to the lake’s ecosystem, chemistry and water levels. 

In Lake Huron, the groundwater that flows through the sinkholes have high levels of salt and sulfur, Ruberg said. 

It’s possible that the groundwater seeping in through the sinkholes will contribute to lake levels in a “very small way, but it’s part of the whole equation we use to see where the lake levels are potentially going to go,” Ruberg said. 

So far, the scientists haven’t found groundwater coming out of the sinkholes that they were able to explore, but they will likely find some when they get out deeper, Ruberg said.

There are dancing bacteria in Lake Huron’s sinkholes?

There sure are. But scientifically, it’s referred to as microbial migration.

At the bottom of Lake Huron’s Middle Island sinkhole, there are mats of purple-colored cyanobacteria that produce oxygen from sunlight. There are also white-colored bacteria that eat sulfur to get energy. The bacteria are a kind of carpet on the lake floor.

Scientists discovered that the bacteria flip-flop twice a day to compete with each other. In the early evening hours, the purple bacteria rise, blocking the white bacteria’s access to sunlight. When the sun comes out, they switch positions; the white sulfur-eaters move below and the purple cyanobacteria can start producing oxygen again. 

The two bacteria do that “tango” every day, completely changing the color of the mat, said Greg Dick, a professor at the University of Michigan and director of the Cooperative Institute of Great Lakes Research.

It’s not uncommon to see animals migrate on a daily basis, Dick said, but it’s not often seen with microbes.

When it comes to the sinkholes in Lake Michigan, scientists have just begun to explore them. So it’s unclear whether bacteria do a daily dance − or other unheard of things.

What will scientists explore next?

Ruberg believes the sinkholes in Lake Michigan are probably similar to the ones in Lake Huron, but scientists won’t know for sure until they investigate further — one of their many next steps. Another avenue of exploration will be to see how much salt and sulfur is seeping into Lake Michigan, he said. 

But Ruberg said there is a chance they will find something that’s never been seen in the Great Lakes before. 

What can we learn from Great Lakes sinkholes?

The sinkholes are valuable natural laboratories. 

“It’s an extreme environment,” Dick 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.”

The dark, low-oxygen and sulfur-rich conditions in the deep sinkholes are similar to early conditions on Earth, Dick said. They can help scientists learn more about Earth’s early history before there was oxygen. 

More: Want to explore a Lake Michigan shipwreck? Wisconsin’s marine sanctuary just made it easier.

Haven’t the bottom of the Great Lakes been explored?

Not really. 

In fact, only 15% of the bottom of the Great Lakes has been mapped in high resolution. Scientists have said they know more about the surface of Mars than they do about the bottom of the largest fresh surface water system on earth.

But that may soon change. 

The Lakebed 2030 Initiative by the Great Lakes Observing System, or GLOS, is an effort by scientists, agencies and other organizations to map and fully explore the lake bottoms. 

And two Michigan representatives proposed a bipartisan bill earlier this year that would authorize $200 million to map the bottoms of all five lakes. The lakes could be mapped within eight years with proper funding, according to a 2021 report by the observing system. 

Until then, there will likely be many more discoveries. 

More: Lake Michigan shipwreck hunters discover historic schooner unseen for 131 years

Caitlin Looby is a Report for America corps member who writes about the environment and the Great Lakes. Reach her at clooby@gannett.com or follow her on X @caitlooby.

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