Eerie structures discovered at the bottom of iconic US lake

Strange circles have been discovered at the bottom of one of America 's most iconic lakes , and experts are struggling to work out what exactly they are.

The enigmatic structures were first spotted in the depths of Lake Michigan two years ago, when sonar images picked them up.

Their identity stumped scientists at the time, but now a new survey has revealed that the shapes are giant holes.

And yet, the mystery by no means ends there.

Researchers discovered the holes in 2022 after launching a mission to map the lakebed inside what is known as the Wisconsin Shipwreck Coast National Marine Sanctuary.

This sanctuary is a protected area of Lake Michigan that is home to 36 known shipwrecks , and may contain many more.

Maritime archaeologist Russ Green, who took part in the project, told Live Science that he and his colleagues were stunned to see eerie circles on their map of the area, which looked natural rather than human-made.

"Any kind of new discovery in the Great Lakes is exciting," Green explained. "But these features really stand out — they are in deeper water – 500 feet (150 metres)-ish and weren't known before, as far as we can tell."

Researchers were stunned to spot the strange circles while mapping the lakebed (Wisconsin Shipwreck Coast National Marine Sanctuary)

Interestingly, at around the same time Green and his colleagues were conducting their work, Brendon Baillod, a local shipwreck hunter, also spotted the peculiar formations.

He told Live Science that, to his eyes, they were clearly depressions, or craters, measuring between 20 and 40 feet (6 to 12 metres) deep.

"There were dozens of them in our search grid," he said. "Most were 500 to 1,000 feet (150 to 300 metres) in diameter and of irregular shapes."

Green, Baillod and their respective teams eventually contacted scientists at the National Oceanic and Atmospheric Administration's (NOAA) Great Lakes Environmental Research Laboratory (GLERL) to present their findings. And, this year, they conducted a joint survey to examine the circles more closely.

On 21 August, they used a remotely operated vehicle to confirm that the shapes are giant, naturally-occurring craters and, so far, they have found around 40 of them – although more are likely to be identified.

The craters are like "perfect, little circles" lying on the lakebed around 14 miles (23 kilometres) southeast of Sheboygan, Wisconsin, extending southwards in a line towards Port Washington, researcher Steve Ruberg told the Milwaukee Journal Sentinel .

A photo, taken by a remotely operated vehicle, of the lakebed inside one of the holes at the bottom of Lake Michigan(Brendon Baillod and Dusty Klifman)

Scientists previously found similar depressions at the bottom of Lake Huron, which borders Michigan and Canada, but these turned out to be sinkholes .

Sinkholes are holes which form when water dissolves surface rock, often limestone, causing the surface layer to collapse.

Since Lake Michigan partly sits on limestone, it's likely that the craters on the lakebed are also sinkholes, Ruberg said. Nevertheless, other experts are reluctant to brand the circles sinkholes until further analysis has been carried out.

"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," Baillod said.

It remains unknown what impact the holes might be having on Lake Michigan, but researchers feel confident that this will become clear in time.

"We'll be exploring them for years to come to learn more, and sort out how they got there and what role they play in Lake Michigan's ecosystem," Green said.

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