In 2022, a team of researchers set out to map the floor of Lake Michigan and search for shipwrecks. They expected typical rocky terrain and sandy patches. Instead, their sonar screens revealed vast, perfectly circular craters scattered across the lakebed, sitting about 500 feet beneath the surface.
The formations are enormous, ranging from 300 to 600 feet wide and plunging nearly 40 feet deep. Dozens of them are arranged in a loose line that extends generally southward, almost like footprints. In August 2024, a remotely operated vehicle captured a closer look at roughly 40 of these craters, and no one had ever seen anything like them in the Great Lakes.
Scientists have offered several theories, but none fully explain the mystery. One possibility is that they are sinkholes, or dolines, which form when surface layers collapse into voids created by water dissolving limestone bedrock. Lake Michigan does sit on limestone, which erodes easily, and similar formations exist in Lake Huron. However, it is rare for sinkholes of this size to occur in a lake bed, and the fact that they form a loose, straight line defies usual geological rules.
Another theory points to glaciers. During the last ice age, massive rivers of ice covered the region, acting like colossal bulldozers. As they moved, they could carve deep gouges into softer or fractured bedrock, leaving depressions behind. But glacial erosion typically creates rounded, crescent-shaped grooves known as chatter marks, not massive circular craters. Once the glaciers melted, the Great Lakes formed, and these craters would have simply filled with water.
The craters may hold more than geological curiosity. The dark, low-oxygen conditions of deep Lake Michigan resemble ancient Earth environments where some of the earliest life forms thrived. Scientists believe the craters could host specially adapted organisms, such as opossum shrimp, deepwater sculpin fish, and invasive quagga mussels, as well as microbes and bacteria. Studying these creatures could offer insight into how life flourished in Earth’s ancient, oxygen-poor oceans.
Lake Michigan has long carried a ghostly reputation. It is home to roughly 6,000 shipwrecks, preserved by cold, fresh water. Over the years, the lake has also been linked to unexplained disappearances of boats and planes, sudden storms, eerie fogs, and ghostly lights. In 2007, researchers discovered a prehistoric structure in Grand Traverse Bay, lying 40 feet below the surface, that has been compared to Stonehenge.
That structure is estimated to be about 9,000 years old, thousands of years older than England’s Stonehenge. Massive stones are arranged in a careful, winding pattern stretching over a mile, with one boulder etched with the image of a mastodon, a creature that went extinct around 11,000 years ago. The site likely sank during the end of the last ice age, when the land was still dry. It may have served as hunting blinds or an ancient calendar, but no one knows for sure.
Only about 15% of the Great Lakes floor has been explored. Scientists are now planning deeper dives and initiatives like Lake Bed 2030, which aims to map the entire underwater landscape, hoping to uncover what other secrets the dark waters hide.
Other discoveries in the Great Lakes have reshaped understanding of ancient life. In Lake Huron, researchers found two small flakes of obsidian buried in mud 100 feet underwater, near the Alpena-Amberly Ridge, a land bridge that once connected Michigan and Ontario. The flakes came from central Oregon, about 2,500 miles away. They were clearly crafted by human hands and were part of a vast trade network that spanned the continent around 10,000 years ago.
The same submerged site has yielded ancient trees with intact roots and peat bogs filled with preserved seeds and leaves. Researchers are working with scientists in Cambridge, England, to extract DNA from the remains, which could confirm what animals lived there. The cold, fresh water acted like a time capsule, preserving evidence of an ice-age civilization that researchers have been studying for more than a decade.
The lakes have also produced dozens of ancient canoes. At the bottom of Lake Mendota near Madison, Wisconsin, researchers found 11 canoes, with the oldest dating back to 2500 BCE. The discovery suggests an ancient village may be hidden underwater, along with stone tools like net sinkers used for fishing.
Across the world, other underwater sites have sparked similar fascination. Archaeologists believe the oldest sunken city ever found is Pavlopetri, off the coast of Greece, which existed around 5,000 years ago. The Bronze Age city lies under 10 to 13 feet of water and once had a population of 500 to 2,000 people. Scientists believe an earthquake sank it, and some have connected it to the legend of Atlantis.
Near Japan, the Yonaguni Monument sits 82 feet underwater, a massive stone structure with perfectly shaped steps and sharp edges. Some experts believe it may be more than 10,000 years old, possibly built by a lost civilization. Skeptics argue the formations are natural, caused by earthquakes and erosion in the tectonically active Pacific Ring of Fire. One marine geologist who explored the site claims to have found tool marks and symbols carved into the rock, suggesting an ancient culture from mainland Asia. He has identified ten different structures, including what he believes are a ruined castle, a triumphal arch, and five temples.
Meanwhile, researchers have found another stone circle in Lake Michigan’s Grand Traverse Bay, which some have compared to Stonehenge. The array of massive stones predates the Egyptian pyramids and England’s Stonehenge by about 4,000 years. One stone bears a clear carving of a mastodon, meaning the builders lived at the same time as those creatures. Some believe the circular formation was a hunting trap used to herd large animals, while local Native American tribes say the stones are sacred and were placed by their ancestors to honor the land.
Lake Michigan continues to give up its secrets. In 2024, the Wisconsin Underwater Archaeological Association announced the discovery of the wreck of the Margaret A. Muir, a 130-foot schooner that sank in 1893 during a sudden storm. All crew members survived, but the ship sat undisturbed on the lake bed for over 130 years until researchers found it using historical research and high-resolution sonar.
The Great Lakes have even produced structures similar to ancient monuments in Europe. Under the waters of Lake Constance in Switzerland, archaeologists found a line of 170 man-made stone cairns dating back about 5,500 years, around the same time England’s Stonehenge was being built. Some of the formations are several dozen feet wide and up to 6 feet tall, likely built by several villages for a collective purpose.
Beyond the Great Lakes, the world’s waters hide other enigmas. In Mexico, the Yucatan Peninsula is riddled with cenotes, bottomless sinkholes filled with crystal-clear water that the Mayans believed were gateways to the underworld. Thousands of collapsed limestone bedrock formations create one of the world’s longest underwater cave systems.
Off the coast of Belize lies the Great Blue Hole, a giant sinkhole about 1,000 feet across and more than 400 feet deep. It was once a cave until rising waters filled it and caused it to collapse. Divers can see stlagmites and stalactites in the dark depths, but entering the tunnels is extremely dangerous.
Off the Pacific Northwest coast, nearly 500 streams of methane bubbles have been discovered, nicknamed the champagne seeps. Scientists are still puzzling over where the methane comes from and why it is warmer than the surrounding ocean. Methane is highly flammable, which has led some to wonder what would happen if a bubble were ignited.
In Iceland, divers can enter Silfra, a fissure filled with pure glacier water, the only underwater place on the planet where you can touch two continents at once. The water temperature hovers around 35 degrees Fahrenheit year-round, and divers risk hypothermia or worse.
Scotland’s Gulf of Corryvreckan contains the third-largest permanent whirlpool on the planet. Water moving through the narrow strait at breakneck speed crashes against underwater rocks, creating waves up to 15 feet high. The roaring maelstrom can be heard from miles away and has swallowed dozens of sailors over the centuries.
In Antarctica, researchers drilling through the Ross Ice Shelf, roughly the size of Spain, found dozens of mysterious species living in the extreme conditions below, including upside-down swimming fish and sea anemones hanging in the frigid waters. The discovery showed that life can thrive even in the most hostile environments.
The Bermuda Triangle, the area between Bermuda, Florida, and Puerto Rico, has been linked to countless disappearances of ships and planes. The mystery gained attention in 1945 when five US Navy planes vanished during a training flight, and a rescue plane with 13 people on board never returned. Some blame magnetic anomalies or massive pockets of flammable methane gas, while experts argue the area simply has heavy traffic, making accidents more likely.
Off the coast of Cuba, divers have found circular columns, paved floors, and symmetrical stone structures more than 2,000 feet below the surface. Some believe these are the ruins of a long-forgotten civilization, but scientists say the disc and donut-shaped structures are natural phenomena created by gases breaking free from the seafloor.
The Sargasso Sea, in the middle of the North Atlantic, is surrounded by powerful ocean currents and filled with dense brown seaweed. In the 19th century, a French ship was found there with its sails set but no crew on board, fueling rumors that the seaweed was carnivorous. The sea remains eerily calm and unnaturally warm.
Some have wondered whether the megalodon, the largest marine predator ever, could still lurk in places like the Mariana Trench, Greenland, or Mexican cenotes. But the giant shark preferred warm, shallow waters, and no proof exists that it survives today.
While mysteries of the deep continue to intrigue, there have also been strange phenomena on land and in the sky. The Kawah Ijen volcano in Indonesia produces electric blue lava, the result of high levels of sulfur interacting with scorching air and molten rock. The volcano also contains the world’s largest acid lake. The Dead Sea is so salty that no macroscopic organisms can live there, only bacteria and fungi. An underground crystal cave in Mexico holds some of the largest crystals in the world, with spikes measuring up to 35 feet long and weighing 55 tons.
The Georgia Guidestones, a collection of giant stones in a star pattern with inscriptions in eight languages, were built in 1980, but no one knows who built them or why. Easter Island’s famous stone heads have stood for hundreds of years, and soil analysis suggests the local people believed the statues would make the soil more fertile. In Saudi Arabia, the Al-Naslaa rock is split perfectly in half by nature. Blood Falls in Antarctica is a red waterfall caused by extreme salted water mixed with iron oxide.
About 4% of people around the world report hearing a strange, persistent low-frequency hum. Known as the Hum, it has been reported in cities like Taos, New Mexico, and Bristol, England. The sound follows people everywhere and disrupts sleep and concentration. Scientists have been unable to confirm a cause, though theories range from seismic activity and tectonic plates to submarines, lightning strikes, and the jet stream. Some believe the hum is simply a hallucination created by the brain, a response to the constant background noise of modern life. No one has found the truth yet.
