Stonehenge has some company in the department of mysterious stone structures: In a new paper, researchers are now revealing that an unusual rock formation was discovered via sonar in the Sea of Galilee a decade ago. Divers who have since gone down to inspect it say it's a 32-foot-tall cone-shaped formation made of "unhewn basalt cobbles and boulders," reports LiveScience, which describes it as a cairn—a stack of rocks piled on top of each other. Its diameter measures about 230 feet (double that of Stonehenge's outer circle), and it's estimated to weigh 60,000 tons.
Though the researchers admit they don't know what the structure's purpose was (LiveScience notes similar cairns have been used to mark graves), they have determined it was definitely made by humans, likely on land; a rising sea would have later submerged it. "The boulders have natural faces with no signs of cutting or chiseling," researchers wrote in the International Journal of Nautical Archaeology. "Similarly, we did not find any sign of arrangement or walls that delineate this structure." Up next: The team hopes to conduct an underwater archaeological expedition in order to mine for artifacts and pinpoint the age of the structure—which one researcher believes could date back more than 4,000 years. (Click to read about another mystery, this one in Africa.)