Multibeam sonar and remotely-operated-vehicle footage collected during the April–May 2025 dive season have mapped the entire formation that sits about 40 feet (12 m) below the surface of Lake Michigan’s Grand Traverse Bay. The scans, led by underwater archaeologist Dr Mark Holley of Northwestern Michigan College, sharpen earlier side-scan images from 2007 and document two concentric granite rings—roughly 40 and 20 feet across—connected to a serpentine line of stones that runs for more than a mile across the lakebed.
One table-sized boulder within the inner ring carries a relief of a mastodon about one metre high. Because mastodons disappeared from North America more than 11,000 years ago, the carving is treated as a terminus post quem, anchoring the construction of the stone array to roughly 7000 BCE. Radiocarbon samples taken from nearby organic deposits in earlier coring campaigns match that late-Ice-Age chronology.
Holley’s team reports that each block is quarried local granite weighing up to 3,000 pounds and shows tool-mark patterns inconsistent with glacial shoving. The exact coordinates remain undisclosed; the Grand Traverse Band of Ottawa and Chippewa Indians received them first to ensure tribal oversight and to deter souvenir diving.
Interpretations of the feature remain divided. University of Michigan archaeologist Prof John O’Shea notes its similarity to a 9,000-year-old drive lane on the Alpena–Amberley Ridge in Lake Huron that was designed to funnel migrating caribou, suggesting a large-game hunting function. Other researchers point to possible sunrise alignments that would place the structure among the oldest known calendrical markers in North America; a minority of geologists still argue for a purely glacial origin.
The campaign used side-scan sonar, multibeam bathymetry, sub-bottom profiling and 4K photogrammetry rigs mounted on ROVs, technologies not available during the initial 2007 discovery. Holley’s group plans to extract microfossil cores later this summer to clarify when rising lake levels inundated the site, a step that could show whether the stones were first assembled on dry ground or moved after submersion.
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