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The Great Serpent Mound
An effigy mound is an earthwork shaped as an animal or a figure, meant to be seen from above or from high ground nearby. The dating is contested: radiocarbon dates from the 1990s gave around 1070, while older and newer samples point to Adena, over a thousand years earlier. The dispute is unresolved, and the figure is a good example of how hard it is to date a structure containing nothing directly datable. Coincidence with the supernova of 1054 and Halley's Comet in 1066 has been proposed but not established.
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