Serpent Mound in Adams County, Ohio, is the largest surviving serpent effigy in the world: 1,348 feet long, about 20 feet wide, with seven coils, a triple-coiled tail, and open jaws facing a 120 foot oval. It was built by hand with bone, stone, and wooden tools and baskets of yellowish clay and ash, faced with rock against erosion. Squier and Davis mapped it for the Smithsonian in 1848 and read the oval as an egg, while J. P. MacLean saw a frog. The bluff over Ohio Brush Creek sits on the Serpent Mound crater, a nearly nine mile impact structure formed 252 to 330 million years ago that shaped the ridge.

Charcoal dated in 1996 pointed to about 1070 CE and the Fort Ancient culture, but a 2014 study of soil cores argued for the Adena around 300 BCE. Brad Lepper countered that organic sediment can read thousands of years too old and that the Adena did not build animal effigies. Theories link the mound to Halley’s Comet in 1066, to solstice alignments published in 1987, and to a Dhegiha Siouan serpent lord of the Beneath World. Frederic Ward Putnam and Alice C. Fletcher raised funds to buy the site in 1886, and it passed to what is now the Ohio History Connection in 1900.

• How the meteorite’s shattered bedrock guided the creek that carved the bluff
• Why dating a soil core is like dating the heartwood of a log cabin’s timber
• The Hardmans’ finding that the head aligns with summer solstice sunset and the tail with winter solstice sunrise
• Self-styled light warriors burying quartz crystals in the effigy in 2012
• A 2011 dig that found a Fort Ancient ash bed and burials from several eras