Serpent Mound

39.0287°N, 83.4304°W.

Effigy mound in Adams County, Ohio, depicting a serpent 411 metres long and averaging 90 cm in height, sculpted from yellow clay and ash. The serpent's head opens toward the summer solstice sunset; its undulating body coils through seven curves to a triple-coiled tail aligned with the winter solstice sunrise and the equinoctial sunrise. Radiocarbon dating from charcoal within the mound has yielded conflicting results: charcoal from 300 BCE (Adena culture) and from 1070 CE (Fort Ancient culture), suggesting either two construction phases or significant disturbance. The site sits on the edge of a 14-km-diameter cryptoexplosion structure — a buried impact or volcanic disturbance approximately 320 million years old, identified in 2003. The mound was first surveyed in 1846 by Squier and Davis and protected by Harvard's Frederic Putnam in 1887.

The largest effigy mound on Earth coils across the rim of an ancient impact scar visible only from satellite.