Serpent Mound's coiled form is not just a shape; part of it lines up with a specific point on the horizon that only happens once a year.
An alignment built into the curves
The mound's head and the oval form near it are positioned such that the head aligns with the sunset position at the summer solstice, an alignment argued by some researchers as deliberate astronomical design rather than coincidence, though as with many such claims at effigy and earthwork sites, intentionality is inferred from geometry rather than confirmed by any accompanying written record.
An effigy, not a burial mound
Unlike many prehistoric North American mounds built primarily for burial, Serpent Mound contains no significant burials of its own, distinguishing it as an effigy mound, a large-scale earthen sculpture built in the shape of an animal, a form found at a number of other sites in the Ohio and upper Mississippi valleys but rarely at this scale or level of preservation.
As this entry's companion article covers
The unresolved debate over which culture actually built the mound, with radiocarbon evidence pointing variously toward the Fort Ancient culture or the earlier Adena culture, and what that uncertainty means for dating the site with confidence, is addressed directly in this entry's companion article rather than repeated here.