The largest effigy mound in the world is also one of the hardest to date, and one of the most fought-over in the present.
The dating problem. It has no burials and no artefacts of its own, so it cannot be dated from its contents. Nineteenth-century investigators tied it to the Adena culture of around two thousand years ago, on the strength of nearby burial mounds. Radiocarbon work in the 1990s, on charcoal from the body of the mound, pointed instead to about 1070 and the later Fort Ancient culture. More recent excavation has reopened the older date, and proposed that the mound was built in stages. The honest statement is that its age is unsettled and the argument continues.
The alignments. The one thing most investigators agree on is that it is oriented: the open jaws and the oval align with the summer-solstice sunset, and the coils have been argued to track other solar and lunar events. What the serpent meant to its builders (a specific being, a calendar, a boundary), is inference.
The contest now. Serpent Mound has become a magnet for New Age and neo-pagan gatherings, drawn by ideas about earth energies that have nothing to do with the people who built it, and these proceed without the sanction of the Shawnee and other descendant nations who hold ties to the Ohio-valley mound builders. When the Ohio Hopewell Ceremonial Earthworks were inscribed as World Heritage in 2023, Serpent Mound was deliberately left out, because it postdates the Hopewell; it is now being advanced for a listing of its own.