Who Built the Great Serpent Mound in Ohio?

The Short Answer

The Great Serpent Mound in Adams County, Ohio, is the largest serpent effigy on Earth, stretching 1,348 feet long — and despite more than a century of research, archaeologists still debate who built it and exactly when.

What Is the Great Serpent Mound?

Coiled across a hilltop in southern Ohio, the Great Serpent Mound is a massive earthwork shaped like an uncoiling snake. At 1,348 feet from tail to jaw, it dwarfs every other serpent effigy ever discovered on the planet. Unlike burial mounds, it contains no artifacts or human remains, which makes determining its age and purpose unusually difficult. The entire structure averages about three feet in height and roughly four to five feet in width — modest dimensions that make it nearly impossible to recognize from ground level. From the air, however, the shape is unmistakable.

The Dating Controversy

For over a century, archaeologists attributed the mound to the Adena culture, a prehistoric people who inhabited the Ohio Valley roughly 2,000 years ago. Then radiocarbon dating conducted in the 1990s upended that consensus, pointing to a construction date of around 1070 CE — squarely within the era of the Fort Ancient culture. Just when that theory gained traction, a 2014 re-analysis of additional samples pushed the date back dramatically, to approximately 321 BCE. That would make the mound a contemporary of ancient Greece — as old as Aristotle. Scientists are still actively debating which date is correct.

The Meteorite Crater Connection

One of the most striking features of the Serpent Mound’s location is what lies beneath it. The site sits directly above a roughly eight-mile-wide geological formation known as the Serpent Mound Cryptoexplosion Structure, widely believed to be the remnant of an ancient meteorite impact. Whether the ancient builders were aware of this unusual geology — and whether it influenced their choice of location — remains an open and genuinely unsettling question.

Astronomical Alignment

The mound’s orientation is difficult to dismiss as coincidence. The serpent’s open jaws point directly toward the sunset on the summer solstice. The oval shape positioned within the jaws — long interpreted as an egg — may instead represent the sun itself, captured at its northernmost point on the horizon. If so, the entire structure functions as an enormous astronomical calendar, built from earth and aligned with the sky with extraordinary precision. Some researchers have also identified potential alignments with the winter solstice sunrise and lunar cycles, suggesting the builders possessed sophisticated knowledge of celestial movements.

How the Mound Was Almost Destroyed

By the late nineteenth century, the Serpent Mound had fallen into private hands, and the farmers who owned the land were preparing to plow it flat. In 1886, Harvard archaeologist Frederic Ward Putnam visited the site and immediately recognized its significance. He launched a personal fundraising campaign, eventually collecting enough money to purchase the land and transfer it to the Ohio Archaeological and Historical Society for preservation. Without his intervention, one of the most extraordinary ancient monuments in North America would likely have been erased within a single growing season.

Why It Still Matters

The Serpent Mound sits at the intersection of archaeoastronomy, Indigenous history, and geological mystery. It raises questions that modern science has not yet fully answered: Who built it? Why here, above an ancient crater? How did its builders calculate solstice alignments with such accuracy? The mound is a designated National Historic Landmark and remains open to visitors today, though its full story is still being written.