Constructed by a mysterious civilisation that left no written records, the massive site is now open to the public for the first time in more than a century.

Autumn leaves crackled under our shoes as dozens of eager tourists and I followed a guide along a grassy mound. We stopped when we reached the opening of a turf-topped circle, which was formed by another wall of mounded earth. We were at The Octagon, part of the Hopewell Ceremonial Earthworks, a large network of hand-constructed hills spread throughout central and southern Ohio that were built as many as 2,000 years ago. Indigenous people would come to The Octagon from hundreds of miles away, gathering regularly for shared rituals and worship.

"There was a sweat lodge or some kind of purification place there," said our guide, Brad Lepper, senior archaeologist for the Ohio History Connection's World Heritage Program (OHC), as he pointed to the circle. I looked inside to see a perfectly manicured lawn – a putting green. A tall flag marked a hole at its centre.

That's because from 1910 through 2024, The Octagon had been used as a golf course. But on 1 January 2025, this ancient, enigmatic site reopened to visitors for the first time in more than a century.

All these prehistoric ceremonial earthworks in Ohio were created by what is now called the Hopewell Culture, a network of Native American societies that gathered from as far away as Montana and the Gulf of Mexico between roughly 100 BCE and 500 CE and were connected by a series of trade routes. Their earthworks in Ohio consist of shapes – like circles, squares and octagons – that were often connected to each other. Archaeologists are only now beginning to understand the sophistication of these engineering marvels.

Built with astonishing mathematical precision, as well as a complex astronomical alignment, these are the largest geometrical earthworks in the world that were not built as fortifications or defensive structures. And while most people have never heard about the sites or its builders, that may be about to change.

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In 2023, eight of Hopewell's earthworks were inscribed as a Unesco World Heritage site. These include The Great Circle and The Octagon in Newark, Ohio, as well Ohio's first state park, Fort Ancient (not an actual fort). The other five are part of the Hopewell Culture National Historical Park: Mound City, Hopeton Earthworks, High Bank Works, Hopewell Mound Group and Seip Earthworks.

Lepper told me The Octagon and The Great Circle were once a larger, single Hopewell complex spanning 4.5 square miles and connected by a series of roads lined by earthwork walls. Walking through both sites today, there is an immediate shock of scale. The Great Circle, where the museum for Hopewell Ceremonial Earthworks is found, is 1,200ft in diameter. Its walls rise up to 14ft high and are outlined on the inside by a deep ditch. The Great Circle was once connected to a square and a burial ellipse, with only part of the square still visible today. The Octagon........

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