The US' 2,000-year-old mystery mounds

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, the 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.  

The Octagon is currently being used as a golf course.

All of these 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.

You could put four Roman Colosseums inside just The Octagon

The US Department of the Interior has nominated eight of Hopewell's earthworks for consideration in 2023 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 sq 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 sprawls a massive 50 acres and is attached to the 20-acre Observatory Circle, a large earthwork circle for gathering and rituals connected to the observation of the night sky.

"You could put four Roman Colosseums inside just The Octagon," Lepper told me. Stonehenge would fit within just that small circle now serving as a putting green. He added that 2,000 years ago, indigenous workers built these earthworks without modern tools, digging up soil with pointed sticks and hauling it in wicker baskets on their backs. One estimate, he noted, is that they moved seven million cubic feet of dirt. 

The achievement of the Hopewell Culture, however, is not simply in creating large, precise shapes, which they did without the vantage point of hills for an aerial view. They also embedded a sort of hidden geometry within these structures. Until the mounds were measured and compared, it was thought that the builders didn't have any mathematical and geometrical sophistication, as there are no written records to testify to their level of knowledge. It was eventually discovered, however, that they made precise measurements across their earthworks and connected them in unsuspecting ways. 

Lepper explained that the circumference of The Great Circle "is equal to the perimeter of the perfect square that it was connected to", and that "the area of that perfect square is equal to the area of the [Observatory Circle] that's connected to The Octagon". 

He added: "If you draw a square inside The Octagon by drawing a line from alternate corners of The Octagon, the sides of that square [1,054ft] are equal to the diameter of the circle that it's attached to [1,054ft].

Examples of this interplay between earthworks have been found repeatedly by archaeologists. According to Lepper, that measure of 1054ft, whether halved or doubled, is found in other indigenous earthworks across the country, and served as a common unit of measure. 

While the Hopewell Culture's geometrical and mathematical knowledge astonished scholars, another level of sophistication appears when the layers are peeled back further: astronomical alignment.  

In the 1980s, two professors at Earlham College in Indiana, Ray Hively (a physicist and astronomer) and Robert Horn (a philosopher), decided to pay a visit to The Octagon and its attached Observatory Circle. As astronomical monuments like Stonehenge had received great attention, they wondered if these earthworks were also aligned to a solar calendar.  

Hively and Horn found no solar connections, but they then considered an alternative purpose: the lunar cycle.

"We thought deliberate lunar alignments unlikely at Newark," they wrote, because while the Sun can be tracked over a year, a complete lunar cycle takes 18.6 years. Even so, the lunar cycle proved to correspond to the position of the Observatory Mound at The Observatory Circle. There, one can watch the Moon rise over the exact centre of The Octagon in the distance every 18.6 years. 

"Astronomical alignments are only relevant and useful if they somehow tie the celestial orbs to belief systems and understandings of life," said Timothy Darvill, professor of archaeology at Bournemouth University who has researched both Stonehenge and the Hopewell Ceremonial Earthworks. "The ceremonies around the observation of the skyscape could well have a secondary function in terms of fostering community."  

That ancient community and culture is part of the case being made to Unesco. 

A Unesco site needs to show that it has "outstanding universal value", said Jennifer Aultman, director of historic sites and museums at Ohio History Connection and the Ohio lead for Unesco consideration. One criterion for this, she said, "is that these are masterpieces of human creative genius", which is where these mathematical, geometrical and astronomical features are important. The other, "is that they bear really exceptional testimony to the cultural tradition that produced them".

-bbc