In Ohio, something mysterious is on the ground. There’s an undulating mound in the middle of a green field. People call it the Serpent Mound, and it’s been there for generations.

Located in southwestern Ohio, Serpent Mound is a giant earthen mound — the largest serpent effigy in the world — thought to have been constructed by the Fort Ancient people around 900 years ago, although some argue that the site is much older and that the Fort Ancients did not build it, but actually refurbished it.

Although no human remains or artifacts have been found in the sinuous, grassy hillock that is Serpent Mound, some graves and burial mounds stand nearby, probably built by the Adena culture — the Fort Ancient people’s predecessors in the area — around 500 C.E.

Regardless, Serpent Mound belongs to a class of structures called effigy mounds, which were commonly built in the shape of animals like bear, lynx, bison or birds, and often served as burial sites for ancient people.

Serpent Mound sits on the edge of a meteorite impact crater, and the serpent itself is between 19 and 25 feet (6 and 7.5 meters) wide and rises around 3 feet (1 meter) from the surrounding landscape, with its head formed by a rock cliff overhanging a nearby creek.

Although it’s difficult to know what its purpose was since it wasn’t used for burials, it acts as a calendar — the sunset on the summer solstice lines up with the serpent’s head. The three eastern-facing curves of the snake’s body line up with the sunrise on the equinoxes, and the serpent’s tail coils align with the winter solstice.

According to Ohio History Connection, Serpent Mound and eight other Ohio American Indian earthworks were chosen in 2008 by the U.S. Department of the Interior for inclusion on the United States’ tentative list of sites to be submitted to UNESCO (the United Nations Educational, Scientific, and Cultural Organization) for inclusion on the prestigious World Heritage Sites list.

If it is eventually inscribed on the list — possibly in 2023, according to Ohio History Connection’s World Heritage Director Jennifer Aultman — Serpent Mound will join the ranks of the pyramids of Egypt, the Great Wall of China, Pompeii, Stonehenge and the Taj Mahal as World Heritage Sites.

The Ohio History Connection notes that several archaeological digs have attempted to figure out the mound’s origins. In the late 19th century, the first few digs, lead by Frederic Ward Putnam, found no artifacts to point to one culture. But since there were two Adena burial sites nearby, they assumed that the Adena built the serpent.

A 1991 expedition upended that theory. Carbon dating put the mound around 900 years old, suggesting it was the Fort Ancient, who flourished later than the Adena, who made the serpent. A second round of carbon dating indicated that parts of the mound were older, once again pointing to the Adena. Basically, no one knows for sure, so more excavations are still being planned.

The mound may also hold celestial secrets. According to History Daily, researchers in 1987 said the head of the serpent aligns with the sunset during the summer solstice, and the coils of its body correspond with the moon’s phases.

This heavenly effect attracts many visitors to the site, reported ABC. But researchers said this is probably not the only reason why the mound was built.