Earlier this fall, a group called "Unite the Collective" posted an online video showing members dancing across Serpent Mound in southern Ohio, and talking about planting hundreds of so-called "orgonites" inside the formation.
The muffin-shaped objects are made of resin and metal shavings, which believers say radiate positive energy.
Jane Mason of the Ohio Historical Society says while it was believed hundreds of orgonites were dug into the mound, the Adams County Sheriff's Department says so far, only about four have been found.
"Unfortunately at this point, the thinking is perhaps there is not enough vandalism to continue to pursue it," says Mason. "The mounds are very important sites in the state, and to the country, and to the world. So we really want to discourage any type of vandalism. We want people to enjoy the mounds, but we want them to enjoy them in a respectful way."
Native Americans built such mounds, known as "effigy mounds," all across the continent over the past few thousand years.
Serpent Mound, believed to date back to the 11th century, is among the most famous. The snake-shaped formation has been likened to the Great Pyramids of Egypt and England's Stonehenge.