Duncan adds that the drawings hold clear cultural significance.

“The artists who put them on the wall did it with a great deal of ritual, and I’m sure there were prayers, singing—and these images are alive,” he says. “And the interesting thing about them as far as artists

are concerned is the tremendous amount of detail and the quality of portraiture of the faces. Most of them are people—humans—but they’re not of this world; they’re supernatural.”

The artwork may represent an early achievement of the Mississippian culture,

which spread across much of what’s now the southeastern and midwestern United States between about 800 and 1600 C.E., writes Kaitlyn Alanis for the Kansas City Star.

During this period, people in the region increasingly based their economies on the cultivation of corn

and other crops, leading to the creation of large towns typically surrounded by smaller villages.

Per Encyclopedia Britannica, Mississippian people adopted town plans centered on a plaza containing a temple and pyramidal or oval earth mounds.

These designs were similar to patterns adopted more than 1,000 years prior in parts of Mexico and Guatemala.

Among the most prominent surviving Mississippian sites are the Cahokia Mounds earthworks, which are situated just outside of St. Louis in Illinois.

The city flourished from 950 to 1350 C.E. and was home to as many as 20,000 residents at its height.

In 2008, Duncan told the Columbia Missourian’s Michael Gibney that the Picture Cave artists probably had ties to Cahokia.

He argued that some of the drawings depict

supernatural figures, including the hero known as Birdman or Morning Star, who was known to have been important in Mississippian culture.

The cave system and 43 acres of surrounding land were sold by a St. Louis family that had owned them since 1953.

The sellers mainly used the land for hunting.

In addition to its cultural significance, the cave system is home to endangered Indiana bats.

Laughlin tells the AP that the auction house vetted potential buyers.

He believes the new owner will continue to protect the site,

pointing out that, as a human burial site, the location is protected under state law. It’s also fairly inaccessible to would-be intruders.

“You can’t take a vehicle and just drive up to the cave,” Laughlin says. “You have to actually trek through the woods to higher ground.”

Only then can visitors squeeze through the 3- by 3-foot cave opening.

Livia Gershon is a daily correspondent for Smithsonian.

She is also a freelance journalist based in New Hampshire. She has written for JSTOR Daily, the Daily Beast, the Boston Globe, HuffPost and Vice, among others.