They dot the arid landscape of Anza-Borrego Desert State Park, some 500 of them in an area that’s less than 10-square-miles: ancient rings of rock and cleared circles in the soil that experts can’t explain.

The mysterious archaeological features elude identification thousands of years after they formed. In fact, experts have a hard time even nailing down their age, beyond ranges of a few thousand years.

There’s at least one scrap of consensus, though: The shapes didn’t appear on their own.

“There’s no way the rock line circles are naturally occurring,” said Hayley Elsken, an associate state archaeologist in Southern California. In her phone interview with FOX5SanDiego.com last week, that statement was about all Elsken could say with confidence.

One of hundreds of mysterious rock circles found in the desert of northeastern San Diego County. (Photo: California State Parks)

She explained that the circles in Anza-Borrego, located in far northeastern San Diego County, stand out for their sheer concentration in one area.

But the features are not unique in their existence. Pioneering archaeologist Malcolm J. Rogers, who worked across Southern California during the early 20th century, documented thousands of similar features elsewhere in state deserts.

The circles range in diameter from about 5 to 12 feet. They are often lined with carefully arranged rocks. Other times, the circles are comprised only of a neat patch of soil, cleared of rocky jumble.

Experts attribute the circles to Native Americans. Anza-Borrego is the traditional homelands of the Cahuilla, Kumeyaay and Cupeño, Elsken said.

But why form circles in the first place? No one best guess has risen to the top, according to Elsken, but a few theories are most popular.

Rogers dubbed the features “sleeping circles,” and suggested early hunters used them as a place to hunker down at night. Elsken said a single person could probably fit snugly in each circle, but it wouldn’t make a very roomy bedroom.

The archaeologist notes that researchers haven’t found much in the way of artifacts buried in the soil near the circles. Maybe a handful of stone tools, but nothing to suggest that people set up in the circles for the long haul.

At the site of ancient villages, by contrast, archaeologists find thousands of artifacts. If the circles served more than a transient purpose, they should have more signs of pottery, food processing and other daily tasks scattered nearby.

Other popular theories revolve around ceremonial or ritualistic purposes.

They look like they’d be more comfortable to sit in than an uncleared surface, Elsken says. So, she reasons, it’s possible they were a place for trance or meditation. They’re often found on elevated surfaces, so the view would be particularly nice from most circles, too.

Whatever the circles’ purpose, they serve as an indelible reminder of the land’s history.

Elsken said the features are typically in remote and elevated parts of the park, so they’re unlikely to be disturbed by hiking or campers. On the off-chance that someone did come across a circle — or any other ancient feature in the park — Elsken asked that people respectfully leave the items untouched.

“This is the Kumeyaay land and we’re guests on it,” Elsken said. “Respect that land, especially when we see cultural sites.”