From the ground, it just looks like a small slope at Blacklick Woods Metro Park, hardly noticeable as you walk past, at most maybe 10 inches tall -- the height of a pitcher's mound.

Ah, but from the air. From the air, a circle is plainly visible. Or it was back in 1954 when an aerial photo was taken of the property when it was part of a farm and eastern Franklin County was rural.

It certainly popped out to David Lamp, a former archaeologist who lives in Lancaster. He was looking at aerial photos earlier this year, trying to spot Native American earthworks.

"It was one of the first ones I found," he said.

Lamp passed along the photo to local archaeologist Jarrod Burks, who, on a recent hot afternoon, used magnetometers to confirm that a Native American earthwork exists in the park, likely about 2,000 years old.

"We completed the survey right on top of where we thought the earthwork would be," said Burks, who works for Ohio Valley Archaeology but voluntarily did the work at the park.

How did he determine it was there? By measuring the magnetic content of the soil.

Topsoil is more magnetic. And over the centuries, topsoil filled the ditch that ringed the earthwork, likely just inside a 3- to 5-foot circular embankment.

Burks estimated the diameter at slightly less than 100 feet, with the structure likely built by the Adena or Hopewell people who once populated Ohio. The Adena people lived in Ohio from 800 B.C. to A.D. 1, while the Hopewell lived in the area from 100 B.C. to A.D. 500. The cultures created the raised mounds as burial sites after cremations, and for ceremonies, often on terraces overlooking rivers. This site is just west of Blacklick Creek.

Why there? No one knows for sure, Burks said. The cultures left no written record. It could have been because they were prominent spots marking territory, Burks said. "A fixed part of the landscape," he said.

Metro Parks cleared brush and trees from the area so Burks could do his work with a cart with four magnetometers attached to it.

Topsoil is more magnetic for several reasons, Burks said. It's more exposed to the air and changes in moisture, for one.

"Ground is full of iron," he said.

Also, animals live in topsoil, chewing on organic materials -- grass, leaves, twigs, you name it -- and producing magnetic particles, he said.

The find has piqued the interest of those who work at the park. They were unaware there was something like this in their midst.

"Everyone has had a lot of energy from what's happened here," said Beth Renner, a Blacklick Woods park naturalist.

Josh Laughbaum, Blacklick Woods' park manager, said a trail will lead to the mound.

"Anything we can do to continue or embrace the cultural heritage of central Ohio," said Tim Moloney, Metro Parks executive director.

Ohio has 456 documented earthworks sites, said Emmy Beach, a spokeswoman for the Ohio History Connection.

Lamp, who is now a toolmaker in Lancaster, said he has pored over aerial photos from Fairfield and Pickaway counties and has spotted "a couple dozen" apparent earthworks.

"Some of them have all the hallmarks," he said.

Lamp said he has talked to younger people who don't realize that so many Native Americans lived in Ohio 2,000 years ago, nor the scope of the earthworks they produced here.

"I think that in a lot of cases, they don't realize how many there were," Lamp said.