SPIRO, Okla. (KFOR) – Their trading empire stretched over more than half of North America.

The ancient Caddoans built a city and religious center seven miles from town that lasted from 700 A.D. until around 1450.

“It incorporated 60 different tribes and 30 different language groups, more than 6 million people, and two-thirds of the U.S,” says site manager Dennis Peterson. “This was its heart, a perfect location.”

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The Spiro Mounds. Image KFOR

He settled here in 1979 and has spent his professional life walking among the Spiro Mounds.

“The burial mound is over here to the west of us,” he tells his tour group. “It’s over 350 feet long and 150 feet wide. The highest point is 34 feet high.”

The state historical society owns about 150 acres just south of the Arkansas River where a series of 12 mounds make up where the religious elite controlled a city that occupied as much as five square miles.

“Farming the bottoms and living on the upper terrace,” Peterson explains.

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A piece of Pre-Columbian history in Oklahoma. Image KFOR

In late June, on the longest day of the year, he conducts visitors around the site and tells them about the rich culture that existed here, and of the cleansing celebrations that would culminate with the summer solstice.

“The high priest would give them something equivalent to an Easter sermon. He would tell them, ‘Now your sins are gone. Don’t dirty things up, and let’s go home and feast.'”

On a sultry evening as the sun sets, Peterson paints a picture in the fading light of a culture that thrived here for centuries and faded with changing weather conditions.

The mounds, two of which line up perfectly on three days in June, connect the dots of Pre-Columbian history in Oklahoma, and shed light on a sacred place where you can still see the sunset today.

For more information about the Spiro Mounds Archeological Site go to www.okhistory.org/sites/spiromounds.