The ancient earthworks of the Ohio River valley are mostly buried beneath modern-day civilization. But archaeologists know their significance.

"This is like having the Great Pyramid of Egypt or Stonehenge or the Great Wall of China here in our back yard," says Brad Lepper, archaeologist with the Ohio Historical Society in Columbus.

Among the things that make the earthworks special:

• Monumental scale. The Newark Earthworks, built by the Hopewell people, include the Great Circle. At 1,200 feet in diameter, four football fields placed end to end could fit inside it. It was built using an estimated 7 million cubic feet of earth, "by people using pointy sticks and baskets," Lepper says.

• Geometrical sophistication. Many of the circles, squares and octagons are "as perfect as we could build them to that scale today," Lepper says. And many were duplicated across hundreds of miles.

• Their astronomical precision. "The circle and octagon at Newark are lined up to the four major rise and set points of the moon, in a lunar cycle that takes 18.6 years to complete," Lepper says.

The Hopewell people might have regarded the moon as a spirit or deity, Lepper says. "Building (the earthwork) with the lunar cycle embedded in its architecture was a way, I think, of bringing that celestial magic down to earth."

John Johnston