The Newark Earthworks in Newark and Heath, Ohio, consists of three sections of preserved earthworks: the Great Circle Earthworks, the Octagon Earthworks, and the Wright Earthworks. This complex, built by the Hopewell culture between 250 AD and 500 AD, contains the largest earthen enclosures in the world, being about 3,000 acres in extent. Today, the preserved site covers 206 acres, and is operated as a state park by the Ohio Historical Society. It has been designated a National Historic Landmark. In 2006, Newark Earthworks was also designated as the "official prehistoric monument of the State of Ohio."
This is part of the Hopewell Ceremonial Earthworks, one of 14 sites nominated in January 2008 by the U.S. Department of the Interior for potential submission by the United States to the UNESCO World Heritage List.(Wikipedia)