The Great Circle

The Newark Earthworks in Newark and Heath, Ohio, consist 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 100 BCE and 400 CE, contains the largest earthen enclosures in the world, and was about 3,000 acres in total extent. Less than 10 percent of the total site has been preserved since European-American settlement; this area contains a total of 206 acres.

Source: Wikipedia