Over three hundred million years ago a meteor crashed in what would become Adams County, in southwestern Ohio. This created a 5-mile wide crater, upon which ancient Native Americans built a massive mound in the shape of a serpent about 900 years ago. Semi-nomadic descendents of the Hopewell culture, known as the Fort Ancient peoples, settled in this part of Ohio between AD 1000 and AD 1650. They had an acute interest and understanding of both solar and lunar alignments.
Although originally attributed to the Adena peoples, this has no been accepted as false, based on radiocarbon dating tests completed in the mid-1990s, which suggest the mound was built by the Fort Ancient peoples. The purpose of the serpent mound, called a "Cryptoexplosive Structure" by the United States National Parks, is still considered quite mysterious. Though there are nearby burial grounds, there have been no human remains discovered in or directly by the mound itself. The National Park Service states that it is a structure of "undetermined origin exposed by differential erosion."
Many anthropologists believe that the shape of the Serpent Mound is actually a massive lunar calendar, built to align with solstices and equinoxes. Animal mounds are widely-considered to be effigies reflective of cosmic alignments. In fact, the towns and surviving structures also reflect this interest in the cosmos. However, who exactly built it is still open to conjecture (it's attributed to three different prehistoric indigenous cultures, but it's actual purpose is still heavily-debated).
Serpent Stats: Discovered in 1847 by Ephraim Squire and Edwin Davis. Measures 1,348 feet long, 1-5 feet high. 120 foot long snake head, with an open mouth. Serpent's head is aligned with the summer solstice sunset. Serpent's tail coils aligned with the winter solstice. Currently the Ohio Historical Society maintains the mound. Radiocarbon dating places the creation of the mound to AD 1070, four years after Halley's Comet. Some historians believe it was a ceremonial site. -Roadtrippers