I am reminded of Spinal Tap guitarist Nigel Tufnel's remarks during the break of their heavy-metal anthem "Stonehenge": "No one knows who they were ... or what they were doing ..." Which isn't quite true, of course: the 'henge has long been considered a site of both astronomical and ritual importance to the Bronze Agers who visited it. The strange and impressive Serpent Mounds of Ohio, however, are another story altogether.
Mound-building native Americans were active in many parts of what is now the U.S., but usually their earthen structures were just that: hill-like mounds, often used as burial sites. Not so the Serpent Mound. While there are a number of animal-shaped effigy mounds, as they're known, in the upper Midwest, the Serpent is by far the world's largest: it measures about 1,370 feet in length and between one and three feet high. Its beautiful and precise shape are also a marvel (drawing inevitable comparisons to Peru's Nazca Lines and other such ancient super-structures), often attributed to the Adena culture (800 BC - 100 AD).