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).

But what's it for? No burial sites were found within the mound, and while it does seem to have some astronomical significance -- the oval-to-head area of the serpent is aligned to the Summer Solstice sunset and the snake's coils align with the Winter Solstice sunrise -- but considering the extraordinarily elaborate nature of its design, experts believe that its usefulness as an enormous calendar was merely secondary. Also adding to the mystery is the serpent's open jaws, which surround a 120-foot hollow oval feature, thought variously to be an egg, the sun, the body of a frog, or merely the remnant of a platform serving to support something. So what does it all mean? Your guess is as good as ours!