On a lush hilltop deep in Southern Ohio, a giant snake slithers through the grass, its intentions a mystery. Despite more than a century of study, we still don't know who built the Great Serpent Mound, or why. That's part of what makes a visit here so fascinating, and also a little bit frustrating. There are still questions that can't be answered through a Google search or more than a century of research. This much is known: At 1,348 feet long, the serpent is the largest effigy mound in the world -- that is, an earthen creation in the shape...