FAR AS THE EYE CAN SEE, the sand shore runs 20 miles along the seaside on Hilton Head Island. “The beach,” as we call the flat, surf and tide-washed, pearl-colored, shellstrewn, stream-riddled, shorebird refuge and place of play for people on Hilton Head Island, is not one place. It is a seamless strand of seven beaches stitched together on the outskirts of this foot-shaped isle. Each shoreline is sculpted by distinct natural forces, and significantly altered by human history. Every beach holds its own secrets, including buried treasures of sorts. You can unearth these secrets — but you need to know where to look.