The Margate Shell Grotto is one of Kent, England's greatest cultural treasures. Some 4.6 million seashells line the walls of the subterranean passageway in ornate mosaics—but just as interesting as the craftsmanship on display is the mystery behind it. The grotto was discovered accidentally in the 19th century, and to this day no one knows who built it ... or why. According to most accounts, farmer James Newlove and his son Joshua were digging a duck pond in 1835 when they struck upon the underground cavern. A closer look revealed that they weren't the first people to enter the space,...