In 1991, acting on a tip from a local fisherman, two scuba divers discovered a sunken German U-boat–complete with its crew of 60 men–not too far off the New Jersey coast. The divers, realizing the momentousness of their discovery, began probing the mystery. Over the next six years, they became expert and well-traveled researchers, taught themselves German, hunted for clues in Germany, and constructed theories corrective of the history books, all...