In the tradition of Shadow Divers, this is the gripping true account of the search for German U-boat U-550, the last unfound, diveable wreck of a U-boat off the United States coast, and the battle in which it was sunk. On April 16, 1944, the SS Pan Pennsylvania was torpedoed and sunk by the German submarine U-550 off the coast of Nantucket, Massachusetts. In return the sub was driven to the surface with depth charges, and then sent to the bottom of the ocean by three destroyer escorts that were guarding the naval convoy. For more than sixty years the location of the U-boat’s wreck eluded divers. In 2012, a team found it—the last undiscovered U-boat in dive-able waters off the Eastern Seaboard of the United States, more than three hundred feet below the surface. This is the story of their twenty-year quest to find this "Holy Grail" of deep-sea diving and their tenacious efforts to dive on this treacherous wreck—and of the stunning clash at sea that sealed its doom and brought the Battle of the Atlantic to America’s doorstep.