The German submarine U-111 sits in icy dry dock at Maine’s Portsmouth Naval Yard in March 1921. It’s flanked by two U.S. submarines, USS S-15 and USS S-16, to enable American engineers to easily compare hull designs. SM U-111, the last World War I-era German submarine to be discovered in U.S. waters. The sleek tube of riveted iron had been part of the Unterseeboot (U-boat) fleet that struck terror in Allied sailors.