During the last year of World War II the once surface-bound diesel-electric U-boat ushered in the age of 'total undersea war' with the introduction of an air mast, or 'snorkel' as it became known among the men who served in Dönitz's submarine fleet. U-boats no longer needed to surface to charge batteries or refresh air; they rarely communicated with their command, operating silently and alone among the shallow coastal waters of the United Kingdom and across to North America. This exhaustive study, the first of its kind, draws upon wartime documents from archives around the world to re-evaluate the last year of the U-boat's deployment, all its key technological innovations, the evolving operations and tactics, and Allied countermeasures.