Hitler's submarines - the dreaded U-boats - played a vital role in Germany's offensive operations across the world's oceans during the Second World War and could have brought Britain to the brink of capitulation. To support his deadly undersea fleet, Hitler ordered monolithic bunkers to be constructed at strategically crucial sites, designed to withstand the most aggressive attack from the Allied forces. The author has uncovered previously unpublished accounts and visited many of the sites in France and Germany to detail the exact purpose of each base and reveal important new information about what remains at some of the most closely guarded sites.