During the Second World War, Saint-Nazaire was one of five French ports chosen by the Germans on the Atlantic coast to become the home port of their submarines. In order to put these U-Boote away from the mission, a gigantic bunker will be built right in the middle of the harbor: the underwater base. How will it be built, by how long and by whom? How does it work and what are its defenses? What types of submarines will come to be repaired? What actions will be taken by the Allies to try to stop it from working? This is what you will discover in this handy guide illustrated by 200 photos, documents and plans.