In December, a mission of the Organisation Todt (Oberbauleitung Süd) inspected the harbour to study the possibilities to build a submarine pen invulnerable to air bombing from England.

Between late 1943 and early 1944, a fortified lock was built to protect submarines during their transfer from the Loire river and the pens.

The attack successfully disabled the dry-dock by ramming an explosive-filled destroyer into it, and by demolition teams wrecking the pumps and electrical system.

The base was equipped with 62 workshops, 97 magazines, 150 offices, 92 dormitories for submarine crews, 20 pumps, 4 kitchens, 2 bakeries, two electrical plants, one restaurant and a hospital.

The base now features several museums, including a mockup of a transatlantic liner called the Escal'Atlantic, and the French submarine Espadon, built in 1960.