A submarine pen (U-Boot-Bunker in German) is a type of submarine base that acts as a bunker to protect submarines from air attack. The term is generally...

as Bremer Vulkan, Deschimag AG Weser with its bunker Hornisse, Kriegsmarinewerft Wilhelmshaven with bunker Wespe and Deschimag Seebeckwerft in Bremerhaven...