U-boat pens were fortified structures built during World War II to protect German U-boats from air attacks and enemy submarines. These pens were typically constructed in coastal areas and featured thick concrete walls, making them resistant to bomb blasts. The design allowed for the safe docking, maintenance, and repair of submarines. The most famous U-boat pens were located in Brest, France, and La Rochelle. These facilities played a crucial role in the Battle of the Atlantic, enabling the German navy to launch surprise attacks on Allied shipping while minimizing the risk of damage to their vessels.