U-Boat pens were fortified structures built during World War II to protect German U-boats from air attacks and enemy naval forces. These pens were typically constructed in coastal areas and featured thick concrete walls, making them resistant to bombings. The design allowed for the safe storage and maintenance of submarines, ensuring they could be quickly deployed for missions. The most famous U-Boat pens were located in Brest, La Rochelle, and Saint-Nazaire in occupied France. These facilities played a crucial role in the Battle of the Atlantic, providing a secure base for German submarines to launch attacks against Allied shipping.