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Submarine pen
Bunker housing U-boats / From Wikipedia, the free encyclopedia
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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.
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The term is generally applied to submarine bases constructed during World War II, particularly in Germany and its occupied countries, which were also known as U-boat pens (after the phrase "U-boat" to refer to German submarines).