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U-boat - Wikipedia
Category:World War I submarines of Germany - Wikipedia
U-boat | German Submarine Warfare in WWI & WWII | Britannica
Type UB I submarine - Wikipedia
How German U‑Boats Were Used in WWI—And Perfected in WWII
Unrestricted U-boat Warfare - National WWI Museum and …
Steel predators: The silent threat of German U-boats in WWI
German and Austrian U-boats of World War One - Kaiserliche …
The U-boat War in World War One (WWI) - Kaiserliche Marine - uboat…
U-boat summary | Britannica