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U-boat - Wikipedia
German submarine U-864 - Wikipedia
Type VII submarine - Wikipedia
U-Boat Tactics: Submerged Torpedo Attacks - uboataces.com
German U-Boat - How a U-Boat works - uboataces.com
U-boat | German Submarine Warfare in WWI & WWII | Britannica
The Schnorchel - Technical pages - German U-boats of WWII ...
Phantom of the Deep: Germany’s Underwater Wonder Weapon - HistoryNet
How German U‑Boats Were Used in WWI—And Perfected in WWII
Germans unleash U‑boats | January 31, 1917 - HISTORY