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Type XXI submarine - Wikipedia
The Type IIB U-boat U-21 - German U-boats of WWII - uboat.net
German submarine U-21 (1936) - Wikipedia
SM U-21 (Germany) - Wikipedia
Phantom of the Deep: Germany’s Underwater Wonder Weapon - HistoryNet
German U-Boat, U-21. Found in the North Sea - YouTube
U-21 - National Underwater and Marine Agency
U 21 - German and Austrian U-boats of World War One
Type XXI U-Boat Diesel-Electric Attack Submarine - Military Factory
Type XXI Elektro boats - U-boat Types - German U-boats of WWII ...