German submarine U-864 was a
Type IX U-boat of
Nazi Germany's
Kriegsmarine in World War II. She departed from
Kiel on 5 December 1944 on her last mission, to transport to Japan a large quantity of mercury and parts and engineering drawings for German jet fighters. While returning to
Bergen, Norway to repair a misfiring engine,
U-864 was detected and sunk on 9 February 1945 by the British submarine , killing all 73 on board. It is the
only instance in the history of naval warfare where one submarine intentionally sank another while both were submerged.