High Quality Content by WIKIPEDIA articles! German submarine U-864 was a German Type IX U-boat of World War II. It departed from Kiel on 5 December 1944 on its 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 HMS Venturer, 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.