First submarine to be sunk by another submerged submarine

On 9 February 1945, towards the end of World War II, HMS Venturer, a British ‘V’-class submarine, made history by sinking a German submarine (U864) while both were submerged in the North Sea west of Bergen, Norway. Venturer acquired the German submarine on her Anti-Submarine Detection Investigation Committee (ASDIC, now known as sonar), which was being used in passive mode, and tracked it for one hour and then, having ascertained the course and speed of the target, she launched 4 torpedoes. The German submarine sank with loss of all crew.