HMS Venturer's underwater attack on U-864

U-864 was carrying materials toward Japan on Operation Caesar, and the Royal Navy placed submarines along its anticipated route to intercept it. The projected passage ran north along Norway before turning into the Atlantic and continuing around Africa. Venturer detected a faint noise near Bergen on 9 February 1945, then acquired a visual indication of a snorkel as the contact became clearer. Passive hydrophones allowed the crew to follow the zigzagging submarine without active transmissions, but interpreting the noise depended on the operators' skill. After roughly three hours of tracking, Lieutenant James Launders faced a choice between attempting a submerged attack and surfacing to recharge, with the risks of retaliation or losing contact. The crew estimated the missing elements of the firing solution while manoeuvring quietly and conserving battery power. Venturer fired all four bow torpedoes with delays and slightly different depth settings, then dived to avoid retaliation. U-864 avoided the first three while trying to change from snorkel-and-diesel operation, but the fourth hit and split her in two. Launders received a bar to his Distinguished Service Order, and other crew members also received awards for the collaborative effort. Venturer achieved the only confirmed submarine-versus-submarine sinking fought wholly underwater.