Action of 9 February 1945 The Action of 9 February 1945 refers to the sinking of the German U - boat " U - 864 " in the North Sea off the Norwegian coast during the Second World War by the Royal Navy submarine HMS " Venturer " . This action is the first and so far only incident of its kind in history where one submarine has intentionally sunk another submarine while both were fully submerged . Background . " U - 864 " was a Type IX U - Boat , designed for ocean - going voyages far from home ports with limited re - supply . She was on a long - range , covert mission codenamed Operation " Caesar " to deliver highly sensitive technology to Germany ' s wartime ally , the Empire of Japan . This included parts for jet engines and missile guidance systems , and 65 tonnes of mercury . She also carried Tadao Yamoto ( a Japanese acoustic torpedo expert ) and Toshio Nakai ( a Japanese fuel expert ) . Also included were two Messerschmitt engineers , Rudolf ( " Rolf " ) von Chlingensperg and Riclef Schomerus . On 6 February 1945 , " U - 864 " passed through the Fedje area ( off the coast of Norway ) without being detected . During this voyage a normally quiet engine began making an abnormally loud and rhythmic noise that could be easily detected by any ASW equipment in the area . Since there were many Allied ( primarily British ) ships , submarines and aircraft in the area on anti - submarine patrol , " U - 864 " s commander , Ralf - Reimar Wolfram , decided to return to the pens at Bergen to repair the problem . By this stage of the war the German naval code , Enigma , had been broken by British mathematician Alan Turing and was being decrypted by a dedicated team at Bletchley Park using a device called the Bombe . The German naval command was unaware that the British were reading communications sent to the U - boat fleet and so aware of Operation " Caesar " . Wanting to avoid giving the Japanese any advantage that might allow them to extend the duration of the war in the Pacific , Royal Navy submarine command dispatched " Venturer " to intercept and destroy " U - 864 " . HMS " Venturer " was now on her eleventh patrol out of the British submarine base at Lerwick in the Shetland Islands . Under the command of 25 - year - old Lieutenant Jimmy Launders , " Venturer " had sunk thirteen German vessels during ten patrols over the previous 12 months , including the destruction of the Type VIIc U - boat " U - 771 " off the Lofoten Islands on 11 November 1944 , seven nautical miles ( 13 km ) east of Andenes . Launders received a brief message from Royal Navy Submarine Command as to the estimated whereabouts of " U - 864 " , that she was somewhere near the island of Fedje , off Norway ' s southwest coast , just north of the pens at Bergen , along with instructions to destroy her . Launders set about the task , making one risky but calculated decision : he decided to switch off " Venturer " s ASDIC ( ASDIC being the British term for sonar ) . They would rely solely on " Venturer ' s " hydrophone , a common and long - used , though far less sophisticated underwater acoustic detection device than ASDIC , to try to detect " U - 864 " along the course that the Enigma - encoded traffic suggested . ASDIC / Sonar is an active echo location system ; it can quickly provide enough data on range and course for a target ' s speed and bearing to be calculated , and a firing solution for a weapon to be arrived at , even against a vessel taking evasive action . However – the ping given off can be heard by the target as well , it can also be heard much further away than the returned echo that indicated a target . Hydrophones are passive devices , very sensitive underwater microphones that an operator can use to tell where a noise is coming from ; working out distance and speed are much more difficult , especially if a vessel takes evasive action . In using hydrophones , the hunting vessel has to move quite slowly so as not to mask out any external noises by its own sounds . Launders ' choice was to hide and see if he could hear the U - Boat in the area . His plan was to listen for the German vessel and stalk her rather than risk alerting the enemy craft , although it would improve his chances of detecting her . The intelligence derived from the German messages led Launders ' commanders to direct him to search for the German U - Boat near Fedje but " U - 864 " had already left the area on her mission to Japan . Wolfram ' s decision to return for repairs at the U - Boat pens at Bergen to fix the abnormal engine noise problem brought " U - 864 " back past Fedje and the area where HMS " Venturer " was located . The Action . As " Venturer " continued her patrol of the waters around Fedje , her hydrophone operator noticed a strange sound which he could not identify . He thought that the noise sounded as though some local fisherman had started up a boat ' s diesel engine . Launders decided to track the strange noise . Then the officer of the watch on " Venturer ' s " periscope noticed what they thought was another periscope above the surface of the water . However it is highly likely that what was seen was the U - Boats snorkel . The snorkel was still a new device , at that time , it was only used by U - Boats . It was probably unknown to Launders and his crew , which is why it was thought to be a periscope . The snorkel was a rigid ' breathing tube ' raised to the surface to allow the U - Boat to run on her diesels while submerged , saving precious battery life . It allowed air to enter both for the engines and the crew , plus it was also used as an outlet for exhaust fumes . Thicker and bulkier than a periscope , it also limited the U - Boat ' s speed and depth . For Launders ' hydrophone operator to hear diesel noises from a submerged U - Boat , the snorkel would have had to be in operation . In addition , the noise of the diesel engines made the U - boat ' s own hydrophones much less effective and it is doubtful that the German hydrophone operator would have heard " Venturer " running slowly on her electric motors . Combined with the hydrophone reports of the strange noise , which he determined to be coming from a submerged vessel , Launders surmised that they had found " U - 864 " . Launders tracked the U - Boat by hydrophone , hoping she would surface and allow a clear shot . But " U - 864 " remained submerged , and as the hydrophone plot emerged , she was seen to be zigzagging . This made the German submarine quite safe according to the assumptions of the time . Launders continued to track the U - Boat . After several hours it became obvious that she was not going to surface , but he needed to attack her anyway – his batteries had only a limited life . It was theoretically possible to compute a firing solution in all four dimensions – time , distance , bearing and target depth – but this had never been attempted in practice because it was assumed that performing the complex calculations would be impossible , plus there were unknown factors that had to be approximated . In most torpedo attacks , the target would have been visually acquired ; the target ' s angle relative to the attacker and its bearing would be observed , then various range finding devices used to establish the distance to the target ; from this speed could be derived , and a basic mechanical computer would offset the aiming point for the torpedo . In addition , any torpedo depth had to be set based on target identification . Too deep and the torpedo would pass under the target ' s keel , too shallow and it would not do as much damage , especially if the target was a warship with an armoured belt at the waterline . Launders could only estimate the depth of his target – and even then his torpedoes could go over or under the U - Boat ' s hull . In terms of a challenge , this was far outside what they had trained for , as they tried to manoeuvre into a firing position without giving their own position away by creating excessive noise , or exhausting their own batteries . Nevertheless , Launders and his crew made the necessary calculations , made assumptions about " U - 864 " ' s defensive manoeuvres , and ordered the firing of all torpedo es in the four bow tubes . As a small , fast - attack boat , " Venturer " was equipped with only four torpedo tubes , all in the bow , and carried only eight in total . The torpedoes were fired with a 17 . 5 second delay between each pair , and at variable depths . " U - 864 " attempted to evade once it heard the torpedoes coming , but the Type IXD2 boats were large and not quick when diving or turning ; additionally , time would have been needed to drop the snorkel , call the crew to action stations , disengage the diesel , and start the electric motors . The fourth torpedo struck the target . Her pressure hull punctured , " U - 864 " instantaneously imploded with the loss of all hands . Aftermath . " U - 864 " sank from the relative safety of the U - Boat pens in Bergen . Launders was awarded a bar to his DSO ( Distinguished Service Order ) for this action , while several members of " Venturer " s crew were decorated by the Royal Navy . Launders ' career in the Navy continued well after the war . The action was the first and so far only battle ever to have been fought entirely under water . The wreck of " U - 864 " lies in of water west of the island of Fedje , and was discovered in 2003 . She remains an environmental hazard , due to her toxic cargo , but is classed as a War Grave , and all maritime operations relating to the wreck , including environmental clean - up efforts , must adhere strictly to the international protocols dealing with treatment of such sites .