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The German submarine U-864 was a Type IXD2 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, the 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.

The British submarine HMS Venturer, commanded by Lieutenant James "Jimmy" S. Launders, was sent on her eleventh patrol from the British submarine base at Lerwick in the Shetland Islands to Fedje, north of Bergen. After German radio transmissions regarding the U-864 were decrypted, she was rerouted to intercept the U-boat. On 6 February the U-864 passed the Fedje area without being detected, but one of her engines began to misfire and she was ordered to return to Bergen. A signal stated that a new escort would be provided her at Hellisøy on 10 February. She made for there, but on 9 February the Venturer heard the U-864's diesel engine noise (Launders had decided not to use ASDIC since it would betray his position) and spotted the U-boat's snorkel.

In an unusually long engagement for a submarine and in a situation for which neither crew had been trained, Launders waited 45 minutes after first contact before going to action stations, waiting in vain for the U-864 to surface and thus present an easier target. Upon realizing they were being followed by the British submarine and that their escort had still not arrived, the U-864 initiated zig-zagging evasive manoeuvres. Each submarine risked raising her periscope. The Venturer had only eight torpedoes (four tubes and four reloads) as opposed to the U-864's total of 22. After three hours, Launders decided to fire a spread of torpedoes at the U-boat's predicted position. The torpedoes were released beginning at 12:12 and then at 17 second intervals after that (taking four minutes to reach their target), and Launders then dived suddenly to evade any retaliation from his opponent. The U-864 heard the torpedoes coming and also dived deeper and turned away to avoid them, managing to avoid the first three but unknowingly steering into the path of the fourth. Imploding, she split in two, sinking with all hands and coming to rest more than 150 m (500 ft) below the surface on the sea floor, 2 nmi (3.7 km; 2.3 mi) west of the island of Fedje.


The Norwegian Navy had known about the U-864 since 1946. HMS Venturer was transferred to the Norwegian Navy when the war was over. They knew about the wreck but not its exact position.


Wolfgang Lauenstein, a German amateur historian, contacted the Norwegian authorities in 1997 for information on the submarine's sinking. He insisted that the U-864 could contain substances such as uranium oxide, heavy water and mercury.

 In February 2001, Kjelstrup recommended that they could look for U-864 using KNM Tyr. They got the permission, but due to many missions they could only use it when the ship was passing Fedje for another mission. 


 "In February 2003 there was an opportunity. I had then obtained topographic maps from the Norwegian Defense Research Institute which showed a seabed with mountains and valleys on the spot. It helped, and in the evening of February 22, I received a telephone from the shipper on board Tyr who could tell that the submarine was found", says Kjelstrup.


The only parts that was notified was the Norwegian Climate and Pollution Agency, the costal administrations and the legal owners of the U-864. The information about the discovery was kept a secret and not told to the public until October 2003. Then, Lauenstein had written a letter that ended in the Ministry of Fisheries. It was found by the newspaper "Strilen" and distributed to the people. Lauenstein wrote that the submarine was supposed to ship 1,870 steel bottles of mercury from Germany to Japan. He found the information in an archive located in Washington.

In October 2003, the Coastal Administration took over responsibility for further work on the wreck. The Coastal Administration carried out investigations that year, but only in the case of a major survey in the fall of 2005, mercury bottles were actually found. Meanwhile, Kjelstrup and other submarine-interested amateur historians found documentation in other archives that the U-864 contained at least 64.000 kilos of mercury.


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