'Cäsar' (iii) was a German unsuccessful attempt to transfer technical plans and strategic materials (turbojet engine parts, V-2 guidance packages and 65 tonnes of mercury) to Japan in Korvettenkapitän Ralf-Reimar Wolfram’s U-864 (5/9 February 1945).
On 5 December 1944 U-864 departed Kiel on its maiden voyage, but while transiting the Kiel Canal, the boat grounded and sustained damage to its hull, and Wolfram decided to take the boat into the U-boat pens at Bergen on the west coast of German-occupied Norway. On 12 January 1945, while U-864 was still under repair, the pens were attacked by British bombers, and thus further delayed the completion of the repairs. The work was completed early in February 1945, and Wolfram’s boat departed for Japan. However, British codebreakers had deciphered Enigma-encoded transmissions and were thereby alert to U-864's mission. The Admiralty accordingly diverted the submarine Venturer, which had just left Lerwick in the Shetland islands group off the north-east coast of Scotland, to the coast of Norway, in the area of the island of Fedje, to search for U-864.
On 6 February the German boat began to suffer problems with one of its Diesel engines, which began to misfire and thereby greatly increase the boat’s sound signature. Wolfram informed the commander of the German base at Bergen that he would be returning for further repairs, and was informed that an escort would be waiting for his boat at Hellesøy on 10 February. At the same time Venturer, under the command of Lieutenant J. S. Launders, arrived off Fedje and started to search for the U-boat.
On 9 February Venturer's hydrophone operator detected a noise that sounded like a Diesel engine. Launders raised periscope and surveyed the horizon, spotted another periscope and began stalking it, assuming that this was U-864. As Venturer followed the U-boat, it became apparent that the British boat had been spotted, for the U-boat began to steer an erratic zig-zag course. After following the German boat for three more hours, Launders made a calculated decision based on U-864's movements and fired a salvo of all four of his boat’s bow torpedoes. Hearing the incoming torpedoes, U-864 performed evasive manoeuvres, successfully avoiding the first three torpedoes but unknowingly steering into the path of the fourth. The torpedo struck the U-boat, splitting her in half and sending it to the bottom with all 73 of its crew.
This was the first and, as far as is known, the only time that a submerged submarine has sunk another submerged submarine.