Operation Caesar () was a secret mission carried out by

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...

in the

Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposi ...

to supply

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...

, with advanced technology. The operation failed due to the

sinking of U-864 During the action of 9 February 1945, , a V-class submarine of the Royal Navy, which was patrolling the waters around Fedje Island, off the Norwegian coast in the North Sea, attacked and sank the German U-boat . The sinking is the only inci ...

by a British submarine, the only known example of a submerged

submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

sinking another submerged submarine.

The operation

On 5 December 1944, ''U-864'', on her maiden voyage, was dispatched on a secret mission to deliver supplies to Japan. ''U-864''s primary cargo was advanced

Messerschmitt Messerschmitt AG () was a German share-ownership limited, aircraft manufacturing corporation named after its chief designer Willy Messerschmitt from mid-July 1938 onwards, and known primarily for its World War II fighter aircraft, in parti ...

jet engine parts for use in Japanese aircraft and

V-2 The V-2 (german: Vergeltungswaffe 2, lit=Retaliation Weapon 2), with the technical name ''Aggregat 4'' (A-4), was the world’s first long-range guided ballistic missile. The missile, powered by a liquid-propellant rocket engine, was develope ...

missile guidance systems. Her secondary cargo comprised of the

strategic material Strategic material is any sort of raw material that is important to an individual's or organization's strategic plan and supply chain management. Lack of supply of strategic materials may leave an organization or government vulnerable to disru ...

mercury in 1,857 canisters for use in weapons production. She left

Kiel Kiel () is the capital and most populous city in the northern German state of Schleswig-Holstein, with a population of 246,243 (2021). Kiel lies approximately north of Hamburg. Due to its geographic location in the southeast of the Jutland ...

and headed north. While going through the

Kiel Canal The Kiel Canal (german: Nord-Ostsee-Kanal, literally "North- oEast alticSea canal", formerly known as the ) is a long freshwater canal in the German state of Schleswig-Holstein. The canal was finished in 1895, but later widened, and links the N ...

, ''U-864'' grounded and damaged her hull. Her captain, Ralf-Reimar Wolfram, decided to sail to the

U-boat pen A submarine pen (''U-Boot-Bunker'' in German) is a type of submarine base that acts as a bunker to protect submarines from air attack. The term is generally applied to submarine bases constructed during World War II, particularly in Germany an ...

s at

Bergen Bergen (), historically Bjørgvin, is a city and municipalities of Norway, municipality in Vestland county on the Western Norway, west coast of Norway. , its population is roughly 285,900. Bergen is the list of towns and cities in Norway, secon ...

Norway Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of ...

. On 12 January 1945, while ''U-864'' was still undergoing repairs, British bombers attacked the pens, delaying the repairs further. The repairs were completed in early February and Wolfram embarked on his mission once again. British code breakers at

Bletchley Park Bletchley Park is an English country house and estate in Bletchley, Milton Keynes (Buckinghamshire) that became the principal centre of Allied code-breaking during the Second World War. The mansion was constructed during the years following ...

deciphered

Enigma Enigma may refer to: *Riddle, someone or something that is mysterious or puzzling Biology *ENIGMA, a class of gene in the LIM domain Computing and technology * Enigma (company), a New York-based data-technology startup * Enigma machine, a family ...

encoded transmissions and were alerted to ''U-864s mission. To counter the German U-boat, the

Admiralty Admiralty most often refers to: *Admiralty, Hong Kong *Admiralty (United Kingdom), military department in command of the Royal Navy from 1707 to 1964 *The rank of admiral *Admiralty law Admiralty can also refer to: Buildings * Admiralty, Traf ...

diverted the submarine (Lieutenant James Launders), which had just left

Lerwick Lerwick (; non, Leirvik; nrn, Larvik) is the main town and port of the Shetland archipelago, Scotland. Shetland's only burgh, Lerwick had a population of about 7,000 residents in 2010. Centred off the north coast of the Scottish mainland ...

in Scotland, to Fedje Island, off the coast of Norway to search for ''U-864''.

Action of 9 February 1945

On 6 February 1945, ''U-864'' experienced trouble with one of her engines, which began misfiring, greatly increasing the sounds the U-boat made. Wolfram contacted Bergen, informing them that he would be returning for further repairs. Wolfram was told that an escort would be waiting for them at Hellisøy on 10 February. At the same time, ''Venturer'' arrived off Fedje Island and began searching for the U-boat. On 9 February, ''Venturer''s

hydrophone A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...

operator detected a noise that sounded like a diesel engine. Launders raised periscope and surveyed the horizon, he spotted what he believed to be another periscope (in fact now thought to be the U-boat's snorkel) and began stalking it, assuming that it was ''U-864''. As ''Venturer'' followed the German U-boat, it became apparent that they knew they had been spotted due to the erratic, zigzag course ''U-864'' began to take. After following the German submarine for three more hours, ''Venturer's'' crew made a calculated decision based on ''U-864's'' movements. At intervals of 17.4 seconds, ''Venturer'' fired all four of her torpedoes. Hearing the incoming torpedoes, ''U-864'' took evasive manoeuvres, avoiding the first three torpedoes but unknowingly steered into the path of the fourth. The torpedo struck the U-boat, splitting her in half and sending her to the ocean floor. All 73 of her crew and passengers perished.

Aftermath

Analysis

The operation was a failure for the Germans. Had it not been for the damage the U-boat sustained or the British bombing of Bergen, ''U-864'' would have had a month's lead on ''Venturer'' by the time the Enigma code was broken and she was dispatched, which would have made a significant difference to the outcome.

Casualties

The Germans lost ''U-864'' and all 73 of her crew, including her captain, Ralf-Reimar Wolfram. The British suffered no casualties or damage. For his actions at Fedje, Lieutenant Launders received the

Distinguished Service Order The Distinguished Service Order (DSO) is a military decoration of the United Kingdom, as well as formerly of other parts of the Commonwealth, awarded for meritorious or distinguished service by officers of the armed forces during wartime, ty ...

Subsequent events

After coming across documents listing mercury as one of the cargoes on board ''U-864'', the

Royal Norwegian Navy The Royal Norwegian Navy ( no, Sjøforsvaret, , Sea defence) is the branch of the Norwegian Armed Forces responsible for naval operations of Norway. , the Royal Norwegian Navy consists of approximately 3,700 personnel (9,450 in mobilized state, ...

began searching for the wreckage. After searching for five years, the U-boat was found in October 2003 by the mine control vessel , off Fedje. After examining the site for two years, it became apparent that the mercury was indeed leaking into the ocean at increasing rates every year. Due to the hazards of mercury, fishing is now prohibited in the region. One study recommended entombing the wreck under a layer of sand, gravel and concrete. The Norwegian government instead awarded a contract to a salvage company to raise the wreck but the proposed operation was postponed pending additional studies. The Norwegian government spent 15 years assessing possible risks of raising the wreck, with its dangerous unexploded torpedoes inside. Finally, in February 2017, it was decided to cover the ''U-864'' with 50 centimetres of sand and of rock to seal the mercury inside the wreck.