U-864 was carrying approximately 67 metric tonnes of mercury because it was a critical raw material for weapons production. The submarine was on a secret mission, codenamed Operation Caesar, to deliver strategic war supplies from Germany to Japan in the final months of World War II. Mercury was one of the most important items in the cargo hold, stored in 1,857 steel canisters tucked into the submarine’s keel.
What Mercury Was Used for in Wartime
Mercury had dozens of military applications in the 1940s. It was essential for producing detonators and fulminate primers, the tiny explosive charges that set off larger munitions like artillery shells, bombs, and torpedoes. Without a reliable supply of mercury, a country’s ability to manufacture functioning ammunition would grind to a halt. Mercury fulminate was the standard detonator compound used by every major military power during the war.
Beyond explosives, mercury was used in electrical switches, barometers, and other precision instruments that military hardware depended on. By late 1944, Japan’s supply lines for strategic raw materials had been devastated by Allied naval superiority in the Pacific. Submarine runs from Europe were one of the few remaining ways to get materials Japan could no longer source on its own.
Operation Caesar and the Full Cargo
Operation Caesar was a covert German mission to resupply its Japanese ally. U-864 departed on her maiden voyage on 5 December 1944, carrying far more than mercury. Her primary cargo was advanced Messerschmitt jet engine components intended for Japanese aircraft development, along with V-2 missile guidance systems. Germany was the world leader in jet propulsion and rocket technology, and Japan wanted to reverse-engineer both.
The mercury was classified as secondary cargo, but 67 tonnes of it represented an enormous strategic value. For context, that is enough mercury to produce millions of detonators. Germany had access to mercury through mines in Spain and Italy, while Japan’s sources had largely been cut off. The exchange was meant to benefit both sides: Japan would gain technology and materials it desperately needed, and Germany would strengthen an ally that was tying down massive American and British forces in the Pacific.
Why the Mission Never Reached Japan
U-864 never made it out of European waters. On 9 February 1945, the British submarine HMS Venturer detected U-864 while patrolling near Fedje Island off the Norwegian coast. What followed was an engagement unlike anything before or since in naval history: the only time one submarine has sunk another while both were fully submerged.
Venturer’s crew picked up faint sounds on their hydrophone at 9:32 a.m. and tracked U-864 for roughly three hours, waiting for the German boat to surface. It never did. Running low on battery power, Venturer’s commander, Jimmy Launders, made a calculated gamble. He fired all four bow torpedoes at staggered intervals and depths, predicting how U-864 would try to evade. The German submarine dodged the first three torpedoes but turned directly into the path of the fourth. U-864 broke apart and sank with all 73 crew members to a depth of about 150 metres.
The Mercury Still on the Seabed
The wreck of U-864 lies in two main sections on the seafloor west of Bergen, Norway, and those 1,857 canisters of mercury are still down there. Over the decades, some canisters have corroded, and mercury has leaked into the surrounding sediment. Norwegian authorities have spent years studying the site and debating how to contain the contamination.
The current approach involves stabilizing the seabed slope around the wreck and building a protective counter fill over it, essentially capping the site to prevent further mercury from spreading into the marine environment. The engineering challenge is significant: the wreck sits on an unstable slope at depths between 155 and 177 metres, and any disturbance risks releasing more mercury. Raising the submarine was considered but ultimately deemed too risky because of the likelihood of rupturing additional canisters during salvage. The mercury that was meant to arm Japanese weapons in 1945 remains one of the largest known point sources of mercury pollution on the Norwegian continental shelf.