It was not quite the deadly legacy the Germans had in mind when they deployed a U-boat on a daring mission to Japan in the last desperate months of World War II.
Robotic excavations have found some of the canisters of mercury
When it set sail in December 1944, U-864 was packed with 65 tonnes of weapons-grade mercury destined to help the Japanese win back supremacy over the US in the Pacific - and divert American attention away from Europe in the process.
Neither the cargo nor the 73 men on board made it. The U-boat was torpedoed to the bottom of the North Sea floor by a British submarine.
More than 60 years on, its toxic cargo is slowly leaking into the waters off the coast of Norway, an ecological time bomb threatening marine - and potentially human - life.
Now the Norwegian government is set to act, following recommendations that the wreck be hermetically sealed to prevent any more of the mercury from escaping.
"We are worried about the long term consequences of the contamination," says Ane Eide Kjeras, spokeswoman for The Norwegian Coastal Administration.
"We need to do something as soon as possible."
The fateful mission of the submarine has been documented by a forthcoming BBC Timewatch programme.
By December 1944, the Germans were hemmed in on all sides and one of the only possible operational routes left was through the North Sea.
Plans were laid for Operation Caesar - which would see the U-864 embark on 5 December, 1944 from the German port of Kiel on its underwater mission. It was loaded with 1,857 canisters of mercury to be used in the production of weapons at Japanese sites, as well as a variety of parts for jets.
But British code breakers at Bletchley Park learned the details of the operation - even the names of the German and Japanese scientists and engineers on board.
The British submarine Venturer was deployed to intercept the vessel after it left the Norwegian port of Bergen.
Aware it was being followed, the U-864 desperately tried to trick its stalkers by zigzagging. The Venturer's commander, 25-year-old Jimmy Launders, took a chance by setting all four of his vessel's torpedoes off at once.
As the U-864 dived to miss one of the oncoming missiles, it headed straight into the fourth. The submarine was split into two parts and as such went to rest more than 150m (500ft) below the surface on the seafloor.
There it lay unknown until the Royal Norwegian Navy, alerted by local fishermen, found the wreck in early 2003, just off the island of Fedje.
A no-fishing zone was imposed around the wreck site after the discovery of documents listing mercury as part of the vessel's cargo. Tests were carried out on the water and silt, with alarming results.
The Norwegian coastal authorities have decided against raising the wreckage, deeming it too dangerous, and are recommending the two parts should be sealed.
Ms Kjeras said an area of about 150m in diameter would be covered with up to 12m (40ft) of material. It is thought a special type of sand or gravel could be used.
Nearly 2,000 eroding flasks of mercury will be covered as a result.
But it will also seal up what is the watery grave of 73 men.
One of them was 18-year-old Willi Transier, who had just asked Edith Wetzler to marry him before he set off on a mission he suspected he might never return from.
"It still hurts," says Mrs Wetzler, who is now 84.
"But I am so thankful that we had the few years we did have together."
Timewatch: The Hunt for U-864 will be broadcast on BBC2 on 5 January at 2100