U-864 was torpedoed off the coast of Bergen, Norway, by British sub HMS Venturer in 1945. Among U-864’s cargo were 1,800 canisters – around 65 tonnes – of mercury.

An estimated four kilograms of mercury has been oozing into the water every year – forcing authorities to ban boating and fishing in the area. The mercury has caused high levels of contamination in cod, torsk and edible crab.

The Norwegian government now plans to cover the 11 acres of seabed around the 2400-tonne wreck with up to 40ft of rubble to contain the leaking mercury. Expected to cost up to $50 million, the operation will start next year and finish in 2020.

Researchers say similar entombing projects to contain mercury-contaminated sites have been used around 30 times over the last
20 years and have worked well. Campaigners wanted the submarine salvaged and brought ashore but the authorities say the canisters could break if they are moved.

The shipwreck was first discovered in March 2003 by a Royal Norwegian Navy minesweeper after being alerted by local fishermen.