In 1945, U-864 Nazi submarine carrying 67 tonnes of mercury sank near an Scandinavian island, 81 years later it's still threatning Norway

More than eight decades after the end of World War II, a German submarine sitting on the seabed off Norway has become the centre of a very modern environmental dilemma.The submarine is U-864, a German Type IXD2 U-boat that sank in February 1945 near the Norwegian island of Fedje. It was carrying a secret cargo intended for Japan, including what could have been as much as 67 tonnes of liquid mercury.

Today, the wreck lies roughly 150 metres below the surface. Its cargo has not disappeared with time. Mercury does not break down, and Norwegian authorities have found elevated concentrations of the toxic metal in sediment around the wreck. The problem became even more complicated in 2026. An underwater operation succeeded in reaching parts of the submarine and recovering mercury containers, but the work also released some mercury into the surrounding environment.

Now Norway faces a difficult choice: remove the remaining mercury or leave the wreck in place and permanently seal it beneath a protective cover.

A secret World War II mission ended off Fedje

U-864 left Germany during the final months of the war on a mission that was far more important than a routine submarine patrol. The submarine was travelling towards Japan, carrying military technology, specialists and material that Germany intended to transfer to its Japanese ally. Its cargo reportedly included components and technical information connected with jet aircraft.

But the voyage went badly. After developing problems with its diesel engine, U-864 returned to Bergen and waited for an escort. British intelligence had already intercepted German communications. A British submarine, HMS Venturer, was sent to find it. On February 9, 1945, the two submarines eventually came into contact. The British crew fired torpedoes at U-864 while both submarines were underwater. One struck the German vessel, breaking it apart and sending it to the seabed.

All 73 people aboard U-864 died, including two Japanese engineers. The encounter has a remarkable place in naval history because it is regarded as the only confirmed case in which one submerged submarine sank another submerged submarine.

The mercury cargo remained hidden for decades

The unusual cargo was not immediately known. It was only in the late 1990s that a German researcher began investigating clues about the submarine's cargo. Norwegian authorities were alerted in 2003, and the wreck was located that same year by the Norwegian naval vessel KNM Tyr.

There was one major problem: nobody had the complete shipping records. Norwegian authorities believe U-864 may have been carrying between 65 and 67 tonnes of metallic mercury, based partly on a Japanese wartime order discovered in British archives. The document referred to 1,867 containers.

That figure has helped investigators estimate the size of the cargo, but it cannot establish the exact amount that went down with the submarine. Some recovered containers also carried the name Almadén, pointing to Spain's historic mercury-producing region and providing clues about where at least part of the cargo originated.

Why mercury makes the wreck dangerous

The concern is not simply that a toxic substance is sitting underwater. Mercury can remain in the environment for extremely long periods. Under certain conditions, microorganisms can transform it into methylmercury, a particularly hazardous form that can accumulate in aquatic food chains.

Fish and other marine organisms can absorb it, allowing contamination to move from the seabed into the wider ecosystem and eventually towards humans. That makes U-864 more than a historical wreck. The submarine is sitting in waters used by fishing and shipping, while Norwegian authorities continue to monitor the surrounding seabed and sediment.

For now, measurements have not shown unusually high mercury levels in fish and shellfish from the broader area compared with other parts of the Norwegian coast. A warning, however, remains in place around the immediate wreck site.

Underwater robots finally reached the cargo in 2026

The biggest development came in June 2026, when remotely operated underwater vehicles were used to reach the part of the wreck where the mercury was believed to be stored. Workers pumped sediment away from the submarine's bow and exposed sections of its keel.

A keel plate measuring about 4.2 metres was removed, revealing several compartments. Inside were strange dark materials, glass-like fragments and liquid mercury. Investigators also located mercury containers. A total of 22 containers were brought to the surface during the operation.

The recovery provided an important technical breakthrough. It demonstrated that remotely operated equipment could reach the cargo area and remove some of the mercury without sending divers into the dangerous wreck. But the operation also exposed the central problem.

About 80% of the recovered containers were leaking

The recovered containers had deteriorated badly after more than 80 years underwater. According to the Norwegian Coastal Administration, about 80% of the containers recovered during the operation were damaged and leaking. The recovery produced roughly half a tonne of mercury.

But removing the containers did not mean that all the contamination stayed safely inside the recovery system. Monitoring during the operation indicated that mercury escaped into the surrounding area. The system designed to filter sediment did not work as intended and had to be disconnected. The authorities estimate that around 3.2 kilograms of mercury escaped during the work. That result has become one of the strongest arguments against attempting a large-scale salvage operation.

The wreck is also a war grave

There is another complication beneath the environmental debate. U-864 is not simply a container of hazardous waste. It is the final resting place of 73 people and an important archaeological record of the closing months of World War II. The wreck also contains military equipment, including torpedoes. After more than eight decades in seawater, their condition is difficult to determine.

Any major intervention therefore carries several risks at once: disturbing the mercury, damaging the wreck, disturbing unexploded or deteriorating weapons and potentially releasing contaminated sediment. This is why the Norwegian authorities have struggled to find a solution acceptable from both environmental and historical perspectives.

Should Norway remove the mercury or bury the wreck?

The debate has continued for years. Since 2006, several investigations have examined what should happen to U-864. The Norwegian Coastal Administration has repeatedly argued for placing a protective cover over the wreck and contaminated seabed. The basic idea is relatively simple: instead of disturbing the wreck further, engineers would create a barrier capable of containing the pollution. But the proposal has faced opposition. A government-appointed expert group recommended in 2022 that Norway should first recover as much mercury as possible and then cover the remaining contaminated area.

The 2026 recovery operation has now changed the calculation. After assessing what happened during the June operation, the Coastal Administration submitted its final recommendation to the Norwegian Ministry of Trade and Fisheries on August 28, 2026. Its latest position is that the wreck should be covered rather than subjected to a major mercury salvage operation. The agency's reasoning is that trying to recover the remaining mercury could release more contamination than the operation ultimately removes.

Fedje residents want the wreck dealt with

For people living around Fedje, this is not an abstract environmental argument. A local citizens' campaign has pushed for the wreck and its hazardous cargo to be completely removed. Residents have also raised concerns about the torpedoes still lying inside the submarine. The authorities, meanwhile, have to consider what happens after either option is chosen.

Covering the wreck would not make the problem disappear. The site would still need long-term monitoring. Removing the mercury would be a much larger and potentially hazardous engineering project, and the 2026 operation showed just how easily the cleanup itself can disturb contamination.

A World War II problem with no easy ending

The story of U-864 began as a secret military mission during the final months of Nazi Germany's war. It has since turned into something very different. The submarine is now an environmental time capsule, a war grave and a hazardous-waste site all at once. Its cargo was intended to help Japan's wartime industry, but decades later the mercury is creating a problem for Norway.

The Norwegian government must now decide whether to disturb the wreck again or attempt to isolate it from the surrounding environment. Either way, the consequences will last far beyond the decision itself. The war ended in 1945. But on the seabed off Fedje, one of its most dangerous remnants is still waiting to be dealt with.