In 1945, the German submarine U-864 was on its way to deliver mercury for weapons production to war-time ally Japan, when it was torpedoed off Norway's western coast. All of the 73 men on board went down with the U-boat, as did the steel containers filled with 65-tonnes of weapons-grade mercury.

The Norwegian Navy spent years looking for the submarine. They finally located it in 2003, lying in 150 meters of water near the island of Fedje, some 70 km from Bergen.

Over the next three years, the coastal authority conducted field studies looking at how best to deal with the ecological time-bomb ticking in the North Sea.

Contaminated ocean

Speaking to DW-WORLD.DE, Ane Eide Kjeras, a spokesperson with the Norwegian Coastal Administration, said the sea bed surrounding the wreck was "heavily contaminated" with mercury. There was also "a low but increasing level of mercury in fish and sea food," she said.

Fishing is banned within a one kilometer zone around the wreck

Not only is mercury a danger to marine life, it can also pose a hazard to humans who eat the contaminated sea food.

The Norwegian Coastal Administration has now recommended hermetically sealing off the submarine and surrounding silt in a layer of sand, gravel and concrete -- an underwater version of the Chernobly nuclear reactor.

There are several good reasons not to attempt to salvage the U-boat, Kjeras said.

"When divers tried to dig into the keel of the U-boat, the wreck started to move because the ground there is very unstable," Kjeras said. "There was also crack in the keel so we are afraid to salvage the U-boat in case it splits and all of the mercury spills into the ocean."

WWII weapons on board

And then there is also the danger that the World War II submarine is still armed.

"We have to assume there were at least a couple of torpedoes on board," said Kjeras, adding that the Norwegian Army were closely involved in the investigation.

"They advised us that the danger of the torpedoes going off is much larger if you try to lift the wreck," Kjeras told DW-WORLD.DE.

The Norwegian government hasn't decided on the exact details of how to seal off the submarine. They also aren't revealing any details of the estimated project costs. But if the measure is passed by parliament, it could begin as early as next summer.