“The idea was to keep the Americans tied down in the east” to draw forces away from the campaign in Europe, said Eric Grove, a British naval historian at the University of Salford in northwest England. “Hitler was getting increasingly desperate.”

The long saga of U-864 is far from over. Many of the canisters containing the liquid mercury are now corroding. Small amounts have seeped out, and Norwegian government tests around the wreck have detected slightly raised amounts of the metal in crabs and fish — the country’s second biggest export, after oil and gas.

Indeed, Kristian Hall, an environmental consultant with a Norwegian engineering firm, said the corroding canisters could produce a threat comparable to the disaster at Minamata, Japan, where some 27 tons of industrial mercury compounds were released into a bay between 1932 and 1968, causing nerve and brain damage to hundreds of townspeople whose diet consisted in large part of local seafood.

“If it is not taken care of properly it could develop into a catastrophe, with corroding canisters beginning to fail one after the other,” Mr. Hall said in an interview.

Last month the Norwegian authorities proposed entombing the submarine in a watery sarcophagus, as has been done elsewhere with underwater hazards.

But Lisbeth Stuberg, an environmental protection officer who is one of the 630 year-round residents here, says many islanders want the wreck removed altogether and are planning a torchlight procession on Thursday to protest the government plan.

Image This image from the Norwegian government shows part of the wreck of the German U-boat U-864, which is now an environmental threat. Credit... Norwegian Coastal Administration, via Scanpix Norway

“There are so many questions and no answers,” Mrs. Stuberg said in an interview on this damp outpost north of Bergen, where the local newspaper has taken to printing a front-page tally of consecutive days of rain (73 by Monday). “If you cover it you do not know the consequences. You are only postponing the problem.”