"Sometimes, we discover historical remains. But I've never found anything as exciting as this one," Ole Petter Hobberstad, a chief engineer at Norway's power network operator Statnett, told AFP.

The German navy ship Karlsruhe, measuring 174 metres (571 feet), took part in the World War II invasion of Norway.

After troops had disembarked on April 9, 1940 the ship was hit by Norwegian artillery then torpedoed by a British submarine. Badly damaged, it was finally ordered sunk by the German captain off the port of Kristiansand, at Norway's southern tip.

Three years ago, Statnett's sonars detected an unidentified wreck close to a high-tension cable between Norway and Denmark, but the company's engineers did not have time to investigate further at the time, Statnett said.

But on June 30, after a storm in the area, a team was sent out to inspect the wreck with a remotely-operated vehicle (ROV).

About 15 metres from the underwater cable, the ROV "showed a huge shipwreck that was torpedoed. But it was not until the cannons -- and Nazi symbol -- became visible on the screen that Ole Petter Hobberstad and the crew understood it was from the war," Statnett said in a statement.

Norway's Maritime Museum later confirmed that there was no doubt: the wreck was indeed that of the Karlsruhe, which had never been found.

The sunken ship is located 13 nautical miles off Kristiansand. It lies upright on the seabed, a rare sight for warships with a high centre of gravity which normally list over, according to experts.

Built in Kiel in northern Germany, the cruiser was launched in 1927.