No one was injured.

The 5.4-tonne bomb was found in September 2019 beneath a waterway leading to the port of Szczecin during work to deepen the passage.

More than 750 people were ordered to leave their homes for the sappers' operation, as it was located on the southern edge of the popular Baltic Sea resort of Swinoujscie, which, like Szczecin, was a busy Nazi Germany military port during the war.

Polish Navy sappers were trying to neutralise it underwater through burning out its explosives but it went off in the process.

A spokesman for the sappers, Grzegorz Lewandowski, told the Associated Press that no one was injured as all the sappers were at a safe distance from the blast, which was felt by local residents in the town of Swinoujscie.

"The operation was carried out perfectly and safely and the bomb is safe now," Lewandowski said.

He noted it was the biggest ever such operation by sappers in Poland, where unexploded wartime bombs, missiles and grenades are still found often.

The Tallboy bomb was designed by British aeronautical engineer Barnes Wallis and was used by the Royal Air Force to destroy large Nazi-controlled objects though underground shocks.

The one in Swinoujscie was probably used in April 1945 on the Nazi German battleship Luetzow.

Experts do not know why it failed to explode at the time.

Australian Associated Press