The Tallboy, also known as the “earthquake” bomb, weighed 12,000 pounds when dropped by the Royal Air Force during the final months of the war.
The bomb is the largest ever found in Poland and is believed to have been used in a raid on the German ship Lützow.
The Polish Navy attempted to defuse it on Tuesday by blasting, a method of heating an explosive until it burns, rather than detonating it, a spokesman for the Polish Defense Ministry told CNN .
But the bomb exploded in the canal near the port town of Swinoujscie, sending out a huge plume of water.
Nazi-occupied Poland suffered repeated bombardments from Allied planes throughout World War II, leaving many cities devastated.
The RAF sent several Lancaster bombers from its so-called “Dambusters” squadron to drop 12 Tallboys on the Lützow in April 1945, but one did not explode.