A 12,000-pound World War II-era bomb exploded in a shipping canal off the Polish port city of Swinoujscie on Tuesday, sending a plume of water high into the air as a team of navy divers were working to neutralize it remotely.

No one was hurt in the operation, which the Polish military forces said they considered a success because the munition, a British “Tallboy” bomb, had ultimately been destroyed.

The day before the operation, the Polish Ministry of Defense announced that the bomb was the largest yet discovered in the country and that divers from the Polish Navy’s Eighth Coast Defense Flotilla would attempt to neutralize the weapon remotely through a process in which special explosive charges are used to punch holes in a bomb’s thick steel case and to light the explosive material inside on fire.

If successful, the procedure would have allowed the 5,200 pounds of energetic material inside the bomb to safely burn itself out underwater, allowing divers to raise the emptied casing out of the water or to tow it farther out to sea.

Poland has been clearing explosive remnants of war from its territory since the end of World War II, according to experts with the International Committee of the Red Cross. From 1944 to 2003, more than 96 million pieces of explosive ordnance had been removed at an estimated cost of $866 million, the experts wrote, and from 1944 to 1989, unexploded ordnance claimed the lives of more than 4,000 people in Poland.

In the field of bomb disposal, any procedure taken to render a munition safe also carries the risk that the item will explode, no matter how closely and properly those procedures are followed.

For a bomb like this one, divers would most likely have surveyed the munition underwater, taking measurements and carefully sketching out key identifying features. Analyzing their findings back on land, the divers would pinpoint which types of fuses would normally be attached to such a bomb and study documented procedures for removing or safely destroying them without causing the munition to explode.

Sometimes, however, such steps are impossible. If the fuse is inaccessible because of deformation upon impact or other obstructions, bomb disposal experts may decide to try a process called deflagration, which involves cutting into the bomb and setting the explosive material inside on fire to safely burn itself out. But rapidly burning explosives still contained inside a thick steel casing can explode just the same, even if additional holes are created to vent the gases produced by the fire.

The Defense Ministry also said the operation was the first attempt to neutralize a Tallboy bomb underwater. The ministry did not respond to messages asking for comment, and efforts to reach the Polish Embassy in Washington were unsuccessful.

Designed by the Royal Air Force in 1943, the Tallboy was used primarily for attacks on high-value targets like submarine pens, viaducts and bridges, and on launch sites for the V-1 cruise missiles and V-2 ballistic missiles that terrorized civilians in England during the war. According to an account written in 1990 by Wing Cmdr. John A. MacBean and Maj. Arthur S. Hogben, 95 Tallboy bombs were also dropped on ships like the German cruiser Lützow, which was moored in Swinoujscie in 1945.

The authors credit Tallboy bombs with sinking the German battleship Tirpitz off northern Norway in November 1944. When dropped on land, Tallboys often created craters measuring 85 feet in diameter and 25 feet deep. It was the second-largest bomb used by British forces during the war, after the 22,000-pound Grand Slam munition, which was also dropped on German targets. Together, the Tallboy and Grand Slam were known as “earthquake bombs” for the devastation they caused on land.

The munition destroyed by divers on Tuesday is thought to have been dropped on the Lützow, but failed to explode on impact. According to Wing Commander MacBean and Major Hogben, the bomb’s fuse was designed to blow up the bomb after a delay as short as a few hundredths of a second, but in this case it apparently failed during the attack, leaving the 21-foot long bomb nestled deep into the mud under 33 feet of water for three-quarters of a century.

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