The five-tonne device — nicknamed “Tallboy” and also known as an “earthquake bomb” — was dropped by the Royal Air Force in an attack on a Nazi warship in 1945.
It was discovered last year during dredging close to the port city of
Swinoujscie — formerly Swinemünde, a part of Germany — in the far northwest of Poland.
“It's a world first. Nobody has ever defused a Tallboy that is so well preserved and underwater,” Grzegorz Lewandowski, spokesman for the Polish Navy's 8th Coastal Defence Flotilla based in Swinoujscie, told AFP.
World War II bombs are still frequently discovered, particularly in east but also west Germany.
The German capital was hit especially hard.
Of the nearly 400 air raids that took place in Berlin between 1940 and 1945, American and British bombers combined dropped around 70,000 tonnes of bombs on the city.
During World War II the area was home to one of the German navy's most important Baltic bases and the area was subjected to massive bombardments, said historian Piotr Laskowski, the author of a book on the Royal Air Force raid on Germany's Lutzow cruiser in April 1945.
The ship's cannons were being used to hold back the advance of the Red Army
in the dying days of the war.
On April 16th, 1945, the RAF sent 18 Lancaster bombers from the 617th Squadron — known as the “Dambusters”.
The bombers released 12 Tallboys on the Lutzow but one failed to explode and one of the planes crashed on the island of Karsibor, killing all seven crew on board.
The ship survived the raid but was eventually seized by the Soviet army and used for target practice after the war. It sank in the Baltic in September 1947.
The Navy announced on its Facebook page that the operation had begun and
that it was planned to take until Friday, depending on the weather.
Michal Jodloski, the head of the diving team, told reporters later on Monday that the operation was “going according to plan” but work was slow as only one diver at a time was being allowed to work on the bomb because of the risks.
A total of around 750 local residents are being evacuated from an area of 2.5 kilometres around the bomb and the operation is expected to last up to five days.
But some residents told AFP they would be staying put.
Halina Paszkowska said the “main danger” for her was the risk of catching Covid-19 in a sports hall where residents are being given shelter during the operation.
Paszkowska said she also had to look after her 88-year-old mother, adding: “I've lived here 50 years and there have been other bombs, but this is the first time there's an evacuation! Before, we just had to stay indoors.”
'A very delicate job'
Maritime traffic on the navigation channel and surrounding waterways will be suspended in an area of 16 kilometres around the bomb disposal operation.
“The first two or three days will be preparations. Our bomb disposal divers will scrape around the bomb, which is embedded in the bottom of the channel at a depth of 12 metres. Only its nose is sticking out,” Lewandowski said.
“It's a very delicate job… The tiniest vibration could detonate the bomb,” he said, pointing out that the option of a controlled explosion has been ruled out for fear of destroying a bridge some 500 metres away.
The navy divers will instead use a technique known as deflagration to burn the explosive charge without causing a detonation, using a remotely controlled device to pierce through the shell to begin combustion.
The bomb is six metres long and has 2.4 tonnes of explosives – equivalent to around 3.6 tons of TNT.
Tallboys were designed to explode underground next to a target, triggering shock waves that would cause destruction.