The device is one of the countless thousands of high explosives dropped across Europe by both sides during World War II.

Bombs are uncovered across Europe

Exactly how many of these remain undiscovered is almost impossible to estimate, but the UK Government know the location of at least 100 beneath the homes, businesses and parks of London alone.

When the whereabouts of these unexploded bombs (UXBs) was made public in 1996, the Ministry of Defence said it did not intend to make safe the explosives unless there was an indication they had become "unstable".

Best left alone

Captain Peter Shields, an army bomb disposal expert for 10 years, says that if left undisturbed these bombs are unlikely to go off, though deeply buried bombs may detonate without anyone noticing.

While each case varies enormously, once uncovered all bombs are treated as if they are unstable.

Bombs can become "unstable"

After more than half a century in the ground, the metal German bomb casing were made from has a reputation for reacting badly with the explosive mixture held inside.

Heat changes, shock, friction and touch can all trigger the bomb fuse to devastating effect.

Because of this real threat, Captain Shields says the army's preferred option is to allow a bomb to explode under controlled conditions.

Damage control

"Technology is really helping out so that we can mitigate the effects of such explosions."

While in peace time the main priority is the preservation of life, should a controlled explosion be felt inappropriate there is no substitute for sending in the experts to attempt a defusing.

"We've tried robotics, but research still hasn't produced a robot able to replicate what a man or woman can do."

Robots cannot take the place of humans

Captain Shields, who received the Queen's Medal for Gallantry last year after spending 24 hours defusing a German bomb in Wiltshire, says the task is not one without its tense moments.

"You've got a job to do and that takes precedence over every other thought. You're too busy to have any personal thoughts of fear, well, except the odd twinge.

"For most of the time it's a team effort and you just rely on your training. But for 5% it does become a bit of a 'lonely walk'."

The job is made no easier by the booby traps the bombs' designers created to stop wartime defusing.

Booby trap

"The bombs were sometimes 'trapped' and set up so they would attract the attention of disposal teams and take out valuable personnel."

Although London was the most heavily bombed part of the country, with some 18,800 tons of high explosives dropped on the capital between September 1940 and May 1941, few British towns escaped totally unscathed.

London endured more than 18,000 tons of high explosives

While industrial centres and ports, such as Liverpool and Coventry, also bore the brunt of Luftwaffe attacks, bombs rained down on even the most unlikely targets.

Some 1,500 people died when Bath was attacked as part of 1942's so-called "Baedecker" raids, named after the famous guide book, when Hitler turned his attentions to destroying places of historical rather than strategic importance.

German bomber pilots unable to find their target were also known to dump their deadly cargo over the British countryside before returning to base.

Hidden danger

Captain Shields doubts the majority of the German ordnance that failed to explode will ever be found.

"The likelihood of finding a German bomb while digging in your garden is almost nil. Because of their ballistic shape and the height from which they are dropped, they tended to bury themselves very deep."

Many children escaped the cities during the Blitz

Captain Shields says no-one can explain why any particular bomb, from the smallest incendiary to a 2,000kg high explosive device, did not explode.

"It may have been wartime mass production. You can't make something as complex as a fusing system in vast numbers without some going wrong."