LONDON, England (CNN) -- A British Army bomb disposal team is working carefully to dismantle a large unexploded World War II bomb unearthed this week near the future Olympic Park in London, officials said Friday.
Army engineers covered the bomb with sand to minimise the risk of an explosion.
Contractors clearing a site in east London unearthed the 2,200-pound German bomb Monday, the Ministry of Defense said, and called in the army experts to make it safe. The bomb measures 4 feet by 2 feet and is the largest found in the capital since 1975, defense officials said.
"It's enormous," said Simon Saunders, the spokesman for the British Army's London district.
The bomb's discovery has forced subway and train lines in the area to shut, transportation officials said.
The contractors who unearthed the bomb at didn't know what it was and pulled it from the ground, Saunders said.
"When they realized what it was, they got out of the way quite quickly," he said. Watch a report on the bomb defusal efforts »
They called in the Army bomb disposal engineers, who covered the bomb with sand to safeguard against an explosion and established a 250-yard cordon around it, Saunders said.
At one point, the bomb started ticking, which suggested a timing device, he said. Disposal experts put strong magnets next to the bomb to shut down the clockwork, and the ticking stopped, he said.
Thursday night, the army team began trying to remove the explosives inside the bomb. They drilled into the shell casing using high-pressure streams of water, Saunders said.
The Army team hoped to find that the explosives inside were in powder form, which could simply be removed with water, but they found that the substance must instead be steamed out, which is a longer process, he said.
"Having steamed it out, that will expose the detonator," Saunders said. "That will have to be made safe with a small charge, which will then make the detonator safe."
The army hopes to complete the process by Friday afternoon, though it could take longer, he said.
"The danger is that because it is old, it is now unstable and unreliable," Saunders said. "The tricky bit, of course, is damaging the detonator whilst they are effectively limiting the explosive charge around it."
The bomb is not in a residential area, but it is near several transportation lines, and that has disrupted service on two subway lines and a rail line, transportation officials said.
"That we can't run a Tube service is always bad. It's difficult," a spokesman for the transportation authority Transport for London said on condition of anonymity. "Obviously, this is no one's fault. It's an historical thing. Everyone just sort of has to bear with us while we get it fixed."
It's not uncommon for World War II-era bombs to be unearthed in Europe. In London, which suffered the aerial bombardment of the Blitz, bombs are uncovered two or three times a year, Saunders said.
The London Blitz lasted from September 1940 until May 1941. German bombers attacked the city every day or night for the first two months, but the worst night was the last: May 10, 1941, when 3,000 people were killed in London, according to the Museum of London.
Much of the Blitz focused on east London.
More than 20,000 people were killed in the Blitz, short for blitzkrieg, the German word for "lighnting war."