The bomb, packed with 3,000 pounds of explosives and believed to have been dropped by the British Royal Air Force, was discovered in the Rhine River after water levels dropped recently because of a lack of rain, Stars and Stripes reported.
The bomb, which officials said was one of the largest unexploded bombs ever found, was discovered along with other unexploded ordnance in the river.
German explosives experts and the German army are to lead the effort to defuse the bomb, which officials estimated could cause a crater more than 20 yards wide and 6 yards deep, cause destruction in a 70-yard radius and blow out windows a half-mile away.
Based on discussions with the British, the Koblenz press office said in an e-mail, "it has been known for some time that this type (of bomb) was dropped over Koblenz."
Plans call for evacuation of city residents in a 1-mile radius as well as a prison and local hospitals.
German news outlets report the bomb lies in about 16 inches of water and is partly buried in mud, making it tough to get to the detonation fuse.
City officials said Wednesday they were still planning how to defuse the bomb.
Explosive ordnance disposal troops from the 21st Theater Sustainment Command in Grafenwoehr and Mannheim will be available to provide support if necessary, a 21st TSC spokeswoman said.
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