German authorities evacuated around 20,000 people from their homes in the western city of Cologne on Wednesday after a World War II bomb was unearthed during construction work.

The unexploded US-made device, which lay five metres (16 feet) underground and was discovered Friday, was set to be defused later in the day, and river traffic on the nearby Rhine was to be halted during the operation, city officials said.

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Residents living within a kilometre (about half a mile) of the site have been told to leave their homes, among them around 1,100 people from a large old people’s and care home, they said.

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Cologne Zoo, also within the danger zone, was to remain shut for the day.

Hundreds of police, firefighters and other helpers were involved in clearing the site for the city’s largest evacuation since the end of the war.

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Seventy years after the end of fighting, parts of Germany remain riddled with unexploded bombs from the Allied campaigns, and construction workers regularly stumble upon them.