Builders uncovered a huge unexploded German World War II bomb in London on Monday, prompting the evacuation of two schools and hundreds of homes.

The bomb, measuring five feet long (1.5 metres) and weighing 1000 pounds (455 kilogrammes) lay undisturbed below a pensioners’ centre for seven decades in a densely populated southeastern part of the British capital.

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“Seems our OAPs (old age pensioners) are hard as nails, drinking tea on top of a 1000lb bomb for 70 years,” Lucas Green, a councillor in the riverside London borough of Southwark, wrote on Twitter.

He added that the bomb was buried two to three metres underground and still had its tail fin intact, and advised residents to open their windows and keep their curtains closed in case of a blast.

“It’s a World War II-era German bomb,” a spokeswoman for the Defence Ministry said, adding that bomb disposal experts were expected to continue working into Tuesday to make the area safe again.

“It has certainly caused a lot of problems. It needs excavating… this is in a tricky location.”

A 400-metre safety cordon was enforced around the bomb’s location, and evacuated residents gathered in a leisure centre where the council provided meals and emergency supplies.

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Traffic was disrupted and authorities advised Londoners to stay away from the area if possible.

The discovery of unexploded bombs is not uncommon in London, which was heavily bombarded by German forces between 1940 and 1941 in the campaign known as “the Blitz”, which killed tens of thousands of Londoners.

The London Fire Brigade said seven unexploded bombs were discovered between 2009 and 2014, as well as five undetonated hand grenades.

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“It’s an unusual atmosphere here: it’s a sunny day, and the roads are deserted. People don’t know what’s happening,” said Green, a councillor for Grange ward in the district.

“They should just keep calm and carry on, as they would say in the Blitz.”