An air raid shelter built in the Second World War was discovered by engineers planning a sewer upgrade in Tottenham.
Thames Water stumbled upon the hitherto unknown shelter believed to have been built around 1940.
Under the ground at Tariff Road in north London, it measures 10 metres long, 2.5m wide and 2m high.
It can only be accessed via manholes and doesn’t show up on any mapping so the new sewer pipes had been due to pass directly through it. This plan has changed.
Mike Lang Hall, archaeological adviser to Thames Water contractors Optimise said: “Our project must go ahead to protect nearby properties from flooding but now we’re doing it in a way so as not to disturb such a precious piece of our heritage.”
There is no record of ownership, he said, adding: “We suspect it was built to serve the local community and possibly workers at the nearby Triumph factory.”
The water company is appealing to the public to see if anyone has any memories of it.