Archaeologists have been left dumbfounded after they unearthed a “mysterious” bunker in a school playing field.

Hendon and District Archaeological Society uncovered the reinforced concrete structure while excavating at Martin Primary School in Plane Tree Walk, East Finchley.

The group can find no records of the hole in any maps or documents and no one it has talked to remembers it being built.

The network of underground tunnels was discovered as the society helped children dig the field during the school’s centenary celebrations.

Society member Bill Bass said he was very surprised when test pits showed what looked like rubble from bomb blasts on what they thought was land that had never been built on.

He said: “As we began to dig through the rubble the top of the wall emerged. We didn’t know what it was. It could have been a cistern or a water feature.

“There’s no record of anyone building on this site and no one knew anything. It’s a mystery.”

Chairman of the society Don Cooper said the structure, which he believes was an air raid shelter, was built in the 1930s and filled in during the 1950s.

He added: “There are a similar structures in Sunnyhill Park in Hendon which were recorded. Lots of these bunkers were dug quickly during the early years of the war. 

"There would have been wartime secrecy but someone must have built it.

“It’s weird that the roof was smashed and torched and it was filled in with rubble at the height of the Cold War when people were worried about the nuclear threat.”

The long, thin tunnels would have been lined with benches on either side, whose positions are still marked with numbers on the walls.

The structure, which archaeologists estimate is 2m deep, 20m wide and 40m long, would have been built with bent tunnels and asbestos doors to limit the damage caused by bomb blasts.

Mr Cooper added: “You could have easily got a couple of hundred children in here. Imagine being the small child farthest away from the door.”

Headteacher of Martin Primary School Helen Morrison said she and the children had been very excited when the bunker was unearthed.

She added: “It was incredible to find this, especially during our centenary year.

“We asked old pupils from the era at an open day we had if any of them remembered it. None of them did, whereas they did remember using the brick air raid shelters that surround the school.

"Maybe it was used by local dignitaries.”

The headteacher added the children had loved getting their hands dirty and unearthing finds such as glass and pottery.

“All the children have been involved in digging the site. I hope the four and five-year-olds will remember finding this incredible bunker.”

Now the Hendon and District Archaeological Society and the school are asking any Times Series readers if they remember using the structure as children.

Mr Cooper added: “Maybe someone can help us solve the mystery.”