The workers noticed a five metre hole in the ground and it is thought to be the roof of a buried air raid shelter which has collapsed.
The only council records mentioning the public bomb shelter are that three large shelters capable of housing 450 people were constructed in the area in 1939/1940.
However, there are no photographs or records of the completed shelters or information kept by the then Sutton and Cheam Council about when they were covered over after the war.
Sutton Council has launched an appeal for information from any residents who may remember when the shelters were open.
Museums Service historian John Phillips said it appeared the shelters had trenches and were made of concrete slabs – common in public bomb shelters. It is unclear at this stage whether this is one large shelter or three separate buildings.