Developers building on a former airfield on Teesside may have uncovered a wartime bunker.
The enormous concrete block, believed to be a gun turret or bunker, was discovered while the site at Martinet Road, Thornaby, was being excavated.
Now Maineside Developments is trying to get hold of old records for the airfield to unravel the mystery of the concrete slab.
Apartments are being built at the site of the former Thornaby Aerodrome.
It came into existence in 1930 when the Auxiliary Airforce 608 Bomber squadron was formed, although the area had been used for flying since 1919.
Commercial manager for Maineside Developments Kevin Slade said: "We've only part-excavated the site, so what the building is quite baffling at the moment. But somebody somewhere must have a record of bunkers and other buildings."
During World War II Thornaby served as an operational training unit, with Hudsons taking part in the 1,000 bomber raids over Germany.
The aerodrome finally closed in 1958 with most of the land being sold for redevelopment in 1963.