It was originally believed to be an observation post, but the project team are not convinced. "Reigate Hill and this unidentified structure are still very much a mystery to us," said Paul Bowen, a military historian and member of the Front Line Surrey Hills team.
"There's 100 years of Surrey's military history here with Reigate Fort, various unidentified structures and the site of a tragic Second World War plane crash.
"Despite this, we still know very little about the area's military history.
"We would be so pleased to hear from readers who remember Reigate Hill during the Second World War; what structures do they recall on the hill? Who was based there? What were the buildings used for?"
The base has two entrances, four windows on the south-facing side and several ventilation holes, and measures roughly 8m long and 3m wide.
According to members of the team it doesn't appear to occupy a defensive position.
Whose idea was it to construct the top secret communications bunker right inside that old chalk quarry on Reigate Hill in the North Downs, as Battle HQ for Operation Overlord? The chalk and old lime kilns were a great disguise; the hideout was gas-proof and bomb-proof with three entrances and a full ventilation system. War Office documents show the plans and the Royal Engineers built it (Welsh miners). The secret was let out by accident when they were having their hair cut. Mr Fone the barber worked out "You don't MINE for chalk". The Reigate Hill site was also used extensively for anti-aircraft guns. Eisenhower was stationed at a mansion to the right, Montgomery at a villa to the left, beside his pigeon loft for sending secret messages. Beeches woods straight ahead were used as tree cover for a camp for the motorbike despatch riders. Churchill occasionally stayed at an equestrian estate just beneath the chalk scar of Colley Hill. Further to the west was Colley mine, still in use during WW2 and also used as shelter during air raids. The Colley area was frequently bombed. Montgomery insisted on regular 5 mile cross-country runs with his officers.