Archaeologists believe they have unearthed an ancient fortified settlement at a hilltop paddock in Buckinghamshire.
Eddie Stratford and Darren Towers at the dig site Taplow
A dig on the site of a new £30m Thames Water main near Taplow has uncovered finds thought to date to the Bronze and early Iron Ages.
Pieces from Roman and mediaeval times have also been found.
The finds suggest it would have been a settlement on a key route to the centre of the country between 700BC and 400BC.
But despite the fortifications, experts called in by the water company believe the site would not have had a military purpose.
Project manager Mark Collard from archaeological contractors Cotswold Archaeology said the settlement, although itself previously unknown, fitted into
a pattern of farms enclosed by fortifications.
"It's not particularly military - it would have been more to say 'This is our bit of land', it's more a sort of status symbol than anything else."
The team was called in ahead of construction work because evidence from nearby sites indicated the area was of historical interest.
But although there is a cluster of other prehistoric sites in the vicinity, the latest find came as a complete surprise to the archaeologists.
"It was totally unknown before work started, Thames Water commissioned a whole series of surveys, it's a success for the system.
"Basically it was a green field before the work started, if they hadn't decided to put a pipeline there we would never have known it was there."
The finds will now be taken away for more detailed study expected to include radio-carbon dating and examinations of the changing styles of pottery to give a
clearer picture of their age.
Mr Collard said: "Any discovery of a prehistoric site is significant particularly one that it relatively well preserved and extensive."
Thames Water is keeping the exact location secret
to prevent any interference at the site but Cotswold Archaeology will eventually produce a full report and register the findings with the Sites and Monuments Records Office so that other archaeologists will know the location.
The Thames Water project is to create a new pipeline to carry water from boreholes near Taplow to a treatment works boosting local supplies.