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Team leader Allyn Gore said they were excavating a trench on a customer's property when they came across the tunnels.
Mr Gore said: 'Nothing had shown up on any of our drawings or records to indicate there was anything unusual about the site.
'Shortly after the excavation work began, the digging team made the extraordinary discovery of what they initially thought to be a cave. Work stopped immediately.
'I have been involved in other excavations where we have discovered old wells and cellars not shown on any plans, but nothing as exciting and impressive as this.'
The company said the four-ft high tunnels were not on any previous Ordnance Survey map but work at the site was halted so heritage chiefs could inspect