A "secret" medieval tunnel system has been discovered by electrical technicians in south Wales.
A team working for Western Power Distribution made the discovery while moving an electrical pole near Tintern, in the Wye Valley, Monmouthshire.
Western Power said the tunnels, near to 12th Century Tintern Abbey, were not shown on any Ordnance Survey map.
After consultation with Cadw, the energy firm has stopped all work to allow archaeological investigation.
The team were excavating a trench on a customer's property, which is in an Area of Outstanding Natural Beauty near the border between Wales and England, when they unearthed the tunnels.
"Nothing had shown up on any of our drawings or records to indicate there was anything unusual about the site," said Western Power technician Allyn Gore, who was leading the team.
"Shortly after the excavation work began, the digging team made the extraordinary discovery of what they initially thought to be a cave."
'Exciting and impressive'
Mr Gore said "work stopped immediately", adding: "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 4ft (122cm) high tunnel was "tucked away" underneath a footpath running parallel to the Angiddy Brook, and appears to follow the brook's route along the valley.
An expert from Cadw, the Welsh Government's historic and cultural heritage service, was said to be "very impressed" and "quite fascinated" by the discovery, Mr Gore added.
Western Power said the network of tunnels did not appear on any maps dating back as far as the 18th Century, and no local residents or authorities had any knowledge of it.
Mr Gore said it "could take years" before investigations were concluded.