The mysterious medieval tunnel found by accident

A routine excavation yielded a surprising discovery when crews unearthed a mysterious medieval tunnel in Wales.

It didn't appear on any Ordnance Survey map, but when technicians in the Wye Valley near Wales' famed Tintern Abbey were called to remove an electrical pole, they discovered something unexpected.

Allyn Gore, the technician for Western Power Distribution working on the project, said, "I had a phone call about two hours into the work … they said, 'Allyn, we've found a cave.'" The digging had revealed a small opening tucked beneath a cluster of rocks and nearly obscured by vines.

Caves aren't unusual in the Wye Valley. But when they looked a little closer, Gore and his team realised it wasn't a cave at all. They had discovered a stone tunnel.

"When we got there, and we'd seen that part of the side of the banking had caved in, you could see straight away that it was something that was man-made," said Gore.

The tunnel followed a public footpath along the Anghidi River towards Tintern Abbey, the 12th-Century abbey built by Cistercian monks whose ruins are visited by thousands each year.

"It was almost as if the public footpath, people had been walking over it for ever how long that footpath had been there and no-one knew what they were walking on top of," said Western Power Distribution wayleave officer Luke Summers.

Western Power Distribution called in Cadw, the Welsh historic monuments body. Cadw estimated the tunnel to have been built around the 14th Century. The energy firm stopped the project to allow for proper archaeological investigation.

Though an adult couldn't walk upright – footage of the section shows a space about 4ft in height – the arching stonework tunnel appears to be large enough to have allowed people to move undetected through the countryside.

Draped in cobwebs and stippled with puddles, the tunnel yielded few of its secrets in the initial exploration. It will take more investigation to accurately date the hidden historical structure and to determine just what this mysterious subterranean passage might have been used for.

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(Video by BBC Reel, text by Christine Sarkis)

This video is part of BBC Reel's Hidden Histories playlist.

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