Discovered by Clonmany man Sean Devlin, the previously unrecorded structure appears to be an underground tunnel or souterrain.

Mr Devlin revealed yesterday that he first discovered the underground chamber several years ago while landscaping his front garden, but didn’t make much of a fuss about his amazing find at the time. The historic significance of the tunnel only became apparent recently after Mr Devlin showed it to amateur archaeologist friends.

“I knew it was an exciting find and I did show it to some people but never to any real experts,” Mr Devlin, owner of Devlin’s Fireplaces in Bridgend, told the ‘Journal’. “I had been doing my lawn and dug it out accidentally with a digger. It was a big round circle with a tiny dark tunnel leading off it which seems to go quite far.”

Souterrains are underground man-made drystone built structures roofed with large lintels, comprising of one or more chambers linked by tunnels called creepways. Their entrance is concealed at ground level. They are usually found in locations near to ringforts, cashels and early ecclesiastical sites. Interestingly, Clonmany means ‘the meadow of the monks’.

Mr Devlin says he may try to improve the underground chamber: “My children couldn’t believe it when we found it - it was great. And the tunnel seems structurally safe and dry so eventually I might do it up and maybe try and put some kind of lights in there to make going in there a bit easier.”

Derry man and long time amateur archaeologist Eddie Harkin, who visited and examined this fascinating structure with colleagues Tommy Gallagher and Brian MacNeachtain, confirmed that it has at least three chambers with a creepway linking each one.

In one chamber Mr Harkin says there is a quantity of bones - which may or may not be human - deposited in niches along one side of the souterrain wall. He also found part of a quern stone as well as a quantity of shells.

According to Mr Harkin, archaeologists believe that sounterrains were used as places of refuge, as many of them have defensive features such as low set lintels built into their roofs. They may have also been used for storing food. Indeed, it is possible that this souterrain continues and may be connected to the sixth century monastic site across the road.

A member of his local heritage group, Mr Devlin says he is delighted to have discovered this ancient monument in his garden and he hopes to learn more about it when an archaeologist from Dublin examines it some time soon.