A souterrain is an underground passage or chamber built from drystone masonry, typically constructed during the early medieval period and associated with nearby settlements. Their precise function is still debated, but they are generally thought to have served as refuges, storage spaces, or both. What the machine uncovered turned out to be one of the more substantial examples recorded in the region.

Following the accidental exposure of the lintel, the site was investigated by Professor E. Rynne, who documented a well-preserved, roughly L-shaped structure running to approximately 28.5 metres in total length. The first and longer of the two chambers, around 19 metres long, 1.9 metres wide, and 1.5 metres high, ran on a northwest to southeast axis, with the entrance located near its northwest end. A second chamber, approximately 9.5 metres long, branched off from the north end of the east wall and ran in a roughly east-northeast to west-southwest direction. Alcoves were noted at the northwest end of the first chamber and at the northeast end of the second, small recesses that hint at deliberate, organised use of the interior space. The souterrain sat within a rath, a type of circular earthen enclosure that would once have enclosed a farmstead, suggesting the underground structure was part of a functioning early medieval settlement.

The souterrain has since been filled in, and only a slight depression in the ground now marks where it lies. There is nothing dramatic to see at the surface, but that understated trace is itself part of the story: a landscape feature that survived intact for over a millennium, only to be encountered almost by accident, documented, and then quietly returned to the earth.