What remains above ground is largely a long depression, the kind of subtle scar in a field that most people would walk past without a second thought. But that hollow marks the ghost of an underground stone structure, nearly seventeen metres of carefully laid drystone corridors that have been slowly filling in for perhaps a thousand years.

A souterrain is an underground chamber or passage built from stone, typically associated with early medieval ringforts in Ireland, where they may have served for storage, refuge, or concealment. This particular example sits within the interior of a ringfort at Lattoon in north County Galway. It consists of two chambers: the first, running roughly north-north-west to south-south-east, measures around 10.5 metres in length, though most of it has fallen in. Only the south-south-east end survives intact, and even that is almost entirely choked with accumulated soil. The second chamber, approximately 6.6 metres long, branches off to the south and runs west-south-west to east-north-east. A gap in its roof at the western end shows that it too has been filled over time. The structure was built without mortar, the stones laid dry against one another in the manner common to early medieval construction throughout Ireland, and the L-shaped plan it follows is a recognised form among souterrains of the period.