A long, curving depression runs northwards from near the southern bank of the enclosure, stretching over fourteen metres in length, narrowing as it goes. It sits no deeper than about half a metre at its most pronounced point, and is just over three metres wide at its broadest. To a passing eye it might seem like little more than a dip in the earth, but its shape and position suggest something more deliberate lies beneath.

Archaeologists believe this hollow may mark the collapsed roof of a souterrain, an underground passage or chamber of the kind commonly associated with early medieval ringforts in Ireland. Souterrains were typically built from stone-lined walls and corbelled or lintelled roofs, and were used for cold storage, refuge, or both. When the roof eventually gives way, the surface above subsides into a characteristic linear depression, which is precisely what appears to have happened here. The ringfort itself, recorded separately, forms the wider context for this feature, and the two together point to a settlement of some complexity, the kind of place where a family or small community would have lived, farmed, and taken precautions against whatever threats the early medieval landscape presented.