That subsidence, a dip or hollow where the earth has shifted downward, is thought to indicate a souterrain beneath, one that has collapsed under its own weight over centuries. A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval ringforts, and used variously for storage, refuge, or both. Where an intact example would have a discoverable entrance and navigable tunnel, what survives at Ballyvoige appears to be the negative impression of one, a absence that only makes itself known by the way the surface has given way above it.