A single stone rests within the depression, sitting roughly 6.5 metres from the ringfort's bank, and the current interpretation is that this dip in the earth indicates the collapsed roof of a souterrain, an underground stone-lined passage or chamber typically built during the early medieval period as a place of refuge, storage, or concealment. The structure, if that word still applies, has long since caved in on itself, leaving only this quiet irregularity in the landscape as evidence.

Souterrains are relatively common finds within Irish ringforts, which were enclosed farmsteads used from roughly the sixth to the twelfth centuries. They were usually constructed from dry-stone walling and roofed with large slabs, then covered over with earth, making them difficult to detect once the ground above had settled or shifted. At Rathduane, the combination of the hollow and the displaced stone suggests the roof at this point gave way at some stage, leaving the telltale depression that archaeologists have since recorded. The ringfort itself, catalogued separately, provides the broader context for this small feature, which would originally have been accessible from within the enclosed area above ground.