That depression, modest and easy to overlook, is all that remains visible of what was once a souterrain, an underground stone-lined passage typically built during the early medieval period and used for storage, refuge, or both. The ground has long since settled over the structure, leaving only this faint signature in the landscape.

The site sits adjacent to the Altanelvick cashel, a circular stone enclosure of the kind that served as a defended farmstead in early medieval Ireland. Cashels were built by enclosing a domestic settlement within a substantial stone wall, and the souterrain would originally have been an integral part of that complex. Here, the hollow measures about three metres wide where it meets the cashel wall, before extending away as the shallower linear depression. Precisely when it was built, or by whom, is not recorded, but souterrains of this type are generally associated with the early Christian centuries in Ireland, when such underground features were commonly constructed beneath or alongside enclosed settlements across the country.