That irregular depression, running across a raised area of earth, is thought to mark the remains of a souterrain, an underground stone-lined passage or chamber built during the early medieval period, typically used for storage or refuge. Most souterrains survive as buried features, invisible until excavated; this one has announced itself through subsidence, its outline traced in the very shape of the land above it.

The ringfort in which it sits, a roughly circular enclosure of the kind built by farming communities across Ireland from roughly the fifth to the twelfth centuries, occupies flat to undulating ground close to Ballysadare Bay. Souterrains were commonly constructed within such enclosures, though their precise function at any given site is rarely easy to determine without excavation. Here, the zig-zag form of the hollow is itself of some interest, since souterrains were often built with deliberate changes of direction, possibly to slow or disorient anyone attempting to force entry. Whether that is the explanation in this case remains uncertain, the feature has not been excavated and its character is inferred from surface evidence alone.