That depression, curving through the northern bank of a rath, is thought to mark the outline of a collapsed souterrain, an underground passage or chamber built in early medieval Ireland typically for storage, refuge, or concealment. Where most souterrains are recorded because someone stumbled into one, or because stonework was exposed during agricultural work, this one announces itself only through the shape of its own failure, the ground having settled over centuries into the void left behind.

The souterrain sits within a rath, the kind of earthen enclosure, roughly circular, defined by a raised bank and internal hollow, that formed the farmsteads of early medieval Irish families from roughly the sixth to the twelfth centuries. Raths are common across Ireland, but the presence of a souterrain inside one adds a layer of complexity. The Y-form of the depression is notable; it suggests the underground structure may have had branching passages rather than a single corridor, a design seen elsewhere in Irish souterrains where separate chambers served different practical purposes. The feature at Altbeagh has not been excavated, so what lies beneath remains a matter of inference from what the ground gives away at the surface.