It is not dramatic to look at, barely a foot deep, but what it may represent is considerably more interesting: the collapsed remains of a souterrain, an underground stone-lined passage or chamber built during the early medieval period, typically used for storage, refuge, or ventilation within a settlement.

Souterrains were commonly constructed within raths, the circular enclosed farmsteads that were the standard unit of rural life in early medieval Ireland, and this feature lies to the west-northwest of the Colgagh rath's centre. The difficulty here is that no lintels, the flat stones that would have formed the roof, and no side walling are visible at the surface. Their absence makes a confident identification impossible. What remains could be the ghost of a passageway whose stone fabric was long ago robbed out or has sunk entirely below the current ground level. The depression itself is the only evidence, a faint signature in the soil where something once existed underground.