On a gentle rise above the undulating pasture of Kerinstown and Balrowan in County Westmeath, this depression sits quietly within the south-eastern quadrant of a ringfort, and the question of what it actually is remains, for now, unanswered.

A souterrain is an underground stone-lined passage or chamber, typically built during the early medieval period in Ireland and associated with ringforts, the circular enclosed settlements that dot the Irish countryside. They were used variously for storage, refuge, or ventilation, and their roofs, when they eventually collapse, often leave precisely the kind of elongated hollow visible here. The depression at Kerinstown measures 3.8 metres wide and extends inward from the inner face of the ringfort's south-eastern bank toward its centre. Along both sides of the hollow, slight raised banks of earth suggest what archaeologists call up-cast, the soil displaced when a passage is dug, which would support the souterrain interpretation. The feature is subtle enough that it is only clearly legible as a slightly curvilinear depression when viewed on aerial photography. There is, however, a competing explanation. The hollow may have nothing to do with early medieval construction at all, and could instead be the result of quarrying carried out after 1700, which would make it a comparatively recent disturbance rather than an ancient structure. The site sits on a slight rise with open views in all directions, which is characteristic of ringfort placement, but the depression itself has not been excavated, and the ambiguity is likely to persist.