The hollow measures nine metres in length and nearly three metres across, and the working interpretation is that it represents the roof-fall of a souterrain, an underground passage or chamber typically constructed from stone and used in early medieval Ireland for storage, refuge, or both. Where a souterrain survives intact, it is usually invisible from above; here, the collapse has made the absence itself the visible thing.

The cashel in which this feature sits is a stone-walled enclosure of the kind common to early medieval Ireland, built as a defended farmstead or settlement. Souterrains were frequently incorporated into such enclosures, often running beneath or alongside the enclosure wall. The depression at Seafield follows that pattern, sitting in the western part of the cashel. Without excavation it is impossible to say how much of the original passage survives below ground, or what condition the stonework might be in, but the surface trace is clear enough to be recorded and measured.