An L-shaped hollow, roughly a metre and a third deep, sits within the south-south-western sector of the enclosure, and its shape strongly suggests it is not simply a dip in the earth but the roof of something that once lay beneath it.

The hollow is thought to mark the collapsed remains of a souterrain, an underground stone-lined passage or chamber typically built during the early medieval period, often in association with raths, the circular earthwork enclosures that were the farmsteads of Gaelic Ireland. Souterrains were used for storage, refuge, or both, and were constructed to last, which is why so many survive, even if only as depressions like this one. The geometry here is unusually legible for a collapsed example: the longer axis runs roughly north to south and measures just over ten metres in length and nearly four metres wide, while a second arm extends east to west from its south-western end, measuring nearly ten metres long and four and a half metres wide. Together they describe an L-shape that mirrors the plan a builder would have followed underground, its outline preserved by the soil settling into the void left behind.