A shallow depression measuring roughly seven and a half metres long, four metres wide, and a metre deep sits inside the fort's bank in the south-western quadrant. That particular sag in the earth is thought to indicate a collapsed souterrain, an underground passage or chamber, typically stone-lined, that early medieval communities in Ireland used for storage, refuge, or both. The roof is gone, the void beneath has caved in, and what remains is this quiet, negative space pressed into the soil.

Souterrains are commonly found in association with ringforts, the circular enclosed settlements that were the typical farmsteads of early medieval Ireland, roughly the sixth to twelfth centuries. The ringfort at Lisleagh already carries its own archaeological record, and the depression in its south-western interior adds another possible layer to the site's story. Without excavation, the identification remains tentative; a collapse of that scale and shape is suggestive, but the underground structure itself, if it ever existed in that form, has not been confirmed by direct investigation.