The collapse left a hole roughly a metre wide and nearly two metres long, and through it archaeologists found the entrance to a souterrain, an earth-cut underground structure of early medieval Irish origin, typically used for cool storage or as a place of refuge. What distinguished this one was its extent: at least four interconnected chambers, dug into the earth beneath pasture on the top of a ridge at Dromnafinshin, and still partly waterlogged when inspectors first entered.

The two chambers that could actually be explored in any detail tell their own story of careful, constrained construction. The first is roughly circular, low-ceilinged at just 0.84 metres high, with a rounded roof and walls cut directly from the earth. From it, a creepway, a narrow crawl-through passage just 0.38 metres wide, connects to a second chamber that opens out slightly: rectangular in plan, about 3.25 metres along its longer axis, and around a metre in height. Both chambers show evidence of construction shafts, vertical openings used during the original digging of the structure and subsequently blocked with stone. From the second chamber, another creepway leads south-east into a third chamber that was flooded and impassable, and through the waterlogged murk of that third space, a further creepway was visible beyond, suggesting the network continues. The full extent of the souterrain remains unknown.