What the bulldozer revealed was a souterrain, an underground structure of the early medieval period, typically cut into earth or rock and used for storage, refuge, or both. This one had been sitting undisturbed beneath the surface, its original entrance unlocated and its existence entirely unknown, until the bucket of a machine punched an accidental opening into its north-western corner.

The structure, investigated by Kiely in 1994, consists of three beehive-shaped chambers carved from the earth and connected by creepways, the narrow low passages that link one chamber to the next and which would have required anyone passing through them to crawl. The first chamber is relatively shallow, only about 65 centimetres high, with a floor that slopes downward to the east and sections of uncoursed drystone walling on the east and west walls. A creepway barely 43 centimetres wide leads from its eastern corner into the second chamber, which sits roughly 30 centimetres lower than the first. From there, a second creepway opens into the third and largest chamber, which reaches nearly 1.6 metres in height and over 3 metres in length, spacious by comparison with what came before it. Traces of charcoal on the floor of the first chamber, close to the creepway entrance, hint at some form of activity within, though the evidence is slight and no firm interpretation has been offered.