The fractured roofing slab gave way under the machinery, and what emerged was a souterrain, an early medieval underground stone passage or chamber typically associated with nearby settlement, used variously for refuge, storage, or concealment. This one had been waiting quietly beneath a field in Kilpadder, in north County Cork, with no surface trace to betray it.

When Twohig investigated the site in 1976, the chamber proved to be a single, roughly rectangular space: 4.2 metres long, no more than a metre wide, and 1.4 metres high, oriented north to south. Seven of its original roofing lintels, the flat stones laid horizontally across the top, were still in place. The walls were corbelled, meaning each course of stone was set slightly inward over the one below, gradually narrowing the space toward the roof, a technique that gives the structure a degree of self-supporting stability without mortar. There was also something unusual in the construction: evidence that some of the wall stones had been roughly dressed, shaped or trimmed, after being set in position rather than before, suggesting a degree of improvisation or adjustment during the build. The original entrance was never located; the most likely explanation is that the debris and spoil thrown up by the collapse buried it before anyone could record it.