What lay beneath was not simply a void in the earth but a structured, stone-roofed underground chamber that had been sealed and forgotten for centuries. This is a souterrain, an earth-cut underground passage or chamber system built in early medieval Ireland, typically associated with settlement sites and used variously for storage, refuge, or concealment. The accidental nature of its discovery is part of what makes it so striking: the collapse exposed something that had been quietly surviving just 65 centimetres below the surface of an ordinary Cork pasture.

The souterrain at Ballynabortagh consists of two chambers connected by a narrow creepway, a low crawl passage cut through the dividing wall. The southern chamber is rectangular, roughly 3.4 metres north to south and 2.2 metres east to west, with a ceiling of large overlapping flagstones resting on lintels and held up by four pillar stones. Two of those pillars, at the north-east and south-west corners, carry ogham inscriptions. Ogham is an early medieval alphabet rendered as a series of notched and slashed lines cut along a central stem, most often seen on standing stones; finding it inside a souterrain, on structural pillars, is considerably less common. The northern chamber is smaller and oval in plan, earth-cut like its neighbour. A blocked creepway in the east wall of the southern chamber may once have served as the original entrance, and a recess in the west wall, about 1.25 metres long, remains unexplained. The site was documented by McCarthy in 2005. Adding further texture to the landscape, another souterrain sits roughly 210 metres to the south-south-west, and a possible third was recorded about 230 metres to the west-south-west in the 1940s, suggesting that this north-facing slope at Ballynabortagh was once a rather busier place than its current pasture would suggest.