Beneath that depression, or what remains accessible of it, lies a souterrain: an underground passage built without mortar, using carefully stacked dry stone, and dating to the early medieval period in Ireland. Souterrains were constructed beneath or beside raths, the circular earthwork enclosures that served as farmsteads for Ireland's early medieval population, and they functioned variously as storage spaces, places of refuge, or both. This particular example, at Carhan in County Kerry, sits centrally within such a rath, which is where these structures are most commonly found.

The visible remains are sparse. Only one of the original roofing lintels, the flat stones laid horizontally across the top of the passage to form a ceiling, survives in position. The accessible section of the passage measures just 1.65 metres in length and 1.08 metres in width, suggesting a relatively modest chamber, though the full extent of the souterrain may once have been considerably greater. The archaeological survey of the Iveragh Peninsula, compiled by A. O'Sullivan and J. Sheehan and published by Cork University Press in 1996, records the site in its current poorly preserved condition. What can be traced at ground level is largely that north-south depression, a ghost of the structure that once ran beneath it.