What lies beneath, or rather what once lay beneath, is a souterrain: an underground stone-lined passage or chamber of the kind commonly built during the early medieval period in Ireland, typically serving as a place of refuge or cool storage. The collapsed roof has left the land above it slightly sunken, and those dips in the earth are now the only outward sign that something was carefully constructed there long ago.

The depressions sit within the bounds of a ringfort, the type of enclosed farmstead that was the standard unit of rural settlement in early medieval Ireland, usually defined by one or more earthen banks and ditches. Souterrains were frequently incorporated into such enclosures, built to be entered from inside the protected space of the fort. At Caherbeg, the underground structure has not survived intact; the weight of time and soil has caused the passage or chamber to collapse inward, leaving the ground above it subtly deformed. It is a common enough fate for souterrains, and in some ways the collapse itself becomes the evidence, the negative space where a structure used to be.