Where the southwestern bank of an ancient circular enclosure has worn away or broken open over time, a shallow linear depression runs inward across the interior, six metres long, three metres wide, and just over half a metre deep. The working theory is that this sunken trace marks the collapsed roof line of a souterrain, an underground passage or chamber, typically built from stone, that was used in early medieval Ireland for storage, refuge, or both. They are common enough finds within raths, the earthen ringforts that once served as farmsteads across the Irish countryside, but they are rarely obvious to the untrained eye. This one is less obvious than most.

The depression sits within a rath, a type of enclosed settlement defined by one or more circular earthen banks and ditches, which was the standard form of rural habitation in Ireland from roughly the fifth to the twelfth century. Souterrains within such enclosures were often connected to the main dwelling and could extend for considerable distances underground, sometimes branching or incorporating tight crawl-ways designed to slow an intruder. What survives at Cartron is not the structure itself but the faint negative impression left where the ground above it has softened and subsided, the roof having long since given way.