But the depression sitting within the south-western quadrant of a rath at Carra in County Galway may mark the ghost of something that once ran underground. Aligned on a north-west to south-east axis, it has the proportions and orientation that archaeologists associate with a collapsed souterrain, an artificial underground passage or chamber typically built from stone and earth, roofed with lintels, and used in early medieval Ireland for storage, refuge, or both.

The rath itself, a roughly circular earthen enclosure of the kind that served as a farmstead during the early medieval period, provides the broader context. Souterrains are frequently found within or immediately adjacent to such enclosures, tucked into their interiors where they could be accessed from a dwelling above. At Carra, the depression falls within the south-western quadrant of the enclosure, which is precisely the kind of sheltered position where these underground features tend to appear. Whether the roof collapsed gradually through centuries of agricultural activity or simply under its own weight over time, the result is the sunken outline visible today, the soil having settled into the void left behind by fallen stonework.