It is the kind of feature that could easily be dismissed as a trick of the terrain, but its regularity and position suggest something more deliberate beneath the surface: a collapsed souterrain, the roof of an underground passage that has given way over centuries and left its outline pressed into the earth above.

Souterrains were stone-lined underground tunnels, typically associated with early medieval ringforts, and were used variously for storage, refuge, or ventilation of adjacent structures. The Ballinlig example sits within a raised rath, a ringfort type defined by a notably prominent bank, and appears to follow a pattern common to such sites. The main passage runs close to the inner face of the northern bank, while two possible side passages, each somewhere between five and six metres long, branch off at right angles toward the west. This T-shaped arrangement, if confirmed, would be consistent with more elaborate souterrain designs found elsewhere in Ireland, where lateral chambers served as additional storage or concealment space. The site has not been excavated, and the word "possible" is doing real work here: the depressions indicate a souterrain rather than proving one beyond doubt.