That quiet label is the cartographic trace of a souterrain, an underground passage or chamber of the kind built in early medieval Ireland for storage, refuge, or escape, and it raises a question the ground itself refuses to answer. When surveyors from the Galway Survey visited the site in July 1984, they found no visible surface trace of the structure at all.

The enclosure in question surrounds Cargin Church, and the souterrain was reportedly located in its north-western quadrant, close to the associated graveyard. Local tradition, recorded as recently as 2004, holds that a tunnel ran from the graveyard to Cargin Castle, roughly 490 metres to the south-south-west. That distance is considerable, and such a passage would have been a substantial undertaking, though long souterrains linking ecclesiastical or domestic sites to nearby fortifications are not unknown in the Irish archaeological record. Whether the tunnel ever existed in full, whether it collapsed long ago, or whether the oral account preserves a memory of something more modest, cannot be said with confidence. The site sits in a landscape where church, graveyard, and castle once formed an interdependent cluster, and the 'Cave' on the old map may be the last legible sign of whatever connected them.