Beneath its surface, tucked under the southern face of the northern road bank at the crest of a steep north-facing slope in Rathbran More, lies a souterrain, the kind of drystone-built underground passage that early medieval Irish communities constructed for storage, refuge, or both. What makes this one quietly odd is that it has been labelled "Cave" in gothic lettering on Ordnance Survey maps since at least 1836, a cartographic flourish that suggests local awareness of the feature long predating any formal archaeological record, and that its entrance remains open to this day, even as the road above it carries on regardless.

When archaeologists recorded the structure in 1969, they described an L-shaped passage running roughly eleven metres in total: south from the entrance, then turning west to end in a beehive chamber, a corbelled, dome-shaped space built without mortar, a construction technique common in early Irish underground and surface structures alike. By that point the chamber was partially collapsed, exposing rock outcrop behind the stonework, though an air-vent extending westward was still intact. The 1836 and 1908 editions of the six-inch Ordnance Survey maps both mark it, which places local knowledge of it comfortably within the nineteenth century at least. Three separate phases of archaeological testing in the surrounding area, carried out by R. Meenan in 2004, C. Gleeson in 2008, and Donald Murphy in 2020, failed to turn up any material directly associated with the souterrain. The most recent of these, to the east of the structure, did find an east-west trench over two metres wide and 1.6 metres deep filled with soil containing plastic inclusions, which is rather more recent than early medieval, and tells us nothing about the souterrain itself beyond the fact that its wider landscape context remains archaeologically blank.