A souterrain, in Irish archaeological terms, is an underground stone-built tunnel or chamber, typically associated with early medieval settlement and thought to have served as storage space, refuge, or both. This one has the distinction of being essentially unfindable. It leaves no trace on the surface, its precise location is disputed, and the one serious attempt to locate it turned up nothing at all.

By 1898, when the Ordnance Survey Name Books were compiled, the structure was already being described as an antiquity that had been closed for upwards of thirty years, meaning access had been blocked since at least the 1860s. The 1921 edition of the six-inch Ordnance Survey map does mark a "Cave (site of)" in gothic script at a particular spot, though the 1838 edition records nothing. Local memory, collected in more recent times, held that the souterrain was last entered in the first half of the twentieth century, and described it as a long, narrow stone-lined passage. In 1988, ahead of house construction, archaeological testing was carried out at the location the 1921 map indicated. The dig, reported by Conroy in 1989, found no evidence of the structure whatsoever. Local tradition, however, suggests the real entrance lies roughly fifty metres to the north-east of the spot the cartographers had marked, which means the 1988 excavation may simply have been looking in the wrong place.