A souterrain is an underground passage or chamber, typically built during the early medieval period, constructed from stone and used variously for storage, refuge, or as an escape route from a ringfort above. The ringfort it once served has itself been erased from the landscape, graded flat at some point in the intervening centuries. What remains is an absence layered upon an absence.

The site appears on the Ordnance Survey six-inch maps of both 1842 and 1901, marked simply as "Cave", which was a common if imprecise label applied by nineteenth-century surveyors to underground features they could not easily classify. The fact that it was recorded twice, across sixty years of mapping, suggests the entrance or some surface indication was still detectable into the early twentieth century. The ringfort to which the souterrain belonged, recorded separately in the Cork archaeological record, occupied the southern half of the same ground. Ringforts, the enclosed farmsteads of early medieval Ireland, were frequently furnished with souterrains, and the pairing here at Shantullig would have been entirely typical of the period. What is less typical is how thoroughly both have vanished. There is now no visible surface trace of either structure.