Nobody nearby seems to know it is there, and nothing on the surface gives it away.

A souterrain is a deliberately constructed underground passage or chamber, typically built during the early medieval period in Ireland, often as a place of refuge or storage. The one at Tonavane is unusual even by the standards of these already enigmatic structures, because of what it apparently connects to. Coleman, writing in 1950, recorded that the souterrain was discovered in the field north-west of a Mr Tim Scanlan's house, and noted the local account that it leads into a natural cave. That combination, a constructed passage merging into a geological one, is a relatively rare arrangement. A subsequent field inspection found no visible surface trace of the souterrain whatsoever, and noted that knowledge of its existence had not persisted among local people. A structure built, perhaps, to be concealed has succeeded, at least in that respect, across the centuries.