At Levallyroe in County Mayo, a souterrain, an underground stone-lined passage typically used in early medieval Ireland for storage or refuge, was recorded within a rath, a type of circular earthen enclosure that served as a farmstead during the early medieval period. The site carries the designation, but the underground structure itself remains stubbornly invisible.

The trail of evidence begins with the historian H. Knox, who in 1911 described two stones in the outer bank of the rath that seemed to hint at something buried. He identified a vertical stone roughly two feet high standing just outside the outer ring, and beside it to the south, a large horizontal stone set into the bank itself, which he suggested might be the lintel of a sealed creep hole, the narrow entrance passage through which a person would have crawled to access a souterrain. It is a plausible reading of the landscape, the kind of observation that requires both experience and imagination. Later examination, however, found two large stones in the eastern external bank, one on the inner face and one on the outer face, but placed in a way that does not align with each other. Neither stone, on current evidence, can be confidently identified as part of a souterrain at all. What Knox saw in 1911 and what survives or remains detectable today are not quite the same thing, and the gap between those two moments is where the uncertainty lives.