The qualification matters here, because no physical trace of it survives at the surface. What remains is something rather more elusive than stone or mortar: a tradition, the local memory or recorded belief that such a structure once existed, or perhaps still exists, unseen, beneath the ground.

The souterrain is associated with a ringfort, the circular earthwork enclosure that was the most common form of rural settlement in early medieval Ireland, built and occupied roughly between the sixth and twelfth centuries. Ringforts frequently contained souterrains accessed through a low entrance shaft, their narrow underground galleries sometimes extending for several metres. At Knocks, the ringfort itself is recorded, but the souterrain attached to it has left nothing visible. Whether it collapsed inward long ago, was deliberately filled, or simply lies intact and undetected beneath the soil is unknown. The tradition of its existence was noted and recorded, but the ground keeps its own counsel.