There is no hollow in the ground, no depression in the grass, no stone jutting at an odd angle to give it away. The field goes by the local name 'poundeen', a diminutive suggesting it was once used to hold stray livestock, and it keeps its secret well.

A souterrain is a man-made underground structure, typically a stone-lined passage or chamber, associated in Ireland with early medieval settlement and used variously for storage, refuge, or ventilation of nearby dwellings. The one at Carrigthomas came to light through local knowledge rather than any surface indication. It was found just inside the south-western fence of the field, and at some point after its discovery that fence was removed, leaving the ground above it unbroken and undivided. What was once a boundary marker has gone; what lies beneath has no marker at all.