No dip in the ground, no hollow, no tell-tale depression. The only reason we know it exists at all is that it appears on the Ordnance Survey six-inch map of 1944, plotted alongside a second souterrain recorded within the same enclosure.

Souterrains are underground stone-lined passages or chambers, typically constructed during the early medieval period in Ireland, and associated with the ringforts, or raths, beside which they were built. Their precise function is still debated, but they are generally understood to have served as cool storage spaces, refuges, or both. The ringfort at Ardrah, a roughly circular enclosed settlement of the kind that once peppered the Irish countryside in enormous numbers, contains not one but two of these subterranean features. Having a pair within a single ringfort is relatively uncommon and suggests a site of some complexity, though the ground above gives nothing away. Both souterrains are recorded as having no visible surface trace, meaning the landscape offers no clue to what lies beneath.