A souterrain is a man-made underground structure, typically stone-lined, associated with early medieval ringforts and used variously for storage, refuge, or as a means of escape. The one at Copsetown exists, if it exists at all, only as a tantalising near-miss: a single flagstone glimpsed and then re-covered, leaving nothing visible at the surface today.

The record of this discovery comes from the antiquarian J.L. Grove White, who documented it sometime between 1905 and 1925. Workmen digging in the ditch of the fort struck a flagstone and, rather than investigate further, covered it back in. Grove White noted it as a possible souterrain lintel, the kind of flat capstone that would sit across the top of an underground chamber or passage. Whether the structure beneath was intact, partially collapsed, or something else entirely, nobody at the time thought to find out. The ringfort itself, a circular earthwork enclosure of the early medieval period, survives at the site, though the potential souterrain beneath it remains unexcavated and unconfirmed.