A souterrain is a man-made underground passage or chamber, typically built during the early medieval period in Ireland, often associated with a nearby settlement and used variously for storage, refuge, or both. The one at Rathduff sits within a rectangular enclosure, but it has been filled in, and the ground above it gives nothing away.

What is known comes from local memory rather than excavation. The description passed down is quietly evocative: an underground cave with steps leading down into it. That detail about steps is worth pausing on. Many souterrains are entered by crawling through a low lintel; steps suggest something more deliberately constructed, perhaps more accessible in its working life. At some point, for reasons unrecorded, the feature was filled in. No date is given for when this happened, no name attached to the decision. The rectangular enclosure that contains the site, recorded separately, is the only structure still legible in the landscape.