A rectangular hollow, roughly one and a half metres wide and running six metres from west to east, terminates in a broader circular dip about three metres across. These are the likely surface traces of a souterrain, an underground stone-lined passage or chamber built during the early medieval period, typically used for storage, refuge, or both. The collapse of such a structure over centuries often leaves precisely this kind of subtle scarring in the landscape, easy to overlook and easy to misread.

The souterrain sits to the north-east of the centre of a ringfort, the circular earthwork enclosure that would once have been the defended farmstead of a local family of some standing. Souterrains were commonly built in association with ringforts across Ireland, and the relationship here follows that pattern closely. The circular terminal depression, wider and somewhat deeper than the passage leading to it, suggests the presence of a chamber at the end of the underground structure, a feature found in many examples elsewhere in the country. What survives at Ballycumber is, as the cautious archaeological language puts it, only a possible site, meaning the ground shapes are consistent with a souterrain but the structure has not been excavated or confirmed by direct investigation.