It is a souterrain, a type of dry-stone-built underground chamber or tunnel constructed during the early medieval period, typically associated with ringforts and used for storage, refuge, or ventilation of food supplies. This one curves in a broad arc from north through east to south, stretching eleven metres in length and nearly two metres wide, though very little of it now survives in any condition to speak of.

Only the extreme northern end remains intact. The rest has collapsed or degraded to the point where the original form is difficult to read. One detail draws the eye even so: a large limestone slab standing upright near the middle of the chamber. It is almost certainly not meant to be upright. The working interpretation is that it was once a roof lintel, one of the flat stones laid horizontally across the passage to seal it from above, that has since tilted or fallen into its current position. It is an accidental monument to the slow failure of the structure around it, the last recognisable piece of the original construction still visible above the rubble.