It measures roughly 1.6 metres long, a metre wide, and only 0.4 metres deep, sitting between the centre of the fort's interior and a gap to the north-northwest. Unassuming as it appears, that sunken patch marks the location of a souterrain, an underground stone-lined passage or chamber built during the early medieval period, typically used for storage, refuge, or both.

A souterrain does not always announce itself. Many have collapsed or silted up over centuries, leaving only these tell-tale depressions as evidence of what lies beneath. What makes this particular site of some interest is a note recorded by Bowman in 1934, who documented two souterrains within the same ringfort and remarked that they could easily be opened. Whether that observation reflected curiosity, concern, or something in between is not recorded, but the phrasing suggests the passages were at least partially intact at that point and perhaps uncomfortably accessible. A second souterrain associated with the fort is recorded separately nearby.