A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval ringforts, used variously for storage, refuge, or concealment. This particular one is notable mostly for its absence: by the time anyone thought to record it properly, it had already been sealed.

The sole reference comes from P. J. Hartnett, writing in 1939, who noted only that the fort once had a "cave" but that it had been closed. No further detail survives as to when it was blocked, by whom, or what condition it was in beforehand. The ringfort itself remains the site's primary feature, and the souterrain exists now mainly as a footnote within it, a structural ghost suggested by a single line in a decades-old observation. Hartnett's careful use of quotation marks around the word "cave" hints that he was relaying local usage rather than making a technical claim, which is itself a small piece of vernacular history, a reminder that souterrains were known to rural communities long before archaeologists got around to cataloguing them.