Measuring roughly six metres north to south and two metres east to west, with a depth of around half a metre, it sits just inside the eastern entrance of the enclosure. Its dimensions and position suggest it is not simply a natural dip in the terrain, but the surface expression of something that once existed underground and has since given way.

The suspected culprit is a collapsed souterrain chamber. Souterrains are stone-lined underground passages and chambers built during the early medieval period in Ireland, typically associated with ringforts, the circular enclosed settlements that were the basic unit of rural life between roughly the sixth and twelfth centuries. They were used for storage, possibly for refuge, and in some cases may have served as escape routes. When their roofing stones or the earth above them fails, the result is precisely the kind of shallow, elongated depression visible here. The ringfort at Caherduggan is recorded separately as a distinct site, and the souterrain, or what may remain of it, sits within that enclosure near its entrance.