The hollow is roughly oval, measuring about 1.7 metres north to south and 2.2 metres east to west, and sinking only around 22 centimetres below the surrounding surface. Unassuming as that sounds, its central position within the ringfort has led archaeologists to suggest it may mark the roof collapse of a souterrain, an underground stone-lined passage or chamber, typically built during the early medieval period to serve as a place of refuge, cold storage, or concealment.

Souterrains are fairly common finds within Irish ringforts, the circular enclosures of earthen banks or stone walls that once served as farmsteads across the early medieval landscape. They were usually constructed by roofing a cut trench with large stone lintels, then backfilling over the top, leaving the passage hidden beneath the living surface of the fort. When the roof eventually gives way, the ground above subsides, leaving precisely the kind of slight, saucer-shaped dip that survives here at Ardglass. Whether this particular depression does represent a collapsed souterrain, or is the result of some other disturbance, has not been confirmed by excavation, and the site retains that particular quality of the genuinely unresolved, something noticed, recorded, and left for the ground to keep its own counsel on.