The depression measures six metres long, four metres wide, and roughly forty centimetres deep, sitting slightly off-centre to the west of the enclosure. That modest dip in the earth is thought to mark the roof of a collapsed souterrain, an underground stone-lined passage or chamber typically constructed during the early medieval period, often beneath or beside a ringfort, and used variously for storage, refuge, or concealment.

Ringforts, the circular enclosed settlements that dot the Irish countryside in their thousands, were the primary farmstead type of early medieval Ireland, broadly spanning the period from around 500 to 1000 AD. Souterrains were frequently built as ancillary features within them, their entrances sometimes concealed and their interiors cool enough to preserve dairy produce. At Ballymague, the ground has subsided in a way consistent with the slow collapse of an underground corbelled or lintelled stone structure beneath, its roof giving way gradually over centuries as the supporting material shifted. The tentative wording, that it "may indicate" a collapsed souterrain, reflects the fact that no excavation appears to have confirmed what lies below.