The evidence is a depression in the ground, roughly five metres long and one and a half metres deep, which archaeologists interpret as the likely position of a collapsed souterrain. Souterrains are underground stone-lined passages or chambers, typically built during the early medieval period in Ireland, and associated almost exclusively with ringforts and similar enclosed settlements. They were used for storage, refuge, or both, and their entrances were often deliberately concealed. When the roof-stones or corbelling of such a structure give way over time, the ground above sinks, leaving precisely this kind of elongated hollow as the only visible trace.

The ringfort itself, recorded as CO027-089, is the broader context for this small anomaly. Ringforts, which are circular enclosed farmsteads defined by one or more earthen banks and ditches, were the dominant settlement type in early medieval Ireland, broadly from around the fifth to the twelfth century. Thousands survive across the country in varying states of preservation, and a significant proportion are known or suspected to contain souterrains. At Manning, the depression sits towards the western side of the fort's interior, which is where ancillary features of this kind are often found, away from the main living area. Whether the souterrain's passage and chambers remain intact beneath the collapsed section, or whether the structure has been entirely crushed by the weight of time and soil, is unknown.