A souterrain, an underground stone-lined passage or chamber constructed during the early Christian period and typically used for refuge or cool storage, had survived here for perhaps a thousand years, only to collapse when heavy machinery passed over it. The cavity simply gave way, leaving behind a subsidence and a question about what, exactly, had been lost.

The structure had been documented as early as 1929, when Gogan recorded a single chamber with an ope, meaning an opening or entrance, at one end. It lay approximately 300 yards from a place known locally as Brother's Fort or Brodar's Fort, a ringfort that has since been levelled and is now marked on record rather than on the landscape. The souterrain sat to the south-west of the ringfort, near Brothersfort House, and the association between the two features is entirely typical. Souterrains are frequently found in close proximity to ringforts, the circular enclosed farmsteads that were the dominant settlement form in Ireland from roughly the sixth to the tenth century. The pairing suggests the chamber was once part of a working farmstead, its underground space offering security or storage for whoever lived within the enclosing bank. By the time the ground collapsed, some four decades before the site was last reviewed, the ringfort itself had already been erased. The souterrain followed it into effective disappearance, though by accident rather than design.