Measured at roughly five metres long, less than a metre wide, and only about forty centimetres deep, this narrow trench running north to south is thought to be the collapsed roof of a souterrain, an underground passage or chamber built during the early medieval period, typically from dry-laid stone and used for storage, refuge, or both.

The souterrain sits within a rath, the circular earthwork enclosure that was the standard form of farmstead in early medieval Ireland, roughly between the sixth and twelfth centuries. Raths, sometimes called ring forts, were defined by one or more earthen banks and ditches, and the one at Keelties is recorded separately as a distinct monument. Souterrains were commonly dug beneath or adjacent to these enclosures, their roofing slabs laid across stone-lined walls to create a dark, cool space at a fairly constant underground temperature. When those capstones fail and the fill above them shifts downward over centuries, the result is precisely what survives here: a tell-tale linear sag in the surface, the outline of a structure that is still technically present but has folded inward on itself. The western portion of the rath in particular is thought to correspond with a collapsed chamber, suggesting that whatever was built here was at least partially roofed and enclosed before it gave way.