The field above it shows nothing unusual at ground level, yet below it runs more than twenty metres of connected chambers and narrow linking passages, the whole thing roofed with massive limestone lintels. By 1915 the Ordnance Survey was marking it on their six-inch maps as a cave, a label that gestures at the local knowledge of the place while understating its true nature considerably.

This is a souterrain, a type of dry-stone underground chamber built in early medieval Ireland, typically associated with ringfort settlements and used variously for storage, refuge, or both. The ringfort that once stood here at Gortaclare has vanished entirely, leaving only the underground component behind. The souterrain has three chambers connected by low, narrow passages called creeps, through which anyone moving between chambers would have had to crouch or crawl. The first chamber, oriented east to west and measuring 7.6 metres long, is entered today through a clay-filled opening at its north-east corner, though this was probably not the original entrance, which the accumulated clay now conceals. A creep 4.1 metres long leads south into the second and largest chamber, 9.4 metres in length. At the southern end of that creep, jambstones pinch the opening to just 0.3 metres wide, barely enough for a person to squeeze through. The second chamber then leads eastward through a shorter creep into a third, smaller chamber. A sillstone, a raised threshold stone, sits midway along this final passage. Almost all of the stone is limestone, but the sillstone and two of the jambstones are conglomerate, a different rock type whose deliberate selection or practical reuse adds a small, unexplained detail to the structure's history.