A souterrain is an underground stone-lined passage or chamber, typically constructed during the early medieval period in Ireland, and used variously for storage, refuge, or concealment. This particular one is unusual not because of what survives, but because of what does not. By 1997, a physical inspection found no visible remains whatsoever at ground level, and yet local people could still recall having been inside it.

The site sits within a large multiperiod field system, meaning the surrounding landscape carries layers of human activity from different eras, each generation of farmers and settlers leaving their mark on the same ground. The souterrain itself was first recorded on a map produced by Robinson in 1977, and later noted again on a working map by the same researcher. What local witnesses described, when asked, was a chamber so small that only a child could fit inside it. The location they pointed to matched Robinson's earlier cartographic note, which at least suggests the memory and the record are pointing at the same spot, even if the ground itself no longer gives anything away.

There is something quietly melancholy about a monument whose existence is now carried almost entirely in the recollections of people who squeezed into it as children. The archaeology has retreated underground or collapsed entirely, and the field system around it continues its long, slow accumulation of time. The souterrain at Ballyganner may be invisible, but it has not quite been forgotten.