The water that had filled it at the time of the most recent survey is part of the reason. The rest comes down to the way these things tend to go: a roof lintel displaced, a passage partially collapsed, a subterranean corridor that gives up its secrets only reluctantly.

A souterrain is an underground passage or chamber, typically drystone-built, associated with early medieval settlement in Ireland. They appear in various contexts but are often found within or close to a rath, a circular earthen enclosure that served as a farmstead. This example at Gorteenanillaun sits in the south-eastern sector of just such a rath, with a children's burial ground, a cillín, lying immediately to its north. The visible portion of the passage runs roughly north to south for around five metres, and surveyors were able to peer into it through one of two exposed roof lintels at the northern end. A second displaced lintel, jutting from the bank of the rath some seven metres further south, suggests the passage continued beneath the enclosure's bank itself. Local knowledge, as recorded by the Galway Archaeological Survey, placed the total length at well over twelve metres, with at least another ten metres extending southward beyond what could be directly examined.

The proximity of the souterrain to the children's burial ground adds an atmospheric layer to the site. Cillíní were used, often from the medieval period into the nineteenth century and occasionally beyond, for the burial of unbaptised infants and others considered ineligible for consecrated ground. Whether the two features share any meaningful spatial relationship, or whether their proximity is simply a consequence of the rath's long use as a focal point in the landscape, is not recorded. What remains clear is that the ground here holds several centuries of activity stacked on top of one another, most of it invisible at the surface, some of it flooded, and all of it considerably longer than it first appears.