A souterrain, which is an underground stone-lined passage or chamber typically associated with early medieval settlement, once warranted enough attention to be marked on an Ordnance Survey map. Today, satellite imagery reveals nothing at the surface, no depression, no trace of stonework, no obvious break in the ground. Whatever lies beneath, the landscape above it has closed over entirely.

The record is thin but telling. The site sits within a cleared area of modern coniferous plantation near the mountain summit, a landscape that has itself been substantially altered by twentieth-century forestry. It does not appear at all on the 1897 edition of the Ordnance Survey 25-inch map, which suggests it was either unknown to surveyors at that time or considered too indistinct to plot. It does appear, annotated simply as 'Souterrain', on the later Cassini edition of the 6-inch map, indicating that at some point the feature was identified and recorded, even if the circumstances of that identification are no longer clear. Immediately to the north lies a possible earthwork, catalogued separately in the Sites and Monuments Record as LI048-066001-, which may or may not be related. Martin Fitzpatrick compiled the current record and uploaded it in September 2021, drawing on Google Earth orthoimages taken between 2011 and 2013, none of which show any surface remains.

For anyone inclined to look, the location presents a few practical realities. Slievereagh is upland terrain, and the cleared forestry ground near the summit can be rough underfoot, with residual brash and uneven soil where trees have been felled. There is no marked path to the site, and the absence of any visible surface feature means there is genuinely nothing to see once you arrive, at least from above. The interest here is archival rather than visual: a place that exists primarily as a dot on an old map and a question about what remains underground. Whether the chamber survives intact beneath the soil, or has long since collapsed, is not currently known.