A souterrain is a man-made stone-lined tunnel, typically built during the early medieval period in Ireland and used for storage, shelter, or refuge. The one at Dromkeen exists now only as an absence, a place defined by what is no longer visible rather than what remains.

The site appears on Ordnance Survey maps from 1841 to 1842, where a circular enclosure is marked, with the word "cave" noted in its interior, the cartographers' term for what was likely this souterrain. The enclosure reappears on the 1916 edition of the OS map, suggesting that some surface trace persisted into the early twentieth century. By 1974, when the Geological Survey of Ireland photographed the area from the air, the enclosure boundary showed up only as a crop mark, the faint differential in vegetation growth that betrays buried features to a camera but not to a passing eye. Today, no surface trace survives at all. The site is documented in C. Toal's North Kerry Archaeological Survey, published in 1995, which drew together evidence from the maps and aerial photography to record what the ground itself no longer shows.

Crop marks of this kind are a common way that vanished archaeology declares itself. Where a buried wall or ditch alters the soil's drainage or depth, the crops or grass above grow differently, and in dry summers the contrast can be sharp enough to read clearly from altitude. At Dromkeen, that fleeting aerial signal is now the primary evidence that anything was ever there at all.