In the gently undulating pasture of Cloonagleavragh in County Sligo, that is precisely what happened. The 1838 Ordnance Survey six-inch map records a small oblong feature, annotated simply as 'Cave', sitting at the centre of a circular ring of trees roughly twenty-five metres in diameter, the whole arrangement set within a square field of about thirty metres on each side. By the 1913 edition of the same map, the parenthetical had arrived, and with it the quiet acknowledgement that whatever had been there was no longer accessible, or no longer there at all.

What the mapmakers were recording was almost certainly a souterrain, an underground passage or chamber, typically stone-lined, built during the early medieval period in Ireland and associated with settlement and storage, sometimes with refuge. Souterrains are frequently found within ringforts, the circular enclosed farmsteads that were the dominant form of rural settlement in Ireland from roughly the sixth to the twelfth centuries. The feature at Cloonagleavragh may have belonged to exactly such a context, as a nearby ringfort has been identified in the same area. The concentric geometry visible on the 1838 map, an underground feature within a ring of trees within a square enclosure, suggests a landscape that had been carefully arranged around this spot, even if the original function of the souterrain had long been forgotten by the time the surveyors arrived. No trace of the underground structure is visible on the surface today.