In a field near Johnstown in County Kildare, there is nothing obvious to see at ground level. No earthwork, no stone, no visible boundary. What gives this site away is a circular shadow in the grass, roughly ten metres across, readable only from the air.

The feature is a ring-ditch cropmark, meaning that the buried remains of a circular ditch cause the vegetation above it to grow differently from the surrounding soil. Ditches, once filled in or simply silted over across centuries, retain more moisture than the compacted ground beside them, and in dry summers that difference becomes visible as a colour variation in crops or grass. Archaeologists and aerial photographers have used this effect for decades to locate sites that have left no surface trace whatsoever. Ring-ditches of this kind are typically associated with prehistoric funerary or ritual activity, often the ploughed-down remains of a burial mound whose central mound has long since disappeared, leaving only the encircling ditch as a ghost in the soil. The Johnstown example, approximately ten metres in diameter, was identified in aerial imagery captured on 25 June 2018 via Google Earth.