In a field in Forenaghts Great, County Kildare, there is a structure that cannot be seen from the ground at all. It exists, for now, only as a ghostly circle in aerial photography, a pale ring roughly 46 metres across pressed into the soil like a watermark in old paper.

What the aerial image reveals is a cropmark, a phenomenon in which the buried remains of a ditch or bank subtly alter the growth of the crop or grass above them. Where a filled-in ditch retains more moisture, plants grow taller or stay greener; where a buried wall or compacted surface lies close beneath the topsoil, vegetation may be stunted. Seen from above, particularly in dry summers when the contrast is sharpest, these variations in plant growth can trace the outlines of enclosures, field systems, or settlement sites that have otherwise vanished from the visible landscape. The circular form here, approximately 46 metres in diameter, is consistent with the kind of ringfort or enclosure that was built across Ireland during the early medieval period, though without excavation the precise date and function of this particular feature cannot be confirmed. The cropmark was identified in Google Earth aerial photography captured on 28 June 2018, when dry conditions apparently brought the feature into relief clearly enough to be noticed and recorded.