A circular enclosure sits beneath a field in Osberstown, County Kildare, invisible to anyone walking across it but readable from the air as a ghostly arc pressed into the earth. What makes it stranger still is that it is incomplete, its circuit broken, with a curving annexe reaching away to the north-east like a tail or an afterthought.

The site is known only through aerial photography, specifically a photograph catalogued as GB89.AF.18, which captured what is called a cropmark. Cropmarks appear when buried features such as ditches or walls affect how the vegetation above them grows; a fosse, which is an ancient defensive or boundary ditch, tends to retain more moisture than the surrounding subsoil, encouraging denser growth that shows up as a darker or lusher stripe in dry conditions. Seen from above, these variations in the crop reveal the shapes of structures long since levelled. At Osberstown, the fosse traces most of a circle, enough to make the original form clear, though the full ring was apparently never completed or has been lost to later disturbance. The annexe to the north-east adds a further puzzle: curvilinear annexes attached to circular enclosures are a recognised feature of early medieval Irish settlement archaeology, sometimes interpreted as stock enclosures or secondary activity areas, though without excavation it is impossible to say what purpose this one served or when it was built.