In the fields around Ballysize in County Kildare, a prehistoric or early medieval enclosure lies completely invisible at ground level. There is no earthwork to walk around, no stones to photograph, no hollow in the ground. The site exists, as far as anyone can tell, entirely as a cropmark, a faint signature pressed into the grass and grain from somewhere below the soil.

Cropmarks appear when buried features such as ditches or walls affect how vegetation grows above them. A filled-in ditch, for instance, retains more moisture than the surrounding subsoil, encouraging crops to grow taller and stay greener longer, while a buried wall has the opposite effect. From the air, particularly during a dry summer when the contrast is sharpest, these differences sketch out the outlines of structures that vanished from the surface long ago. It was precisely this kind of aerial survey that brought the Ballysize enclosure to light. Dr. Gillian Barrett identified it in 1991, and a photograph taken during that survey, catalogued as GB91.EI.28, shows the cropmark of a curvilinear enclosure, meaning one with a roughly circular or oval outline, defined by a fosse. A fosse is simply a ditch, typically the defining boundary feature of an enclosed settlement or ring-fort of the early medieval period, though enclosures of this kind span a wide range of dates and uses. The curvilinear form is consistent with the ring-forts, known in Irish as raths or liosanna, that once numbered in the tens of thousands across the Irish countryside, many of which have been levelled by centuries of agriculture.