Others exist only as a faint trace of colour in a field, visible solely from the air under exactly the right conditions. The ring-ditch at Ballykeating in north County Cork belongs firmly to the second category. The entire record of its existence rests on a single aerial photograph taken in July 1989, in which the outline of a circular ditch, roughly ten metres across, shows up as a cropmark in the soil below.

Cropmarks form when buried features, such as ditches or pits, alter the growth of crops or grass above them. A filled-in ditch retains more moisture than the surrounding ground, causing the vegetation overhead to grow taller or greener, while a buried wall or bank has the opposite effect. From ground level these differences are almost imperceptible, but from the air, particularly in dry summer conditions, the pattern can emerge with surprising clarity. What the 1989 photograph captured at Ballykeating is the ghostly fosse, or enclosing ditch, of a small circular enclosure. Ring-ditches of this kind are generally associated with prehistoric burial or ritual activity, the ditch being the remaining trace of a mound or barrow that has long since been ploughed flat. At approximately ten metres in diameter, the Ballykeating example is notably small, sitting at the modest end of the scale for such features in Ireland.