Others exist only as a faint signal in the soil, detectable solely from the air. At Magherabane in County Offaly, there is nothing whatsoever to see at ground level; the site's entire existence rests on a cropmark photographed from an aircraft in 1973, suggesting the outline of a circular enclosure buried somewhere beneath the fields.

Cropmarks appear when buried features, such as the filled-in ditches of an ancient enclosure, cause crops or grasses above them to grow differently from the surrounding vegetation. In dry summers especially, the variation in moisture retention between disturbed and undisturbed soil produces subtle differences in colour and height that are invisible to anyone standing in the field but can resolve into recognisable shapes when seen from above. The 1973 aerial photographs of Magherabane captured what may be one such shape, a rough circle, hinting at an enclosure of the kind built throughout Ireland from the prehistoric period well into the early medieval era. Circular enclosures served many purposes across those millennia, from the ringforts used as defended farmsteads to earlier ceremonial or funerary sites, and without excavation it is not possible to say which tradition this potential example belongs to, or even to confirm that it is genuinely archaeological rather than a product of natural soil variation.