In a pasture in Clonkeen, County Westmeath, a ringfort that was still legible on the ground in 1975 had, by the time a Digital Globe camera passed overhead in November 2011, been reduced to a faint crop mark, the ghostly outline of an earthwork pressed just barely into the soil beneath the grass.

Ringforts, known in Irish as raths, were enclosed farmsteads typically built during the early medieval period, roughly between the fifth and twelfth centuries. They usually consisted of a circular area surrounded by one or more earthen banks and ditches, and served as the dwelling place of a farming family, offering a degree of protection for people and livestock. The Clonkeen example was a modest one: a sub-circular enclosure roughly 25 metres across, sitting on the eastern side of a slight rise in pastureland, with open views in every direction. Its surrounding bank was already poorly preserved when surveyors visited, and the fosse, a round-bottomed ditch of the kind typical of these sites, had been partially filled in to the south. Bog lay within 135 metres to the north-east and 155 metres to the south-west, the kind of wet, marginal terrain that often framed early medieval settlement in the Irish midlands, where good elevated ground was worth occupying. At some point between those 1975 site visits and 2011, the earthworks were levelled entirely, most likely through continued agricultural activity.

What remains is essentially invisible to anyone walking the field. The crop mark visible in aerial photography occurs because buried features affect how plants grow above them, with soil disturbed by old ditches or banks producing subtly different vegetation that registers from altitude even when nothing can be seen at ground level. The Clonkeen rath is, in that sense, a site that can only be seen from the sky, and even then only under the right conditions.