No earthworks break the surface, no stones protrude from the soil, no signpost marks the spot. The only evidence that something once stood here is a ghostly outline, visible only from the air, where the buried edges of an old enclosure affect the growth of crops above them just enough to show up in an aerial photograph as a faint, sub-rectangular shadow pressed into the land.

Crop marks of this kind form when buried features, walls, ditches, or filled pits, alter how moisture and nutrients move through the soil. Crops growing over a filled ditch, for instance, tend to grow taller and greener than those in undisturbed ground; those growing over a buried wall may be stunted. Seen from the right altitude at the right time of year, these subtle differences in growth resolve into shapes that reveal the outlines of structures long since vanished at ground level. In this case, the mark suggests a sub-rectangular enclosure, the kind of form associated with a broad range of Irish archaeological periods and functions, from early medieval farmsteads to ecclesiastical sites. The record, compiled by David O'Connor and updated by Christine Baker for upload in January 2015, notes the enclosure alongside other possible features flagged in the Sites and Monuments Record file and through a personal communication from T. Condit, though the precise nature or date of the site has not been established from the available evidence. The setting itself offers one small clue to the character of the place: the site sits on low-lying ground that rises steeply to the south, a landscape arrangement that often attracted early settlement.

For anyone curious enough to seek it out, Fosterstown South in County Dublin offers no dramatic payoff at ground level. The value in visiting, if visiting is even the right word, lies in the exercise of imagination required when the land gives nothing away. The crop mark that revealed the enclosure would only be legible from altitude, and would only appear under specific growing conditions, meaning the site effectively disappears and reappears depending on the season and the angle of view. Walking the area, the most a visitor might notice is the topography, that southward rise, and the ordinary agricultural fields occupying land that aerial photography suggests is rather less ordinary than it looks.