Somewhere in the gently undulating pastureland of Calfstown in Co. Kildare, a very large enclosure once occupied the ground, or perhaps still does, entirely invisible from the surface. What makes it quietly remarkable is the shape recorded on an early nineteenth-century map: a D-form roughly 170 metres across at its widest, with a notably straight side running along the south-south-west, spanning around 140 metres. That combination of scale and geometry is unusual. Most earthwork enclosures of Irish prehistory or the early medieval period are roughly circular, and something this size, if it were confirmed, would rank among the more substantial examples in the region.

The enclosure appears on the first edition of the Ordnance Survey six-inch map, surveyed in 1838, drawn with enough confidence to suggest the cartographers could see something on the ground at the time. By the revised edition of 1911, it had vanished from the record entirely, whether because fieldworkers decided it was not a genuine monument, or because the feature had by then been ploughed or otherwise erased, is not known. When the site was visited in 1985, there was no visible surface trace whatsoever. The story did not quite end there, however. An aerial photograph taken in 2005 suggests the outline may survive as a cropmark, the kind of faint discolouration in growing crops that can reveal buried ditches or banks long after they have ceased to register on foot. Cropmarks appear because buried features alter the moisture and nutrient content of soil above them, causing the plants overhead to ripen at slightly different rates, something only visible from the air, and only in the right conditions of drought and crop type.