On the Curragh plain in Co. Kildare, somewhere beneath the grass, lies a circular enclosure that no longer announces itself to anyone walking past. There is nothing to see at ground level; the structure survives only as a cropmark, a faint shadow readable from the air when differential moisture or soil disturbance causes the vegetation above a buried feature to grow at a slightly different rate to its surroundings. In this case, the aerial photograph designated CUCAP BDU 21 revealed the outline of a fosse, a defensive or boundary ditch, tracing a circle with an estimated maximum diameter of around eleven metres.

A fosse of this scale, enclosing such a compact area, is consistent with a wide range of monument types found across Ireland, from small ringforts to burial enclosures, though the notes attach no specific classification or date to this one. What the photograph captures is essentially an absence pressed into the earth, the negative space of a ditch that was dug, used, and eventually filled or silted over, leaving the soil above it subtly different in composition from the surrounding plain. The Curragh itself is one of the few remaining expanses of semi-natural grassland in Ireland, which may be part of why the cropmark registered so legibly; the open, undisturbed sward offers fewer competing variables than ploughed agricultural land.