Somewhere in the townland of Bigbog, in County Kildare, there is an enclosure that you cannot see. Walk the ground and you will find nothing, no ridge, no hollow, no trace of the roughly circular boundary that was clearly visible to the surveyors who mapped this part of Ireland in 1837. It appears on their six-inch Ordnance Survey map as a distinct feature, approximately forty metres across at its widest point, and then, as far as surface evidence is concerned, it simply stops existing.

What the eye misses at ground level, aerial photography can sometimes recover. A cropmark captured in an aerial photograph reveals the ghost of a curvilinear enclosure defined by a fosse, a ditch dug to mark a boundary or provide a degree of enclosure and defence. Cropmarks appear when buried features affect the moisture and nutrient content of the soil above them, causing the vegetation to grow slightly differently, in ways that become legible from the air even when nothing survives above the surface. The enclosure at Bigbog belongs to a well-documented class of site found across Ireland, typically associated with the early medieval period, though without excavation the date of this particular example remains unknown. What the 1837 map confirms is that something was still recognisable on the ground at that point, even if nearly two centuries of agriculture have since erased it entirely.