A field in County Kildare holds something that cannot be seen by simply walking through it. What lies beneath the grass at Killhill only becomes legible from the air, where the soil and roots give away old secrets through the subtle language of crop marks, the slight variations in colour and growth that betray buried features beneath an otherwise unremarkable pasture.

The feature in question appeared in a 2005 aerial photograph as the crop marks of two possible concentric fosses, that is, ditches, enclosing a roughly circular area with an estimated maximum diameter of around 40 metres. It sits on an east-facing slope in the eastern sector of an existing hillfort, a type of prehistoric enclosure typically defined by earthen banks and ditches following the natural contours of elevated ground. The relationship between this smaller enclosure and the hillfort around it is not yet clear, and the concentric arrangement, two rings of ditching one inside the other, raises questions that the surface of the land refuses to answer. Whether the inner enclosure predates the hillfort, postdates it, or was always part of the same complex remains unknown without further investigation.