At Ballydribbeen in County Kerry, on a level stretch of ground near the top of a north-north-east-facing slope, there lies an enclosure that no one standing in the field has ever actually seen. It leaves no impression on the grass, no rise or hollow underfoot, no stones poking through the soil. The only evidence that it exists at all is a cropmark captured in an aerial photograph taken in 1983.

The cropmark in question shows the fosse, or ditch, of what appears to be a circular enclosure. A fosse is a defensive or boundary ditch, typically dug around an enclosed settlement or ceremonial space, and circular enclosures of this type are common in the Irish archaeological record, ranging from ringforts used as farmsteads in the early medieval period to much older prehistoric sites. What made this one unusual was that, when two test trenches were excavated in June 2000 ahead of a proposed housing development, the diggers found nothing. No ditch fill, no soil disturbance, no physical trace of the feature that had appeared so clearly from the air seventeen years earlier. The diameter of the enclosure could not even be estimated from the photograph. Kelleher's report from 2002 records the negative result plainly, without explanation.

Cropmarks form when buried features such as ditches retain moisture differently from the surrounding soil, causing the vegetation above them to grow taller or greener, or conversely to wither, in patterns that are invisible at ground level but legible from above. They can be transient, appearing in dry summers and vanishing in wet ones, and they do not always correspond to features that survive in the ground. Whether the fosse was shallow enough to have been destroyed by centuries of ploughing, or whether the photograph captured something else entirely, the field at Ballydribbeen keeps its answer to itself.