About twenty metres across, it leaves no trace above ground, no earthwork, no stone, no obvious break in the soil. The only way to see it is from above, and only under the right conditions: a dry summer, when crops growing over a buried ditch draw differently on the moisture and nutrients below, turning faintly greener or yellower than the surrounding field, and printing the outline of something old onto the surface of something new.

This kind of faint impression is known as a cropmark, and it is one of the more quietly remarkable tools in the study of buried archaeology. The feature at Rathgarvan is a ring-ditch, a circular enclosure defined by a fosse, which is simply a ditch, cut into the ground. Ring-ditches of this type are most commonly associated with prehistoric funerary or ritual activity, often the eroded remains of a round barrow or burial mound from which the central mound has long since been ploughed flat, leaving only the surrounding ditch as a ghostly signature in the subsoil. The site was identified and reported by Jean-Charles Caillère, who spotted it on satellite imagery via Apple Maps, a reminder that aerial and remote-sensing observation, whether from a plane or a phone screen, continues to add to the archaeological record in ways that ground-level survey cannot always replicate.