At Mullamast in County Kildare, a near-perfect circle roughly fourteen metres across shows up in aerial photography as a cropmark, the kind of ghostly imprint left when buried or disturbed ground affects the growth of overlying vegetation. From above, it resolves into the outline of a small pit or quarry, its geometry just regular enough to catch the eye against the surrounding fields.

What makes this feature quietly interesting is its cartographic history. When the Ordnance Survey produced its first edition of the six-inch map of Ireland in the nineteenth century, the hollow was not recorded at all. It appears only on later revisions, both the revised six-inch map and the larger-scale twenty-five-inch OS map, where it is marked with hachures, the short radiating lines surveyors used to indicate a depression or hollow in the ground. Whether the feature was simply missed in the initial survey, or whether it came into existence or became sufficiently prominent only between editions, is not clear from what survives. The cropmark itself was identified more recently from Google Earth aerial imagery captured in June 2018, which gave the circular form a new kind of legibility that ground-level inspection alone might not provide.