In a field somewhere in the Kildare townland of Grangerosnolvan, a circle is hiding in the grass. It cannot be seen from the ground at all, at least not by the naked eye at ground level. The only way it reveals itself is from the air, and only under the right conditions, when the buried remains of an ancient ditch cause the crops above it to grow differently from the surrounding soil, producing what archaeologists call a cropmark.

The circle in question appears in an aerial photograph catalogued as CUCAP BGN 34, which shows a cropmark tracing a fosse, that is, a ditch, curving around a roughly circular area approximately 25 metres in diameter. A gap in the northern arc of the cropmark suggests an original entrance on that side. The site is tentatively identified as a ring-ditch or ringbarrow. Ring-ditches are circular earthwork enclosures whose original form has been levelled over time, leaving only the buried cut of the ditch to betray their presence; they are often associated with prehistoric funerary or ceremonial activity, though their precise function can vary. A ringbarrow is a related type, in which a low mound at the centre was once enclosed by a surrounding ditch. In either case, the feature at Grangerosnolvan belongs to a class of monument that was once physically present in the landscape and is now invisible except through the technology of aerial survey.