In a field at Churchfield in County Mayo, there is a low, rounded hillock, domed in profile, sitting above a flat spread of bog and reclaimed pasture to the south. What lies beneath or within it is genuinely unknown. It appears in the Record of Monuments and Places, the statutory list of protected archaeological sites in Ireland, not because anyone has excavated it or confirmed its origins, but because an aerial photograph caught something, a roughly circular feature that was enough to prompt a listing as a possible earthwork.

The feature was examined on the ground in 1998, but the inspection was inconclusive. Gorse and brambles had colonised the crown of the hill so densely that the top could not be properly assessed. The circular form visible from the air might indicate a constructed earthwork, the kind of low enclosure or platform raised by hand in the early medieval period or earlier, but the official assessment stops short of any such claim. The nature of this feature is uncertain, which is the phrase used, and it remains so. It was formally listed in the RMP in 1997 on the basis of that single aerial image alone.