There is a ringfort at Knockavannia in County Waterford that you cannot see. Standing in the pasture where it lies, on a gentle north-west-facing slope, there is nothing to indicate that the ground beneath has a history stretching back well over a thousand years. No earthen bank rises to meet the eye, no ditch breaks the surface. The site is, as the record puts it plainly, not visible at ground level.

A ringfort is an enclosed farmstead, typically of early medieval date, defined by one or more circular earthen banks and ditches. They are among the most common field monuments in Ireland, numbering in the tens of thousands, though their survival varies enormously depending on what the land around them has been put to in the centuries since. At Knockavannia, the enclosure was already compromised by the time the Ordnance Survey mapped it in 1840. The six-inch map of that year recorded a circular embanked enclosure with an external diameter of approximately 45 metres, but noted that buildings were encroaching on it from the north-west. Those buildings remain to the north of the site today, and the continued pressure of agriculture and construction has apparently reduced what was once a legible earthwork to something that registers only in cartographic memory, surviving now as a shape on an old map rather than as anything a person walking the field would notice.