There is a cave at Greenan in County Waterford that has, in a very literal sense, disappeared from view. The ground above it is ordinary pasture, and nothing at the surface gives any indication that a cave lies beneath. The only reliable evidence of its existence comes from a cartographic source more than a century and a half old.

The Ordnance Survey's six-inch map, produced in 1840 as part of the first systematic, large-scale mapping of Ireland, marked the feature plainly as a cave. That survey was a remarkably precise document for its time, and features recorded on it, even minor ones like this, were generally the result of careful local observation rather than guesswork. The cave sits within what appears to have been an enclosure, a bounded area of land whose original purpose is not specified but which hints at some earlier, deliberate use of this part of the landscape. Whether the cave was incorporated into that enclosure as a functional feature, a source of water, a place of shelter, or something else entirely, the notes do not say. What the 1840 surveyors saw, or were told about, has since been swallowed by the field above it.