Some, on closer inspection, turn out to be something far more ordinary, and the story of how a landscape feature gets mistaken for a piece of prehistory is its own kind of curiosity. At Lackan in County Sligo, a site that spent years catalogued as a possible enclosure, then upgraded to a rectangular enclosure, was eventually found to be neither.

The site first appeared in the Sites and Monuments Record in 1989 as a possible enclosure, a designation based entirely on features spotted in an aerial photograph. By 1995 it had been carried forward into the Record of Monuments and Places with a more confident label, rectangular enclosure, again on the strength of that same aerial image. A rectangular enclosure would typically suggest an early medieval or prehistoric boundary, perhaps a farmstead or a penned area for livestock, defined by an earthen bank or fosse. When a physical inspection was finally carried out in 1993, however, no such structure was found. What the aerial photograph had captured was a natural terrace with a west-facing slope, the kind of stepped landform that can produce crisp geometric shadows under the right light and angle. Running along the eastern edge of the terrace was a defunct field bank oriented north to south, the remnant of relatively recent agricultural land division rather than anything ancient. The geometry that had looked compelling from the air dissolved entirely at ground level.