Some sites announce themselves with towers or carved stone; others exist only as a faint signal from the air. At Alasty in County Kildare, a circular earthen mound, estimated at roughly twenty metres in diameter, was recorded on the 1939 edition of the Ordnance Survey six-inch map, yet it had been entirely absent from the equivalent map produced a century earlier in 1838. By 1971, it had effectively ceased to exist at ground level, leaving only a cropmark on an aerial photograph. A cropmark forms when buried features, walls, ditches, or mounds, affect how vegetation above them grows, showing up in dry summers as pale or dark patches visible only from altitude. That 1971 photograph, taken as part of the Cambridge University Committee for Aerial Photography series, preserved what the landscape itself had already swallowed.

The mound sat on level, improved pasture, drained to the north by the small Painestown River, which flows to the north-west. The period between the two Ordnance Survey editions spans a century of agricultural change in the Irish midlands, and it is likely that the process of improving and draining the surrounding land gradually eroded or levelled whatever had remained above ground. Whether the mound was an ancient burial feature, a ring barrow, or something of later origin is not recorded. What is clear is that at some point between 1838 and 1939, it was considered significant enough to map, and at some point after that, the land around it was tidied into invisibility.