Others exist primarily as information held in the soil itself, legible only from altitude and only under the right conditions. In a tillage field in Milltown, in the barony of Newcastle, County Dublin, there is an earthwork of the latter kind: a circular feature roughly 30 metres in diameter, defined not by any visible surface remains but by the cropmark of a ditch that appears in aerial imagery when the season and the light align just so.

Cropmarks form when buried features, such as the filled-in ditch of an ancient enclosure, affect the growth of whatever crop is planted above them. Soil that once filled a ditch tends to retain moisture and nutrients differently from the undisturbed ground around it, and those differences show up in the height and colour of the growing crop, particularly during dry spells when the contrast is most pronounced. The circular ditch recorded here was identified on a Google Earth orthoimage taken on 24 June 2018, compiled by Caimin O'Brien from details provided by Jean-Charles Caillère, and uploaded to the national record in May 2023. The feature sits in a tillage field and lies approximately 120 metres to the east-northeast of a previously recorded ring-ditch, catalogued separately as DU017-110. Whether the two are related in date or function is not recorded in the current notes.

There is little for a visitor to see on the ground, which is precisely what makes this kind of site worth understanding. The field is agricultural land, and the feature has no surface expression that would distinguish it from any other part of the field. The best way to appreciate it is to consult the Google Earth orthoimage from June 2018, where the circular cropmark is clearly legible. For those interested in the broader landscape, the proximity of the second ring-ditch to the southwest adds context; ring-ditches are the ploughed-down remains of prehistoric burial mounds or enclosures, and their clustering in a single field hints at a landscape that was used and reused across long periods. The site is a reminder that a great deal of Irish archaeology is invisible at eye level, recorded not through excavation but through attentive reading of the land from above.