A cropmark, the faint differential in how grass grows over buried or disturbed ground, outlines the footprint of a building of entirely unknown antiquity, its long axis running northwest to southeast. It does not appear on any map that might easily explain it, and the aerial imagery that revealed it offers no further clues as to what it once was, or when it stood.

Cropmarks form when buried walls or foundations affect the moisture and nutrient content of the soil above them, causing the grass to grow slightly differently, often appearing greener or more parched depending on the season and conditions. The outline here was identified on Google Earth aerial imagery captured on 29 May 2018, and compiled into the archaeological record by Caimin O'Brien, uploaded in May 2020. What makes the spot stranger still is what lies roughly sixty metres to the north: a second cropmark, this one showing the crenellated lines of First World War practice trenches. The Phoenix Park was used for military training during that period, and these earthwork exercises left their impression in the soil, still faintly legible a century later.

The building cropmark and the trench lines are not visible to someone walking the park; this is firmly a site for the aerial observer or the curious researcher with access to satellite imagery. The Phoenix Park is freely open to the public throughout the year, and the southern sector near Chapelizod is accessible on foot or by bicycle. There are no markers or interpretive panels pointing to either feature. If you want to orient yourself, pulling up the 29 May 2018 Google Earth imagery of the area and comparing it with the ground is really the only way to get a sense of where to look. The building remains unidentified, its age unestablished, and for the moment it sits in that particular category of archaeology, known to exist, unknown in almost every other respect.