A cropmark, the faint differential in vegetation colour and growth that forms above buried structures when soil moisture varies over hidden stonework or earthworks, outlines the ghost of a building aligned on a northwest to southeast axis. No one presently knows what it was, when it was built, or why it stood where it did. It came to attention not through excavation but through a Google Earth aerial image captured on 28 January 2017, one of those moments when dry conditions and the right angle of light render the invisible briefly legible.

About forty metres to the west of this unnamed building, the same imagery reveals a second set of cropmarks, these ones forming crenellated lines. These are the traces of First World War practice trenches, the kind dug on home soil to train soldiers in the techniques of trench warfare before they were shipped to the Western Front. Phoenix Park has a long military history, with the Royal Hospital Kilmainham nearby and various garrison uses over the centuries, so the presence of training infrastructure in the park is not surprising in itself. What is more arresting is the proximity of the two features: the orderly geometry of early twentieth century military preparation laid down just metres from something of entirely unknown age and purpose. The record was compiled by Caimin O'Brien and uploaded in May 2020.

Both features are subsurface and invisible to the naked eye under normal conditions. A visitor walking the southern reaches of the park would pass over them without any indication they were there. The cropmarks are only legible from the air, and only under the right seasonal and meteorological conditions, typically during dry spells in summer when moisture stress in the grass above buried features differs from the surrounding sward. The Google Earth imagery from January 2017 happens to be one such revealing moment. For anyone interested in looking, the orthoimage cited in the original record is the most direct way to see what prompted the entry in the first place.