To most eyes it might read as little more than a dip in the earth, the kind of irregularity you could walk past without a second glance. To an archaeologist, it points strongly toward something that once lay beneath the surface, a souterrain, now collapsed into itself.

Souterrains are underground stone-lined passages or chambers built during the early medieval period in Ireland, typically between the seventh and twelfth centuries. Their purpose is still debated, though most scholars think they served as places of refuge, cool storage for food, or both. They are found across the country, often associated with ringforts or other enclosures, and this one sits within a cliff-edge fort, a promontory or coastal enclosure that uses the natural drama of the cliff as part of its defensive boundary. What remains visible at Dunnamark is not the souterrain itself but the scar of its absence: a depression measuring 2.4 metres by 1.9 metres, with an extension of 1.6 metres by 0.8 metres at the north-east corner, the dimensions suggesting a chamber and a connecting passage whose roof has long since given way. The collapse is itself informative, hinting at the shape and extent of what was once a carefully constructed underground space.