But at Seafield in County Sligo, a dip near the western edge of an ancient cashel has prompted a more considered reading. The depression, sinking to about three quarters of a metre, may be the roof of a souterrain that has given way, leaving the land surface to sag quietly inward over whatever passage or chamber once lay beneath. A souterrain is an underground stone-lined tunnel or chamber, typically built in early medieval Ireland for storage or refuge, and their collapsed remnants are often the only visible trace that one ever existed.

The cashel in which this feature sits is a stone-walled enclosure of the kind used for settlement and defence in early Christian Ireland, and the souterrain, if that is indeed what the depression represents, would have been a functional component of that enclosed world. What makes the Seafield example particularly suggestive is the presence of a few sea-shells worked into the soil of the depression itself. This detail points towards a midden, the term for an accumulated deposit of domestic refuse, often including animal bone, shellfish remains, and other discarded material, that builds up in and around inhabited sites over generations. Sea-shells in the fill of a potential collapsed souterrain hint at layers of human activity on this spot, coastal people making use of what the sea provided and leaving behind the quiet residue of daily life.