This one is remarkable for what almost certainly cannot. Within a ringfort at Glenaglogh in County Cork, there may or may not be a souterrain, an underground stone-lined passage or chamber of the kind that early medieval Irish communities used for storage or refuge. The uncertainty is not a matter of lost paperwork. It is baked into the record itself.

In 1939, the archaeologist P. J. Hartnett observed a slight mound and some exposed large slabs on the southern interior of the ringfort's rampart. He suspected these might indicate the presence of a souterrain beneath, but enquiries at the time failed to confirm it. That qualified observation has remained unresolved ever since. Today there is no visible surface trace of anything at all, which means the question Hartnett raised in a footnote over eighty years ago is still, technically, open. The site sits inside a ringfort, one of the thousands of roughly circular earthwork enclosures that dot the Irish countryside, most dating from the early medieval period between roughly the fifth and twelfth centuries. Ringforts frequently contain souterrains, which makes Hartnett's hunch plausible enough, though plausibility and proof are very different things in archaeology.