On the north-west-facing slope of Croughaun Hill in County Waterford, cut into the bank of a small stream, there is a passage that raises more questions than it answers. Roughly four metres long and shaped like an inverted L, the tunnel is tall enough to stand in and wide enough for a person to move through with some ease. It curves around to the north-west and then begins to descend into the rock. What makes it particularly puzzling is what is absent: there is no spoil heap, no mound of displaced material that would ordinarily pile up beside any serious excavation into solid rock.

The site is classified as a mine, though that label sits a little uneasily given the short length of the passage and the missing debris. When miners extract rock or ore-bearing material, the waste has to go somewhere, and its absence here has never been satisfactorily explained. The passage is cut into the north-east bank of a stream running roughly south-east to north-west, which may suggest some connection to prospecting for minerals carried by water, though no period of working has been firmly established. Croughaun Hill sits within a part of Waterford that has seen various episodes of small-scale mineral extraction over the centuries, but whether this particular cutting belongs to any of them remains unclear.