Cut into the base of the cliff at a bearing of roughly 320 degrees, the opening is barely a metre wide and just over a metre high, with a length of less than a metre. What makes it worth noticing is the evidence of how it was made and, more particularly, why it was left: the miners drove it in the wrong direction entirely, working against the cleavage grain of the rock rather than with it.

This kind of ancient copper mining on Mountgabriel is associated with Bronze Age activity documented by researcher William O'Brien, whose 1987 survey catalogued the site as mine 3b. Fire-setting was the primary technique in use here, a method in which fire was lit against a rock face to heat it, after which the sudden application of water or simply the cooling process caused the stone to fracture and allowed the ore-bearing material to be extracted. The scorched face is still visible. When excavation was carried out some five metres down the slope from the opening, spoil sediment turned up, the discarded waste material from the brief and ultimately futile digging. There are no floor sediments inside the working itself, suggesting it was abandoned before any real accumulation could occur. Mountgabriel is one of several sites in the Mizen Peninsula area associated with prehistoric copper extraction, and this small misfired attempt sits seven metres south of a more substantial working nearby.