Looks like Dave & Yorkie have got it.
The picture is of the Ty-Gwyn Mine tramming adit. It is 534 yards long, driven in the 1840's, mostly through solid rock, It was dug from near the entrance of the present day pier to connect with the Tyn-y-Fron shaft sunk 270 feet from the surface higher up the Orme to the adit level. The adit basically runs under Church Walks. Usually, the adit will flood about two hours after a high tide.
In certain sections the rail lines which the ore wagons traveled along are still in situ.