Also for novelty value, Cae Fali, you can enter an adit, pass under the Ffestiniog Railway, and pop out of a hole on the other side.
where is this Mr B? ...
CAE FALI MINE lead Llandecwyn, Merioneth SH631405
Bick D.E., 2003,
The Old Copper Mines of Snowdonia 3rd Ed. p. p. 26
Boyd, J.I.C., 1975,
The Festiniog Railway, p. 271 (erroneously described as an adit of Catherine and Jane Consols)
Foster-Smith, J.R., 1977,
The Mines of Merioneth (NMRS British Mining 6) p. 8
OS six-inch map of Merionethshire 2nd ed. sheet XI.SE revised 1899, published 1901
CAE VALLEY MINE lead Llandecwyn, Merioneth SH631405
Carnarvon & Denbigh Herald 25 Jul 1863 p. 5
Mining Journal 1863 p. 737
Also, possibly:
CAENANT MINE lead Llanfrothen, Merioneth
Mineral Statistics 1874
Dave