I noticed she was polite enough not to question the mysterious bulge in the guy's cheek throughout the second half of the video - and the fact he seems unable to stop talking. Coffee just doesn't cut it for underground work
Potosi does indeed have a fantastic history of silver production.
A mine up in the hills some miles east of Tan y Bwlch, beyond Blaen Ceulan, usually called Esgair Hir was renamed at one time the "Welsh Potosi" to suggest it was as rich in silver (it wasn't!).