Ahh, the dream of Parc to hafna
As far as I understand it, this is a mixup and the cryptic reference above is a well known previous "classic" through trip, not dramatically long or exotic, but not discussed or encouraged any more due to concerns about cadmium discharge into a lake and the entrance being somewhere that would get sealed up if discussed openly so let's not take the "P" in these discussions. I'm sure it's one you know of.
Anyhow, on the original topic, if the single map of all the mines/lodes existed we'd see possibilities for mega through trips? or more likely weep over ones that might have been possible 40 years ago that are lost now