Parys Mountain Answered
General interest: I went there (surface) this year, it's pretty-much dead (due to heavy-metal contamination), a spooky "outer-space" sort of place.
Near Amlwch on Anglesey (that's Wales) The site contains several mines worked since the Bronze age, it was once the largest copper mine in the world.
This site has some great photos of the underground levels, recently drained:
www.parysmountain.co.uk/ (I nicked an image) note that 1 fathom is 6 feet.
Google Earth: W 4o20 '40", N 53o23'10"