Tamarmole asked "Do Welsh metal mines use fathoms?" - Yes, much more common than yards or feet for depths; especially used to designate level names, usually (though not invariably) as depths below deep adit level.
Lozz observed "The fathom was also used as a cubic measure" Fathoms were also used as square measure, especially when calculating areas of stopeable lode in sq. fms. and expressing ore as tons per [sq.] fm.
Many metal mine plans were drawn at scales of x fms. to an inch. I've never found antique scale rules to 5 or 10 fms to an inch (the most usually encountered scales) and have had to make my own (1:360 and 1:720 respectively).
Sorry: :offtopic: