In reply to Boulderdash86: there are copper mines and slate mines. Both can be lethal to the unwary (particularly the copper mines where sound-looking false floors are often held up by rotting timber stemples). The slate mines tend to be more stable and these are easier to locate - usually in open slate workings. There is at least one absolutely vast cavern which would give Gaping Gill a run for its money. It's almost 20 years since I last went in but I think it is quite close to the main 'tourist' track near Low Water. It can be entered at several levels and really needs good lights to appreciate - a diddy headtorch won't do. Tread carefully and you'll probably survive.