Magic metal is a catch-all term for materials that have been infused with magic in such a way that it can be manipulated easily. While wood is used prominently for this, the term was coined by The Crafters, to whom metal is much more widely used.
Magic metals are mined like so much else, but the operation itself is very dangerous. The Crafters and The House of Three Pennies have both spent copius sums in making it safer, but certain things simply cannot be made safe. The amount of magic in those mines, for example, make monsters and infection very likely. The first can be relieved with guards, however, and this is usually a standard precaution. Even mining foremen who doesn’t care the slightest for his workers deploy some armed forces in there, if only to prevent otherwise minor monster attacks into regular massacres.