Often undertaken by subsistance miners with hand tools .. eg: Coober Pedy is surrounded by a rabbit warren of tunnels 30 ft down and fanning out following opal.

Often undertaken by subsistance miners with hand tools .. eg: Coober Pedy is surrounded by a rabbit warren of tunnels 30 ft down and fanning out following opal. Admittedly that's "soft" rock, but 30 foot is shallow underground mining compared to major league underground mines and open cut super pits.