I read Heinlein's The Moon is a Harsh Mistress some time ago and recall the mining operation but cannot for the life of me remember what they were digging up. Can anyone help me?
At that time, they were (as I recall) mining for two primary things; water (in the form of ice) & space ("cubic" as Mannie calls it; 'digging tunnels', such as for the barracks for the new Peace Dragoons, 1/2 way thru the story. Mannie also talks about expanding farming areas & living space via mining.)
"Project Centerbore was years later" (as nearly as I can remember the phrase; I don't have a copy handy) is mentioned at one point, referencing the fact that mining for Uranium was later a significant thing, but not during this book.
They weren't necessarily mining, they were digging tunnels and caves to live and to farm in. They were shipping back grain to earth, far more than what was sustainable. This was a major factor in the revolution.