Your moles drive drill rigs on their own. Each one digs a patch of ore trip after trip until it runs out, then drops the ore at its level's station. The lift hauls it up and you sell it on a live market for Grubs.

Each level has a quota. Fill it to open the next one. Deeper ore pays more but needs a stronger drill. Go as deep as you can, then Prestige: start over with faster crews. The deeper you go first, the bigger the jump.