Mining levels unlock better pickaxe recipes and the level 5 Geologist or Miner split. Geode processing, gem duplication, and bomb crafting make mining the early-game gold engine; this page maps ore density by mine level.
Plan a mine trip around the ore you actually need: copper and iron support tool and machine upgrades, gold powers quality sprinklers and kegs, and iridium becomes reliable only after deeper mine or Skull Cavern runs. The elevator lets you revisit five-floor checkpoints instead of starting from floor one.
Miner adds extra ore from an ore node, while Geologist increases the chance that a geode contains two gems. Pick Miner for upgrade materials and bomb-focused runs; pick Geologist when gem income, museum donations, or crystalarium inputs matter more.
Bring food, a weapon, and a return plan. Breaking geodes at Clint, saving coal for machines, and carrying bombs for dense rock rooms turns the same energy bar into more usable ore than slowly striking every stone.