In-Game Text
"Now when a miner discovers a vena profunda he begins sinking a shaft and above it sets up a windlass, and builds a shed over the shaft to prevent the rain from falling in, lest the men who turn the windlass be numbed by the cold or troubled by the rain."
– Georgius Agricola, de Re Metallica
In order for ores to be smelted into metal, they must first be extracted from within the womb of the earth. Mining allows you to construct Mine Supports and to mine from softened rock and ore surfaces within the mine. Soften darker rock surfaces by lighting a fire by them and letting it burn.
About
Once a  Mine Entrance is built on a Metal Node you can enter your Mine. A Mine start as 10x10 (100x tiles total) with a ramp which can be right-clicked to exit. You cannot Claim the inside of a Mine over so Larceny and Waste will not leave  Evidence and  Scents inside of a Mine, however Forgery and Poisoncraft still will.
Mining
With  Any Pickaxe equipped clicking the  Mine action will change your cursor into a pickaxe, which can then be right-clicked on any wall within a Mine Entrance to begin Mining. The Phlegm this will drain is dependent on what Mine level you are on and can be lowered by increasing your Mining Artifact Bonus. Once you have finished Mining you will automatically  Lift out the appropriate  Ore Boulder based on the wall type you just Mined. Unlike other walls, Hard Rubble Wall must be softened before Mining by lighting a  Pile of Wood at its base and letting it burn down. Each time any wall is Mined there is a chance to cause a Cave-In or spawn  Bats.
Mine-Downs
WARNING! Triggering a Mine-down may cause a huge amount of Humour Damage.
Mining a Softened Hard Rubble Wall has a chance to cause a Mine-Down, with this chance being increased the older that Mine level is. A Mine-Down will instantly create a new Mine Entrance leading down another level, and clear away a 13x13 hexagonal space around your Mine Entrance. For this reason it is advised to Mine nearby Ore Walls before Mining nearby Hard Rubble Walls. Triggering a Mine-down may cause you to take a large amount of damage to one, or more of your Humours, however if you are Knocked Out from this damage you will not loss any Proficiencies, but you WILL drop your Inventory. This damage can be lowered slightly by increasing your  Mines & Mountains Proficiency. Additionally a Mine-Down may cause you to drop to the next Mine level as well. The new Mine level will likely initially contain a large number of Bats which will eventually despawn if you wait long enough.
Mine Levels
A Mine can be up to 8 levels deep, with each level requiring more Phlegm to Mine on. The first & level are considered safe however Mining anywhere, NOT just Hard Rubble Walls, on Level 8 might randomly result in a Mine-Down to the 9th level, which is known as  Dreamworld. The exact nature of Dreamworld is shrouded in mystery, but what is know is that Dreamworld connects all other Mines that have reached this 9th level. Caution is advised and one should think carefully before Mining down to Dreamworld.
The lower you go, the more phlegm you use per block you mine. As well as, each level you go raises the purity cap of the ore you find by ~10%, and each point in your clothing mining bonus raises the cap by ~.01%.
Iron Purity
The Purity of a  Bar of Pig Iron comes from the  Iron Ore Boulder the  Iron Ore used to make it, was Chipped from. Iron Ore Boulders' get their Purity from a combination of Mine Level, distance from the ramp, and chance. Iron Purity will transfer directly onto  Fish Hooks,  Knife Blades, or any  Wrought Iron Bars made from it.
Cave-Ins
Each time a Wall tile is Mined there is a chance to trigger a Cave-In. A Cave-In will scatter new Rubble Walls randomly in areas that you have already Mined out around the source of the Cave-In. Being under a Cave-In when it falls will instantly cause you to become Knocked Out and will damage nearby Structures. The chance of a Cave-In happening is based off the wall's proximity to other Un-Mined Walls, its proximity to already existing Support Beam and your Mines & Mountains Proficiency. While very rare it is technically still possible to get a Cave-In while mining under the radius of a Support Beam. An impending Cave-In will be evident by a rumbling sound and dust falling from the ceiling and can be stopped by building a Support Beam directly under the exact spot where the dust is falling.
One strategy for finding iron ore is to mine in a straight line until a vein is located since a 1-tile corridor, without any bends, will rarely cause a cave-in, even without mine supports.
Bats
A Bat is an aggressive Creature which live inside of Mine Entrances and appear by chance when Mining walls, when a Cave-In is imminent, or on the next level after a Mine-Down. The chance of Bats appearing is greater with higher levels of Madness.
A Bat, when attacking will fly in close and, if not killed, will attach itself to your Neck detaching anything in that slot and sucking your Blood. To remove a Bat, open your Equipment Window and click the Bat in your Neck slot. This will cause Bat will fly a bit away from you and then turn around to attack you again. Abilities like  Stomp will work well on Bats, even though they are flying.
| 
