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 inisde of a Mine over so Larceny and Waste will not leave scents, however Forgery and Poisoncraft still will.
Mining
Having  Any Pickaxe equipped allows you to  Mine. Clicking Mine will change your cursor into a pickaxe which you can then right-click on the Wall to Mine out and automatically  Lift the appropriate Boulder.
There are 5x types of Walls in a Mine. Light gray Soft Rubble Walls will yield a  Rubble Boulder. This is the most common Wall type. Mining a Rubble Walls will reveal either more Rubble Walls or one of the other Walls types. Metal Ore Walls tend to run in veins, while a Dark Rubble Wall must be softened by setting a  Pile of Wood at its base and letting it burn down. Mining from a softened Dark Rubble Wall will give you a Rubble Boulder or potentially cause a Cave-Down. Each time any Wall is Mined there is a chance to cause a Cave-In or spawn  Bats.
Cave-Downs
WARNING! Triggering a Cave-down will cause a huge amount of Blood Damage.
Mining a Softened Hard Rubble Wall has a chance to cause a Cave-Down. A Cave-Down will instantly create a new Mine Entrance leading down another level, and clear away a 13x13 hexagonal space around it, for this reason it is advised to Mine nearby Ore Walls before Mining any Dark Rubble Walls. Triggering a Cave-Down will drop you down to the next level. This fall will cause a huge amount of Blood damage, however if you are Knocked Out you won't loss any Proficiencies, but you will drop your inventory. The new level may initially contain a large number of Bats which will eventually despawn if you wait long enough.
Mine Levels
There are 8 safe Mine Levels. The 9th level is known as Dreamworld, and connects all other Mines that have reached the 9th level. Caution is advised and one should think carefully before digging into Dreamworld. Mining anywhere on Level 8 might randomly result in a Cave-Down to Level 9, not just Hard Rubble Walls.
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. This chance is based off the Wall's proximity to other Un-Mined Walls, its proximity to already existing Support Beam and your  Mines & Mountains Proficieny. An impending Cave-In will be shown evident by a rumbling sound and dust falling from the ceiling. An impending Cave-In can be stopped by building a Support Beam directly under the exact spot where the dust is falling.
If a Support Beam is not placed in time a Cave-In will randomly collapse some Un-Mined Soft Rubble Walls from the ceiling. These Walls can generally be Mined although if they fall Into other structures, they may make those structures inaccessible.
Support Beams can also be built argumentatively to lower the chance of Cave-Ins within the Support Beams radius. Even within a Support Beam's radius there is still a very small possibility for a Cave-In.
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.
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