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.
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.
Triggering a Mine-down may cause a huge amount of Humour
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.
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.