Mining a cave wall inside a mine will create a tunnel further into the mine. It takes 45 mining actions to produce the message "The wall will break soon", after which it takes average 5 actions to open the tunnel.
Tunnel excavation produces rock does not produce Rock shards on every action.
==== Dropshaft ====
There is no falling [[damage]] inside mines.
I do not recommend doing this as it ruins the structure of the mine. Making some parts unstable and there for not able to be mined.
=== Tunnel exits ===