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Collapsed rocks fall from above when digging a cave (just like roofs that could not be supported by any structure within 6 tiles away) which will prevent obtaining an open air space since mountains have countless layers piling up to their heights. Mining will keep causing more rocks to fall indefinitely.
When rocks fall, anything below it is immediately destroyed and can never be recovered. This can be dangerous to your colonists, but if used carefully, you can make a trap to instantly kill incoming threats.