- A section of the cavern has collapsed!
A cave-in is what happens when there are no walls or supports in a section of cavern 7x7 or larger. Mining out an overly large room will not result in an immediate cave-in - after a short time, the message "You hear a deep rumbling coming from the cavern." will appear as a warning, then the room will eventually collapse. If a support holding up a 7x7 section is triggered by a lever or pressure plate, though, the cave-in is immediate.
A cave-in produces a dust cloud which knocks dwarves unconscious, as well as several tiles of unmined rock (with varying levels of damage, similar to that from mining) such that there are no longer any 7x7 unsupported areas.