- A section of the cavern has collapsed!
A cave-in is when walls, floors, and objects plummet downwards to lower Z-levels under the influence of gravity. A cave-in will occur if constructions or ground tiles are detached from all supporting tiles (bridges and the like do not support constructions). Since it is only a placeholder, the system is highly unrealistic—you can hold up a giant megafortress by a slender pillar of soap. Toady One has stated he intends to implement more realistic cave-ins in future versions.
Cave-ins can be disabled through the init file, by changing [CAVEINS:YES] to [CAVEINS:NO].
 How cave-ins work
Any disconnected construction or section of rock or soil will cave in. The game checks for connections along the X, Y, and Z axes (that's left/right, up/down, and above/below). Any construction, even Stairs (natural or constructed), and supports (naturally) provide support/connections. Upstairs will provide support for the z-level above even if there is no downstairs above, acting as an invisible floor. Diagonal connections and bridges do not provide support.
 Results of a cave-in
- Any creature caught under the falling material is crushed and killed.
- Any item caught under falling natural walls is destroyed completely. Natural floors and constructed walls and floors have a small chance of destroying items.
- Anything standing on the area that caves in falls and may be wounded. The fall victim has a chance of being unable to walk away, somewhat proportional to the distance fallen but not set in stone. No pun intended.
- A large amount of dust is generated. Any creature caught by the dust from the collapse is knocked unconscious and thrown a few tiles, which may cause them to fall off a narrow bridge fifty z-levels above the ground.
- All buildings and non-wall constructions under the falling area are destroyed. Buildings above the cave-in will deconstruct if they are no longer supported.
- Natural terrain will remain intact during the cave-in; the only effect is they are revealed.
- Constructions will deconstruct when they collide with solid terrain.
- Any terrain crashes through multiple floors, and stops only upon reaching solid ground or a constructed wall, where natural terrain piles up and constructions deconstruct.
- Mined stairs and ramps will settle like unmined rock; Stairs down that fall onto previously empty floors will reveal the level below. If there's rock or floor above them, it'll cover the stairs.
- Anything falling into a fluid sinks to the bottom. Therefore, it is not a good idea to punch a skylight into your meeting area if you forgot that e.g. your gem pile was directly below and you had a magma tube three Z levels afterward... you get the idea.
- Any water or magma displaced by falling natural walls is not destroyed, but teleported upwards(!) to directly on top of the fallen walls.
 Avoiding cave-ins
Do not make unconnected sections of rock.
Actually, you're quite unlikely to cause cave-ins unless you are actively trying to cause them. In which case, you'd be wondering how to avoid cave-ins that cause damage to your folks. That's simple: Add a support under the stone mass, and link it to a distant lever. When you're done, hide everyone, pull the lever and watch the fireworks.
One of the more common accidental cave-ins results when you're taking out the floor in a checker-pattern (dwarves channeling may sometimes tend to make this mistake) and the area below isn't supported, resulting in a situation like the diagram below:
Floor -1 ▒▒▒▒▒▒ ▒ ▒ ▒ X +▒ <-- The X is a floor tile. It's not attached, so it will fall down. ▒ +>▒ ▒ ▒ ▒▒▒▒▒▒ Floor -2 ▒▒▒▒▒▒ ▒....▒ ▒...▒▒ <-- Causing this area to receive a cave-in flow and knocking out any dwarves in its reach. ▒...<▒ ▒....▒ ▒▒▒▒▒▒
Another thing to watch out for is if you want to dig away a hill above ground, to make room for your fancy overground fort. You may dig away the hill on one level, and then have a huge platform of "floor" on the z-level above that falls on your miner if they get disconnected from the ground. Easy thing to miss the first time you do it.
The solution here is to dig ramps instead, since these take away both the soil on the level you are digging on and the floor on the level above. This is not foolproof, however, as trees will prevent the floor it's on from being removed, resulting a free-hanging floor when you carve the ramp around it. In addition, ramps do not provide support for other tiles on the higher z-level; depending upon the order they are constructed, cave-ins may still occur.
 Using cave-ins
Intentional cave-ins serve several purposes:
- Dropping a section of natural rock above an underground river can provide a way to dam the river. Similarly, causing a cave-in is probably the easiest way to be able to dig beneath an aquifer you can't dig around. This only works with natural rock; constructions will revert to their components, and not block the river at all.
- Since a cave-in kills all creatures instantly, it can provide a convenient or amusing way to off a group of creatures.
- Removal of floor tiles
- Causing a cave-in will destroy non-reinforced (no wall or support underneath) floor tiles directly underneath the falling terrain - this is a good way to e.g. hollow out a large area. All that's left to do is a little bit of cleanup on the edges, but look at all the channeling you save yourself!
- Destroying items
- Elven siege left behind a bunch of worthless wooden weapons? Got some fire imp fat in your butcher's shop? Need an amusing way to off your stone? Smash it with a cave-in and you'll never see it again.
- Breaking through multiple aquifer levels
- Showcase with two levels: User:Rhenaya/HowtoDualAquifer
- Trapping [TRAPAVOID] creatures
- Since the dust from a cave in knocks out creatures, and any unconscious creature triggers a trap (This includes your dwarves and other friendly creatures), combine a cave in with cage traps for the capture.
 Caving-in the toplevel/terrain from inside
You can cause terrain above you to cave in without going outside by first mining up stairs below the "borderline" you want to channel, and then channeling the tiles above them. The tiles above the up stairs can be mined from below while standing on the stair, so you don't have to go outside. Ramps would also work for that alone, but the ramps would allow enemies to enter, whereas the up-stairs alone do not allow passage to above as there is no corresponding down-stair above them.
 Step 1: Preparation, up-stairs
Z-level -1: The level below the target, part of your tunnels.
▒▒▒▒▒▒▒ ▒<<<<<▒ ▒<...<▒ ▒<.S.<▒ ▒<...<▒ ▒<<<<<▒ ▒▒▒▒▒▒▒
- < = up-stairs
- S = support
 Step 2: Channeling
Z-level 0: Your enemies, trees, etc. are on this level, and none of your dwarves needs to go here.
....... .CCCCC. .C...C. .C...C. .C...C. .CCCCC. .......
- C = Channel
- Dwarves are able to dig the channels by standing below on the stairs.
- Enemies are not able to enter without down-stairs connected to the up-stairs.
 Step 3: Keep your dwarves safe and pull the lever
Note: Your dwarves will not dig the channels if "dwarves stay indoors" is active, but you can lock them in using drawbridges to prevent them from going outside.
Beware, enemies with ranged weapons are able to shoot down the channels at your dwarves below, but they should be safe from melee fighters.
Want an underground forest but no underground river on the map? No problem, just drop the above-ground-forest a few z-levels and build a floor above it.