v0.34:Cave-in

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v50.14 · v0.47 · v0.34.11 · v0.31 · v0.28 · v0.23

This article is about an older version of DF.

More Info · V

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 support (bridges 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 in the d_init.txt 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.

Note that supports and fortifications, but not statues, create an invisible floor on the level above them. No dwarf can enter the invisible floor, but it will hold an area attached to the floor tiles in four directions alongside it or the constructed/natural wall above it.

Results of a cave-in

Any creature caught directly underneath (on the same tile underneath) a cave-in is killed, the only exception being Ghosts.
Plants under a cave-in are obliterated^[Verify]--including fully-grown trees.
Trees, shrubs and saplings that fall are obliterated. Falling grass of all types (including cave moss and floor fungus) gets turned into undifferentiated green "Grass".
Any item caught under falling natural walls is destroyed completely. Natural floors and constructed walls and floors have a chance of destroying items.^[Verify]
Anything standing on the area that caves in falls and may get away with being stunned. 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 can be thrown a few tiles even up z-levels, which may cause them to fall off, say, a narrow bridge fifty z-levels above the ground, or can mash them into a fine paste against the wall. Dwarves will receive an unhappy thought from choking on dust clouds (which won't matter if they're dead).
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.
Supported natural terrain will remain intact during the cave-in (terrain unsupported from below will collapse).
Constructions will deconstruct when they collide with solid terrain.
Any terrain crashes through multiple floors and ramps, and stops only upon reaching solid ground, a constructed wall, or a support. Natural terrain piles up and constructions deconstruct.
Mined stairs and ramps will settle like unmined rock; Stairs that fall down onto previously empty floors will reveal the level below. If there's rock or a 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 afterwards... you get the idea.
Any fluid displaced by falling natural walls is not destroyed, but transported to directly on top of the fallen walls. This principle can be used to construct magma pistons.
Soil layers will change into the lowest soil type on map.
Magma mist will be generated in all tiles of magma that were in the path of the cave-in.
Any mined minerals or stone in the area directly under the cave-in will be forced out from under the cave-in (or even up a few z-levels too, if the cave-in falls a long distance).

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. If you're feeling lazier, use statues to keep dwarves off the wrong squares. Provided they move directly away from the cave-in area, the dust may not catch them - and they don't blunder off edges and die unless the dust catches them.

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. To avoid this, channel from the surface downwards, which doesn't remove anything that isn't supported - though you might still mine out something that was supporting a floor you weren't mining, so be careful. Miners also don't check that there's nobody standing on the floor that will shortly cease to exist - meaning that several miners channelling floors in the same area are a danger to each other. So you should allow only one dwarf to mine out floors in an area at a time.

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:

Defense
Use cave-ins to block off water approaches to underground cavern levels. Combined with walls higher up, a cavern can (with great effort) be rendered completely safe from all intruding vermin.¹
Death
Since a cave-in kills all creatures instantly, it can provide a convenient or amusing way to off a group of creatures. This is also one of the most effective ways of dealing with titans and forgotten beasts with dangerous syndromes, especially airborne contaminants (deadly dust/vapors) and poisonous blood. For certain randomly generated creatures, they may be so indestructible that a cave-in is the only way to kill them. Also, it's a great way to 'spare' an 'injured' dwarf who likes 'laying' in a bed all to himself.
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!
Breaking through multiple aquifer levels
Showcase with two levels: User:Rhenaya/HowtoDualAquifer
Trapping [TRAPAVOID] creatures:
Since the dust from a cave-in can knock creatures unconscious, and any unconscious creature triggers a trap (including your dwarves and other friendly creatures), combine a cave-in with nearby cage traps for the capture. Note that this is only useful for kobolds and gremlins, as all other creatures which avoid traps are also immune to being knocked unconscious.
Moving Water / Magma faster than Pump Stacks
Main article: Magma piston

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, channel the tiles above them, and removing the stairs afterwards. 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.

¹Not literal vermin, those won't be blocked.

Bugs

If a creature gets hit by stone or mineral dust from cave-in, in reports it will be shown as "is caught in a cloud of boiling magma!"^Bug:1638
Rarely, a cave-in may happen right at the start of the game. This can have several effects depending on where it happens: from releasing underground tree spores to revealing an upright adamantine weapon in the Room list.