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40d:Cave-in
Cave-ins occur whenever any 7x7 (or larger) area is fully excavated. Therefore, you must exercise caution when mining large contiguous spaces. If you have mined out too large a space, the game will sometimes provide a warning in the form of "a deep rumbling from the cavern."
Objects and occupants caught in a cave-in can be damaged or destroyed. Cave-ins will scatter about a dozen partly-mined rock tiles across a wide area along with a cloud of dust and debris that can knock out nearby supports, triggering a potential chain reaction. A cave-in that occurs over the magma river can also produce magma mist. The rock left behind will always be gray stone, not light or dark stone, ore, or gems. This rock can be mined out again (sometimes leaving behind new stones), but at least one tile should be left unmined as a pillar to prevent another cave-in.
Avoiding cave-ins
To avoid cave-ins, you must have a minimum of one support (or unmined wall, referred to as a pillar) somewhere in a fully-excavated 7x7 region.
The general rule to remember for all situations is, 7x7 = not safe, 7x7 - 1 = safe.
For instance, the following 7x7 room can be safely created because it has a "notch" in it. This pillar could be located anywhere within the 7x7 area to prevent a cave-in:
For maximal use of space, supports or pillars should be put at each corner of each 8x8 region:
An enclosed 13x13 space could be built like this:
The above example includes two large passageways leading into the room. As long as neither passageway is widened beyond 6 squares, no fully-cleared 7x7 regions will exist, and the structure will remain safe.
Rooms which are 6 squares across or less can be indefinitely long or wide without needing supports:
They are often called "hallways."
Doorways or other passageways can be set into this hallway as long as the opening is never more than 6 squares across:
If any of the rock marked ▓ is cleared, that will create an uninterrupted 7x7 region, which will collapse.
River, chasm, and magma tiles are considered walls by the game, so you can safely excavate up to 6 squares deep along each of their banks.
Creating cave-ins
Cave-ins can be a good thing when used right. For example, they could knock out a multitude of invaders, block off a permaflood, or remove a channel.
There are two ways to make cave-ins. The first and simplest is by simply mining out a 7x7 uninterrupted region. A cave-in trigger will periodically happen. When it happens, all 7x7 regions will collapse.
You can also place a support in the 7x7 region and hook the support up to a lever. When you pull the lever, the support will collapse and a cave-in will happen immediately.
In an extremely long term game, cave-ins can be used to replenish depleted stone supplies.