Dig (not channel) all the floors where you want the hole to be. Go to the top hole, and channel out a "cutout" of what you want the hole to be.
- = channeled
code:--------
- -
--------
Make sure to leave one floor unchanneled, so you don't have a cave in! Now go make sure no dwarves go onto the floors beneath it, then dig the last channel out. Make sure your miner is not standing in the cutout!
There will be a cavein, and the rock will crash through the floors beneath. Instant hole! You don't have to worry about channeling any other floor but the top one! No more stuck miners! It surprised me too when it crashed through the floor (It also surprised my LEGENDARY MINER. :( ).
The completely safe way to do it is thusly:
code:# = wall
_ = channel
. = floor
X = up/down stairway
< = up stairway
I = support (b-S)
At the top level, dig out something like this:XXXXXXXXXXXXXXXXXXXXXX
X____________________X
X_.................._X
X_.................._X
X_.......X.........._X
X_.................._X
X____________________X
XXXXXXXXXXXXXXXXXXXXXXThen for every level you want a hole, do the following in this order:
XXXXXXXXXXXXXXXXXXXXXX
X....................X
X....................X
X....................X
X........X...........X <- Completely mine out the level
X....................X except for the up/down stairs
X....................X
XXXXXXXXXXXXXXXXXXXXXXThen:
XXXXXXXXXXXXXXXXXXXXXX
X____________________X
X_.................._X
X_.................._X
X_.......X.........._X <- Now we add the channels. Note that your miners can
X_.................._X always get to the channels, and out of the area.
X____________________X
XXXXXXXXXXXXXXXXXXXXXXOn the bottom-most level:
XXXXXXXXXXXXXXXXXXXXXX
X....................X
X....................X
X....................X
X........<.I.........X
X....................X
X....................X
XXXXXXXXXXXXXXXXXXXXXXThen connect a lever to that support. When everything is mined out,
remove the lone up stair and throw the lever. Forbidding the area is optional.
This works because stairs provide support. Note that if you really want a hole, you need to mine out all of the raw rock; loose stones are fine, but unmined rough walls will fall and restabilize themselves like Lego, leaving you with really weird things like an entire column of rock ten z-levels high supported by one single rough pillar.