I'm trying to follow the Wikia's description on how to cave-in an aquifer, but it's just not working. Either nothing collapses at all, or when I do get a section to collapse, I end up with a crushed and/or drowning miner and the aquifer still full of water. Is there an easier way to do this, because to be honest, the explanation on the Wikia is pretty poor and confusing in the first place.
Soil/not soil is not the issue. What you have to do is create a solid 1-z-layer plug of completely undisturbed soil 2 layers above the aquifer. At least 5x5 is probably needed but I always do at least 9x9.
Dig a 2-square corridor around the edge of this solid plug.
Dig out all the soil below the plug in the layer that is immediately below the plug and immediately above the aquifer, plus at least an additional 1-square border
In this same layer, channel out a space directly below the plug into the aquifer in the exact shape of the plug- this channeled area will completely fill with water. Channel out one additional square outside the border of where the plug will fall to create somewhere for the displaced water to drain back into the aquifer once the plug falls.
Go back up a to the plug level and channel out all around the edge of the plug. The plug will not fall because it is still attached to the layer above it.
Go up another layer and completely remove all soil above the plug plus a 2-square border (if this layer is the surface you don't have to do anything.) Now channel almost all the way around the top of the plug, leaving a single floor tile on the left side of the plug that connects the top of the plug to the wall. Evacuate everything from the two levels below unless you want things to die.
Now have a miner channel out the remaining floor tile that attaches the plug to the wall. The miner will always channel from the left and will never stand on the plug. Make sure that he has an escape route and is not between a wall and the pit or he will be blown into the pit by dust.
The plug will fall into the pool and completely fill it. There will be about 5/7 water in the bottom of the 2-z-level pit that has been left behind but it will drain back down into the aquifer through the extra channeled tile that you left. You can now dig down through the center of the area where the pool was. The plug will have changed into whatever the material of the aquifer is; don't worry about this. The soil will be damp but this is from the 1/7 water that will still be above it, not from the aquifer. Dig straight through and do not dig out anything except staircases in the aquifer layer. (It's safest to avoid digging around in the layer immediately below the aquifer, too.)
This method is infallible but only works if your aquifer is just 1 z-level.