I took a dwarven year to set up a basic, run-of-the-mill fort just a few Z-levels under the ground, so that my dwarves can survive while working on the largest project I have ever started. It's being protected with cage traps - yes, they're lame, but they work for all intents and purposes. My eventual fort will have a cooler defense system. I'm making rock crafts on repeat to buy stuff like anvils and picks - I don't feel like digging down to magma and mining out ores in a non-permanent fort.
I will build an enormous, deep pit, eventually right down to the magma sea (though I'll probably start digging down to the first cavern layer or something like that). In the middle of the pit, a series of ramps will go down as in a winding spiral staircase. My fort will be built around the deep pit, and connected to the staircase with natural bridges.
My question: what is the most efficient way to quickly dig out a deep pit? I was thinking of first digging out all the layers of the pit, then digging out channels around all the walls, placing supports to avoid collapsing. Then, when that is finished, I want to link all the supports to a lever and pull the lever, ideally causing an enormous cave-in that (after a real-life day of processing by my slow computer) will instantly dig out my pit. Is this even possible, or will I have to channel out everything by hand? The pit has a radius of about 21 tiles, so that would take an enormous amount of designation time, let alone the actual labor by the dwarves...
Digging out a plug doesn't work either, I think, because there's going to be a lot magma-safe stone.