I apologize if this is an obvious question, but: what is the least painful way to dig out large 3D volumes?
For cisterns and magma reservoirs I usually just designate the whole thing as up/down stairs, which I leave in place. The miners can dig that out in any order, no cave-ins, no deaths by dehydration. Hooray! But it leaves the place full of stairs, which is not always what I want.
For my glass-roofed sunroom/statue garden, I channelled out one layer at a time, from the top. Worked fine, and I could designate a whole floor at a time, at least, but I had to start from the top and provide access to every level.
For my obsidian casting room/visitors' lobby, I dug all the floors out and then channelled each layer, but this required channelling a line at a time to avoid cave-ins, and synchronizing the layers so as not to drop junk stone on the heads of miners below.
I tried channelling upward ramps but I kept getting cave-ins - never on top of a dwarf, but they got thrown around pretty badly. The layer above had been dug out, but it still had its floors, so I think it was those floors, somehow falling. This was definitely the problem when there were trees growing above that I couldn't just channel out (woodcutters being slow for some reason, with the fort standing open).
For horizontal water plumbing I designate up stairs to prevent tree growth. Ugly when empty but way less pain than paving the floors. And I can remind myself where to put valves by removing the stairs. Unfortunately I ran into a weird problem where I dug one set of up stairs under another and the water broke through. I thought they were supposed to leave the floors intact? Maybe I had mis-designated the above tile?
For a clean vertical shaft with no stairs or anything, I end up having to run a parallel shaft with stairs, channel each floor, and then wall it back off. I guess there's a do-or-die approach of just channelling straight down and hoping the miner reaches the bottom before getting too thirsty to work, but is there a better option? Can up-down stairs be safely removed (bottom-to-top perhaps) with no side access?