Big Mother F*ing Digging Project, that is.
Through a bit of trial and error I've come up with a pretty foolproof technique for digging, and managing the low FPS that comes from tons of loose stone. I didn't see a decent place to put it on the wiki, so I thought I'd share here. It's more of a strategy than anything.
Hollowing out a mountain:
Step 0: Have a magma pipe. Have temperature turned on.
Step 1: Dig out the bottom floor with ramps (clearing level bottom, level bottom+1 and the floor between them. Take care to discover the magma pipe properly (from the top) or your dwarves might accidentally tap it.
Step 2. Quantum-dump or use up all the valuable stone that is loose.
Step 3. Tap the magma pipe from the bottom level, allowing it to flood that level of your map. When the season turns over, all that stone disappears.
Step 4. Smooth, construct in, whatever with the empty levels of magma pipe.
Step 5. Dig out level bottom+2. Quantum dump or use up all the valuable stone/ore. Channel around the edges of that level, let it drop (along with all the useless stone) into the magma level. You'll have to channel around at least as big as the widest part of your magma pipe.
Step 6. Repeat step 5 until you only have the top level left. Build whatever you like, and roof it, and even after channeling the dirt level away, the indoors will remain subterranean/dark/underground for 'dwarves stay inside' purposes.
Step 7. Profit!
I'm dropping bits of dirt down a few levels to have 'arboretums/greenhouses' in the lower levels, so I'll have a bit of Dwarven Science to show/tell as well, to do with falling levels, cracking levels, and cave ins.