Here I propose a hypothetical mega-project idea, for your consideration. I posit that one could construct a massive fortress within a volcano, by dropping an enormous, ~120 Z-level tall plug, constructed above the lip of the volcano, into the caldera. The plug would need to be tall enough to extend from the very bottom of the map, the semi-molten rock, up to (and probably exceeding) the surface of the lava. The plug would need to be completely solid, but could be built and collapsed in parts. If you wanted to you could probably mod the world gen settings to give you enough height to build it all at once, but in theory the plug could be dropped in a couple layers at a time. Here's the set-up:
....WWWWWWWW....
....WWWWWWWW....
....WWWWWWWW....
....WWWWWWWW....
....WWWWWWWW....
....WWWWWWWW....
....WWWWWWWW....
....WWWWWWWW....
.WWSWWWWWWWWSWW.
.W............W.
.W............W.
XXX~~~~~~~~~~XXX
XXX~~~~~~~~~~XXX
XXX~~~~~~~~~~XXX
XXX~~~~~~~~~~XXX
XXX~~~~~~~~~~XXX
XXX~~~~~~~~~~XXX
XXX~~~~~~~~~~XXX
XXX~~~~~~~~~~XXX
RRRRRRRRRRRRRRRR
X: Natural walls around the volcano
R: Semi-molten rock
S: Supports
W: Player-constructed walls
~: Lava
. : Air
After releasing supports:
................
................
................
................
................
................
................
................
.WW..........WW.
.W............W.
.W..WWWWWWWW..W.
XXX~WWWWWWWW~XXX
XXX~WWWWWWWW~XXX
XXX~WWWWWWWW~XXX
XXX~WWWWWWWW~XXX
XXX~WWWWWWWW~XXX
XXX~WWWWWWWW~XXX
XXX~WWWWWWWW~XXX
XXX~WWWWWWWW~XXX
RRRRRRRRRRRRRRRR
This is scaled down, the real version would be much taller, but recall that you can drop a few layers at a time if you don't want to/can't build it all at once.
Now, with an enormous block of stone spanning from SMR to the surface, you can dig out an entire fortress inside of it. Dropping the plug will displace a preposterous amount of lava, so it doubles as an incredibly powerful/apocolyptic weapon. Just make sure your dwarves hide away underground every time they drop some layers down. You don't actually need to have it reach all the way to the surface either. Just make the first dozen layers or so fill the entire diameter of the volcano, and you can dig out an underground access tunnel and dig out your fortress from below. Just make sure you drop enough layers for your access tunnel to be above the magma sea. Instead of making a towering 120-block high fort that extends to the surface, you could make a more modest structure that's only a few dozen layers. In theory you could also drop a few layers, dig out part of the fort, then used the dug-out blocks to drop the next few layers for more efficiency regardless of what height you want the end result to be.
Potential reason why this might not be possible:
Will the constructed blocks survive the fall? If too much of the plug is damaged while falling this project might not be possible, as lava could leak into any cracks.
Will new layers dropping destroy old already-dropped layers? This might not be a game-breaker if it only destroys one layer at a time; if you drop 4 layers and destroy one, you've still increased the height by a net 3 layers. The big problem would be if RNG is involved: if blocks are destroyed in unpredictable ways, cracks could easily form.
Will the hollowed-out sections of the block attempt to fill back up with lava? What is exactly the trigger for the volcano to start filling? Can a volcano fill into an area completely sealed from the lava? Is there any way to cheese it to make sure your fort doesn't get flooded?
Do collapsing structures survive contact with SMR, or do they just get deleted? If SMR destroys collapsing blocks, it's game over. The plan will not work.
Post your thoughts. Is there any chance this plan could work? Are any modifications to the plan needed?