Messing about with a fort I was about to abandon anyway, but whose Clown Metal was largely inaccessible by magma[1], I hatched a plan by which I would dig into the non-semi-molten rock in an area so as to later drain the current magma[2] down a level or two[3]. And I hit that layer that 'resurfaces itself'.
But I put some more channels in on that lower level, and at least one (probably two) breached "That Zone", within sight of the Clown's base. FPS went from 35 to 8 quite quickly[4] as they wander around. But the resurfacing had already occurred. No access. They wandered around, harmlessly, and eventually all but a couple were outside the limit of my discovery vision. And, what's more, I now knew exactly how far I could exploit (should I ever get to reach it) the raw metal without breaching the usual shaft.
Although I didn't get that far, because a Cyclops came on board, I mustered my entire fighting force (including many who I had had difficulty taking
out of military duties) and sent them along to try to wrestle him to death (hey, why not..? Some were very good, by now...) And then the game crashed. Still, I have a save from just after the most recent wave of immigrants (before encountering the magma sea, at least two of the cavern levels and long before any of the Fun Stuff happened) so I might have another go at it. Or just play it straight.
Although I'd also done some experiments with booby-trapped stalactites which might be interesting weapons in any such war. I'm thinking of something that's a cross between a typical Indiana Jones movie and Total Recall's alien cavern. Pressure-pad initiated, per stalactite, but with a master lever operating them all if it becomes obvious they don't detect floaty/ethereal things too well, or they're all effectively trap-avoid anyway.
[1] Although I later realised that there was a one or two tile opportunity to dig into it while retaining walls against the magma.
[2] My first breach into the sea was via an up-down stairway right on the edge of the sea, and there were around seven 6/7ths perpetually roaming around the top level of one of the pockets of magma, so I surmised it wasn't going to fill up again from anywhere
[3] If I could, and at least give opportunity for some evaporation before I could put in some some throwaway pumps.
[4] Yes, that's a relatively slow machine I'm using. Although it seems to me that the rendering takes up most power, as the processor demands the fan on high speed whenever DF is active, or even paused, but declines to normal whenever a DF is minimised.