Goal: Say goodbye to 31.25. This shall be accomplished by construction & testing of an anti-invasion system consisting of a giant tub hanging over the fortress, filled with magma. System will divulge the magma with a unidirectional spread coving the entire surface edge of the embark.
Steps ...
A: A more or less self sufficient fortress. ( Took a lot longer than such a short description is implied. )
B: Flail around with various concepts, learning trap encrusted hallways & causeways, military management, & the Pissed Off Pastured Predators Perimeter Defence Programme. Build up a monstrous military force as an interim solution. ( Side note: Considered this step an overall failure - the end result is a single legendary militia commander axedwarf / swordsdwarf / givemeanythingdwarf who solos full on goblin invasions, rendering all other methods redundant for almost all hostile surface situations. Hell, I can't even get be bothered to keep this guy out of the line of fire of the ballista battery, it's all the same to him. Loads of fun figuring this all out & occasionally watching hostiles fight hordes of bears was amusing. )
C: Wall off the entire embark area up to the 5x tile no go zone. Wow, this is going to require an awful lot of stone & hauling & ...
D: Discover the concept of ice construction. This is awesome but the murky pools ain't going to cut it.
E: Plow into the caverns & raid them for the water supply. Figure out power generation & pump stacks. A messy step with many causalities but the experience gained is vital for what is to come.
F: Build & deploy a giant fortress overhanging water tank ice generator.
G: Oops. "Cave ins" happen without warning & with unsurprising regularity. Add in some secondary passive flood mitigation measures for the water tank. Very valuable experience gained.
H: From this point on everything that can be made of ice is made of ice. While the primary purpose is still the magma chamber it's all ice all the time everywhere with all sorts of tangential projects.
I: Find magma sea, build & test the magma pump stack, finish construction of the ice floor of the almost embark sized magma chamber above. Construct the ice bridges that will act as the wall of the magma chamber ...
Then spring arrives & all the bridges go "poof." I'm a little disappointed that this happened before I was able to link them all up, close them, & fill the magma chamber. Would have been a much better ending. Can't always rely on the FUN factor I guess.
In spite of the wiki's reluctance to endorse sphalerite I'm satisfied it's magma safe for all things & in all ways. Back to strip mining real rock.
Once this is complete & tested I shall be able to find peace - thru undead Keas & mine carts.