As promised, here are pictures and instructions for caving in SMR. I did it on the map's edge, too, for extra difficulty points. Casting obsidian up there is a real bitch, but some
savescumming trial runs, I got it to work.
Why mess with obsidian casting? Turns out you can't build supports withing 5 tiles of the map's edge, and as you probably knew, you can't build walls or fortifications there, either. Making things even more awkward, you can't build track stops on the top level (so using minecarts to move magma is virtually impossible), and you can't really use buckets of water (since dwarves won't empty a bucket onto the level they are standing on). Basically, your only option is to pump both magma and water up to the top level, then somehow control the flow so you only cast obsidian where you want it, since casting anywhere else will probably block you from casting on the intended tiles.
In the above picture, you see my casting rig just after successfully casting on all necessary tiles. It's a bit of a mess. The pump stacks were located in the lower right corner of the picture. Water went through the long pipe to the left side, magma took the short pipe to the right side.
The plan was to cast the right wall first by flooding the middle area with water to about 2/7 (using a bridge to block the water from entering the magma shaft), then "disabling" that bridge (see
this thread), causing the magma to spill onto the water, casting a line of obsidian just to the left of the bridge. To do the left wall, I carved fortifications in the right wall to let the magma through, then did the same thing, except this time the magma filled up the middle area.
As you might guess from the picture, there was a lot of spillover in the middle area, and it did set a lot of the map on fire. It's not necessary to do it that way.
In each cavern, you need to build a structure to extend the sky support downward. Stairs are easiest. The structure shown extends upward to the cavern ceiling, and I built similar structures in all three caverns.
I built stairs underneath all SMR tiles I wanted to cave in. After building these stairs, removing any last support should be enough to bring down the whole column, SMR included.
In the first shot, you can clearly see that the edge of the map, normally unminable, has collapsed. That edge has been permanently lowered by 5-10 z-levels. Unfortunately, it cannot be further collapsed, because the edge is unminable and it has caved in as far as it can go. An edge created over the magma sea could be totally destroyed, because the magma flow tiles would still swallow the cave-in.
The column is not swallowed by the eerie glowing pit. Instead, it sits on the bottom of the map. Since the bottom of the map is at two different levels in my site, and the column fell on the border between the two areas, the column was split slightly.
At this point, if you did not have too many layers of SMR, you can clear out the natural stone above it to create a large skylight in Hell.
Ok, I had to use dfhack to make it snow, but that's not the point. If you have a cold map, you can literally make Hell freeze over.