So I found my first mysterious structure at my latest fort, and I'm looking for ways to have some fun with it (!!fun!! or otherwise). I already backed up the save and removed the adamantine sword just to see what happens, and what happens is my 154-dwarf fortress gets annihilated in approximately 3 minutes. Now I'm looking for a way to successfully get the sword with minimal damages. I've heard that building a constructed floor on top of the glowing floor seals them off, but that's way too cheap for me.
I'm thinking of digging out a really long channel to divert the river on the surface (I always have my entrance double as a drowning trap, so I've plenty of experience with this) to drown them. I've got the highest stairway nearly walled off (as a guide for the water and also a trap for the demons should something go wrong since walls are immune to building destroyers), so that I just have one wall left to construct from the inside -- I will disable masonry on all but one useless dwarf so that nobody of any consequence is lost. Urist McSacrifice will build the final wall, then go pull the sword and release the demons, then someone in the main fort will open the floodgates to the river so that it flows down to that walled-in basin, down the stairway, through the structure, and drowns the demons (unless they're somehow immune to drowning). Assuming this works, I can then re-engage the floodgates and deconstruct the walls of the basin. However, since slade is indestructible, there would be no way to drain the structure and actually obtain the sword.
I'm definitely doing the river diverting thing, because that sounds like fun, but what other suggestions are there? When I find magma, could I do similar with it and then divert the river to fill the structure with obsidian to mine through, or would the water only obsidian-ize the topmost layer?