Yeah, yeah, I know what you're thinking. "Oooh, someone made an obsidian monolith. Big deal." Well, see, it is a big deal. Or, at least, it's big.
The mold:
The mold is 70x41. Including the bridges, which aren't shown in the 3dwarf screenshot, that were used to scaffold the mold, ~6500 stone were used for the mold and the scaffolding.
The tower in the middle of the mold has a set of magma pumps, and a set of water pumps -- I really like the idea of my monolith having indoor plumbing available on every level if I need it. The secondary tower in the distance is almost directly on top of the volcano, and supplies more lava -- tests showed that the magma pumps in the tower inside the mold was too far from the volcano to pump lava fast enough.
The strange "arms" on the interior tower are to provide support for the retracting bridges that weren't adjacent to a wall, so they didn't fall when the whole thing retracted.
The filling. In a needlessly dwarfy complication, I built retracting bridges at the very top of the map to hold water, which is why it looks like there's a ceiling -- that's all the water waiting to be dropped. Turns out that would crash DF when the retracting bridges would reset. So I dropped the retracting bridges down a level, which works fine.
The big problem with the filling was the magma. Drained the volcano doing this. Twice. And that was just filling a layer with 2/7 magma before dropping the water on it. Took *years*.
Done! This is in the process of removing the mold. The finished product is 68x39x23. That's 60,996 tiles worth of obsidian. Now, the indoor plumbing pipe is 690 of those tiles, so if you were to dig this thing out top to bottom, you'd
only get a maximum of 60,306 obsidian.
Now, the question becomes, what to do with it? To be honest, I hadn't planned that far. I'm thinking of abandoning, and starting a community fort based on new dwarves coming in and finding this...structure...that speaks to them, at night, in their dreams...