Ah crap.
So I've just spent the last week (real-time) carving out a massive space, creating about 300k stone. Literally, the whole week - I've been leaving it running overnight while using Dtil's Unpause. This isn't the first time I've done such a thing on this fort, but I really messed up this time.
See, here's how the first big dig happened: Build temp fort, dig out utterly massive area, use matgloss token editry to boil away the extra stone. The first time this worked quite well, although I lost a handful of masterwork armour stands and the like - definitely worth it, I thought, to remove 300k stone worth of lag in a single frame of computer processing.
However, following this, I set up permanent camp in the newly-dug-out area, got the main part of the fort working, then after a four year break started digging out massive area #2. See, area #1 was the actual fort, and area #2 was for the most part just an aesthetic extension of it despite the amount of stone being used.
So I just finished digging everything out tonight, and did a once-over of the fort before I set up my boiler. Looked like I'd lose a lot of unused masterwork mechanisms, some noble furniture... overall, nothing that special. HA HA HA HA HA HA. I ignore the ominous feeling and take the plunge.
Bam, 300k stone gone. However, my lake is suddenly half empty and continuing its drainage, while engineering is flooded. I start freaking out and investigating in paused mode. It seems I forgot to account for mechanisms already installed into buildings. So my floodgates, despite being made of non-boiled stone, got uprooted and now my lake is draining into the UG river chasm. Also, my recreational pool which I'd spent a long time micromanaging to get it to exactly 4/7 depth all around was draining into engineering because the pump used a microcline block and collapsed. And I can't fix any of this because the twenty levers to operate my massive pump/floodgate setup all used at least one microcline mechanism and deconstructed themselves. Also the orthoclase gear assemblies in the powerplant failed and the waterwheels fell apart. And it seems that every single mechanism my four legendary engineers ever made was of one of the stones destroyed, and I've got a lot of dwarves tantruming - prior to this, my least happy dwarf was at 1400 or so happiness (150 is ecstatic).
Normally losing is Fun, but I'm about to lose four months of IRL tender loving care, and am flipping out. Aaaaaaaaaaugh. I'm really hoping this situation is salvageable. My framerate was already at 0 even before all this water started moving about. Yeesh.