One time, my very first fortress, I flooded almost the entire thing...
That wasn't the priceless bit, the priceless bit is that four years later it's a normal thriving fortress.
One spring, I happened to have a dwarf pull the lever to flood the fields, just as another was going to collect cave spider silk... As it was my first fortress, I hadn't taken the necessary precautions... As a wall of water swept through my fortress I began to despair, having heard of infinite floods and the like. Out of curiosity, I played on to see what I could do.
First thing, take stock of my remaining dwarves- I had 31 (of 84) still alive somehow, but most of them were locked in their workshops or bedrooms, doomed to die of thirst... or the opposite... if I couldn't find a way to save them. Two dwarves were on break outside of my fortress, miraculously one was a miner. Deep inside my fortress, past the chasm (that had doors around it) I had a couple more miners, a metal smith and some random peasant hauling ore.
I also had a jeweler who was hauling ore who was trapped by the chasm... by an unbuilt aqueduct, part of my incomplete world-nuking Lava System...
First thing I tried was to dig into the switch room (miraculously unflooded) with my outside miner to put the floodgate back up. Naturally it was too late, as there was a perma-flood. There was another switch though... The one to activate the Lava System.
It was then that I got an idea. I had the jeweler complete the aqueduct, and the pulled the switch. The Jeweler was trapped, as water quickly crossed the aqueduct... He stood there, a heroic martyr, to be burned to death by the steam as the lava flowed outward...
I still think of him when people say how useless jewelers are...
Unfortunately... The aqueduct over the river hadn't been built yet...
I again began to despair, until I remembered the two miners, a metal smith and peasant by the magma flow. I had another idea. I immediately had the miners dig a tunnel parallel to the lava channel meanwhile, the peasant and metal smith set about building a masons workshop, a mechanic's workshop, a floodgate, and three mechanisms.
This was the tricky bit... I dug a tunnel to within one square of a stockpile I had. I then recruited one miner and stationed him right next to the flooded stockpile, while the other miner sealed him in with the floodgate... He then dug into the flooded room... and died... Another hero...
Meanwhile I had the floodgate hooked up with a lever I built, and designated another channel to be built between the floodgate and Lava System... I realized the miner that would build the channel would die of lava in this case... Thinking twice (every dwarf being so very valuable), I suspended construction and turned off the Lava System (again, from the switchroom near the entrance). Much to my surprise the lava-cauterized channel did not refill with water, making the construction of the final channel simple.
Not everything was simple, as my miner in the switchroom took that time to decide to open the door to the fortress, flooding the outside world... Quickly, the heroic band beyond the chasm built two more mechanisms and attached the floodgate at the magma flow to their switch.
The switch was pulled- lava slowly filled the fortress, and then the outside world (killing a rather soggy band of elf-merchants). The switch was pulled again, and the lava drained...
The aftermath: 19 dwarves had survived... Three surviving heroes deep within the cave, and seven more who had the sense to stay in their rooms... three of those were nobles... One cat had survived in a dwarf's room... it was the only animal left in my fortress...
There was no food or alcohol left, and nine more (already hungry from the ordeal) dwarves died of starvation in the next few months before the human caravan arrived (I still had a bunch of trinkets, and I had a nice road too, so I got plenty of food). All the nobles survived, leaving me with seven working-dwarves and three nobles... They were not happy nobles...
The year was hard with so few dwarves working in such a massive fortress, but next spring fifteen migrants arrived...
From then on things began to regain a sense of normalcy, my dwarves slowly refilling the empty halls... As I said at the beginning, it's four uneventful years later and the fortress is thriving- even larger than before the Catastrophe...
The one lasting scar... Every single engraving has a dwarf drowning or melting or something... This fortress will never forget the Hazards of Irrigation...
Edit: Comma placement
[ February 10, 2007: Message edited by: Eiba ]