So, amusing story time.
I'm on genesis mod and usually play adventure mode, I started up a fortress a while ago, for the sole purpose of having an interesting place to seek out and explore with an adventurer. Usually my fortresses die pretty fast, tantrum spirals explode on one another and things get pretty messy. Amazingly, my fortress is still standing, despite an invasion of ghouls, a shoddy start (gen mod doesn't sem to have set up startout in fort mode, and an extremely on-off brewing/food production line. After digging down and exposing three different layers of caves, I have successfully found some lulz.
My dwarves excavated a mine shaft around the second cavern some time ago, and then continued downward into the depths of the earth in a bid to unearth some interesting things (which I did) and hopefully strike some lava at some point. Now the upper caves are very much flooded, the lower one I'm not sure about. But after hitting the stopping point, the miner went back to clear up all the rest of the rock. I'd forgotten that that included a wall into an underground lake. Naturally he smashed through and made a small trickle of diagonally flowing water. He then sat around panicking for a while and I went to check on the other levels. Seven levels below that, there is a dwarf picking up the herbs and plants immediately around the vicinity of a mine shaft. Suddenly I notice that the water is coming in faster. You guessed it. Mr.Miner went back and took out another chunk of the wall letting in a full flow of water. The poor herb collector seven levels below is now sitting at the back of a cave watching a random torrent of water make its way down the mineshaft and flood the lower levels of the caves. I have a brilliant mental image of this dwarf suddenly being soaked to the skin, temporarily stranded in a subterranean cave by a river that appears out of nowhere, and thinking "What the fuck is this!?"
I can't wait to see what comes of this drainage. I'll probably have somewhere to put a couple of farms, at least.