Found a vertical cavern, which I dug (with appropriate grates) a drain into from my kitchen well cistern. A migrant arrived with a lot of pets, and one was a rabbit. I've wanted to know what was down that endlessly deep cavern for a while... so I penned the buck rabbit in the middle of the kitchen well drain... and flushed.
The kitchen well cistern, incidentally, is a six-by-six-by-four hole which was 7/7 full of water to the brim. Said fluffy buck wabbit went flying out on a huge and epic plume of water, and fell. And fell. And... fell... I kept dropping and dropping and dropping, watching the water. An outcropping of rock to one side did get a splatter of red, but buck kept on reporting back a beautiful waterfall and mostly sheer walls made of ore... I closed the floodgates, then checked on buck, who was still falling. He finally reappeared (after falling past every type of ore in existence, sphaelerite, native gold, garnierite, tetrahedrite, native platinum, etc) at the bottom of the fall, cushioned on a huge stack of water which I don't think had been there before I emptied the cistern. He had fallen on his hind legs: crushed legs, lungs and ribs were unfortunately only part of his injuries, I basically pulped him, but he wandered around down there for a little while, eventually succumbing to his injuries a few months later.
My dorfs haven't made it even a quarter of the way down that terrible descent. I'm considering throwing a bird-type thing next. There's a pterodactyl forgotten beast down there at the moment though, so I'm waiting for it to be out of the way before I pen the next beloved pet intrepid explorer in the water cannon launching tube... Although if I find out who the sodding vampire is, I guess I'll draft him to the army, then station the sodding bastard right there in the drain.