After finding magma and the third cavern, I decided to move my entire fortress down to between those two levels. My worldgen has approximately 20 levels between features, giving me plenty of room.
So everything moved down - stockpiles, meeting zones, everything. The last step was to irrigate the new farms.
I got a new lesson in water pressure.
I tapped the cavern lake from underneath, as I figured the pressure would help fill up my reservoir quickly so I could resume operations with minimal downtime. Oh, it filled up quickly, alright.
I took a lot of precautions: I had multiple doors, an airlock system, and I even modded dwarves to have a speed of ten so I wouldn't lose this particular miner. But as soon as the cavern water was breached - woooosh!
I had to savescum just to see what happened, it moved so fast. First frame, miner breaches the wall. Second frame, water rushes in, the dwarf barely managing to stay ahead of the flow. Fifth frame, dwarf makes it to the first door, 7/7 water directly behind him. Sixth frame, dwarf opens the door. Seventh frame - the door's still open? Eighth, ninth, tenth, eleventh - door's still open, water rushing through. After a long, long while, the door finally shuts, but not before half the cavern water has found a new home in my fortress, having followed the dwarf to the bottom.
The whole fortress drowned.
Next time? I'm using a sacrificial miner dwarf, and I'm walling that sucker in.