I made a moat!
Unfortunately I didn't notice exactly where I channeled and 7FUN7 began...
I managed to wall it off before it got to a point where it would have been really hard to recover from [Those downstairs lead to my main industry]. One of my miners was sleeping in the bedrooms at the time while the other one was above ground for... some reason. So the trapped sleeping miner had to escape; dig his way to the main halls; then dig up to reconnect the surface-level of the fortress to the rest of it.
Oh; and he still chooses to sleep there.
I lost a fisherdwarf to the 7FUN7; and a trade depot is underwater. Traders just stand at the edge of the map now; despite me building a new; closer depot... for some reason.
Also all this 7FUN7 happened just as a SEIGE arrived. Said SEIGE killed 1 dwarf who was wandering around outside; before a goblin got caught in a trap and all the others left the map because my fort was walled off.
Also; while it took a while to save my surfacedorfs; I just made a fishery with some wood still up there; and had a well and surface farms, so they were self-sufficient. Which is good to know that if something goes horribly wrong underground; my fort is in a place where I can wall it off and at least survive above ground...