SUCCESS!!!!!!
I made it. I know it was only a two level aquifer, and not particularly spectacular in any way, but it's a great sense of achievement. A personal best. And as a reward on the underside: two more levels down and it's limonite mixed with bituminous coal! My dwarves are going nuts digging that stuff out. We're going to have some serious armour soon as I can find some goddamn marble or something.
Stuff that scared me the hell out: (And stuff I did/assumed wrong)
1. When the aquifer drained, it filled that drainpipe 7/7 all the way out, and I honestly thought it was going to fill, and start bubbling up, like it stormwater out of a drain. Terrifying! Also, it took an amazing number of cancellations to get the walls done on the lower level of the staircase. I don't really know why. Was I meant to turn off the pumps? I thought that might be it. Anyhow, I took that as PROOF that the WATER WAS RISING!
2. Taking down the waterwheel. There are plenty of tutorials explaining how to build a waterwheel. There are none explaining how to take it DOWN, more than a brief warning that it can fall. So I tried to stop it spinning first. That... that didn't work out too well, but no one was actually injured, as my hospital is still empty. So yeah. I think next time I'll try deconstructing it whilst it is spinning. Oops.
3. I didn't realise that I couldn't dig out the layer below the aquifer. I almost did, due to sheer exuberance at getting there. Um. Warn people. The result is flooding. Got it all fixed up now. Looks like that's why you said to make the drain a few levels lower than the aquifer.
Right. Now I'm off to make a dining hall, bedrooms, and lots and lots of smithy areas. Plus I want to totally rearrange the single level my poor dorfs have been living and working on until now.
THANKYOU!!!