1. Carpet that has shown itself capable of growing mould is probably also capable of growing other fungi, such as mushrooms.
I am told on good authority (ie one of my siblings) that fluorescent mushrooms growing in one's house is a Very Bad Thing. Fortunately, the mushrooms growing in my carpet were not fluorescent. If they had been fluorescent I might have noticed them earlier, because at this time of year I leave the house when it's still dark and get home after it's darkened again so only see things by daylight during the weekends. Thus, this morning I discovered mushrooms growing in my carpet that, while not fluorescent, did not look very edible either.
2. A screwdriver (to pry up the first carpet nail) and thereafter some sturdy pliers (to grip and pull) are a pretty decent way to rip up dry carpet.
There is no decent way to rip up rotten carpet. You still probably can't beat pliers but at a certain point of rotten they're just tearing it apart a few chunks or threads at a time.
3. Sufficiently thin carpet and underlay is indistinguishable from thin carpet.
When I bought the house, the inspection report commented on the thin carpet and lack of underlay. I've been meaning for years to get new carpet and underlay and revel in luxury but first I had to get earthquake repairs finished, and then I've been hunting for some decent carpet in a colour other than grey or beige (that's a rant for another day), and now fixing the roof and whatever wood has rotted in the process is probably going to take priority. But anyway.
Joke's on the house inspector, because when I started ripping up the rotten carpet it turned out there was too underlay, just for some reason it was cut an inch away from the walls.
4. Damp wool carpet smells bad. Rotten wool carpet smells worse. Fungi-ridden carpet smells even worse. But if you want to smell the worst thing of all, that comes when you start ripping it all up.
Yes, I wore a face mask. It was still foul. I think the rotten underlay was even worse than the rotten carpet.
5. The best way to cut through carpet is with a craft knife.
I tried scissors but they didn't seem keen on it and I wasn't keen on dulling my good fabric scissors. But Dad suggested a knife and that cut through both the carpet and the underlay like soft butter.
Bonus discovery: a previous owner appears to have laid the kitchen lino on top of the old kitchen lino. I can only see the edge of the latter, but it appears to be peak 1970s. (Like my carpet, in fact. I actually quite like my carpet, apart from it being threadbare and also now missing a large chunk due to rot and mushrooms. It was good Axminster carpet; not this but very similar to this. --Oh hey, maybe if I can't find any coloured carpet in New Zealand I could simply import some carpet at great expense from the UK, because this one is pretty close to what I'd really like.)
Conclusion: Please let it not rain significantly before I can get the builder to come and figure out what's going on up there. <weep>
 Linguistic sidebar: Is it universal with adjectives to have the absolute ('bad'), the comparative ('worse'), and the superlative ('worst')? I'm wondering because lots of languages have three degrees of distance (Spanish: aquí, allí, allá; Māori: tēnei, tēnā, tērā; even English used to have here, there, yonder) but then there's occasional glorious exceptions like Malagasy which has seven. So now I'm imagining a language with multiple degrees of comparison, kind of like: bad > worse > worser > worst > worstest. Ripping up rotten underlay is the worstest.