As soon as I got home, I went downstairs to check on it.
The extension is flooded; carpet is soaked through on one side, and lifting it up where it wasn't soaked, I could see at least half an inch of standing water under it. The carpet is basically floating on top of water. We'd noticed a few days ago that the extention had a water problem, so this was bad but not astonishing.
Then I went into the rest of the basement.
From the entrance to the rear of the house, north to south, there is a line a couple of yards wide of soaked carpet. Underneath almost all of the rest of the carpet, there's water. Almost the entire basement is flooded, about an inch deep in places. A few spots are still dry. There's so much stuff still packed down there that it's hard to tell the exact extent of the damage. But it's a safe bet that anything in contact with the floor is going to suffer water damage.
On the bright side, the stuff in boxes is stuff that we didn't need for three months, so it's not like these are our most prized possessions.
Lut and I did a bit of bailing, but it's hard even to bail, with the carpet soaking up so much water. And there are large heavy pieces of furniture occupying much of floor that we have nowhere dry we could move it. I suppose we could shift all the remaining boxes off the floor and upstairs. I don't know that it's worth it.
It's still raining, so the problem is going to get worse. Some of the water will go out through the sump pump, so there's a probable limit to how much worse it will get.
Lut's suggesting getting plastic sheeting to put under the furniture, and keep it out of contact with the floor. I guess that's better than sticking my head in the sand and hoping it will go away on its own.
In any event, one thing's for sure: I'll be calling a professional for help next. I've already contacted my agent about what recourse is available with regards to the sellers.
In the meantime ... guess I'll go buy some plastic sheeting.