One of the upstairs toilets soon developed a leak. We fixed it. The other upstairs toilet developed a leak. We fixed it.
A wise man would have inspected the downstairs toilet some seven years ago.
So when we noticed the toilet was seriously leaking and wobbling back and forth when we sat on it, we decided to pull the bowl and take a look (with help from our friend Sam).
It's been leaking into the floor for a long time. The floor is going to have to be rebuilt, because the floorboard is completely rotten.