Work on the bell tower in Pisa began in 1173, on ground that turned out to be soft clay. Around 1178, with three storeys up, it had already started to lean, and then the city went to war and the builders went away. They came back almost a hundred years later. By then the clay had been pressed down under the weight and had firmed up enough to carry more. Engineers who have studied it think that if the work had gone on without a break, the tower would have fallen over before it was finished.

The second crew couldn’t straighten it, so they made the storeys taller on one side, trying to bring the top back over the middle. That’s why the tower isn’t straight but curves slightly, like a banana. Then there was another war and another long stop, and the bells went up in 1372, about two hundred years after the start.

What I like is that the interruptions did the real work. Nobody planned to wait a century. The wars that stopped the building were what saved it. When it was finally made safe, in 2001, the engineers did it slowly too: they dug soil out from under the high side, a little at a time, over about ten years.