Why does the Leaning Tower of Pisa lean, and why hasn't it fallen?

The Leaning Tower of Pisa leans because it was built on ground too soft to hold it. In 1178, when the builders had reached only the second floor, the tower began to sink: its foundation was just three metres deep, in weak, unstable subsoil. The design was flawed from the start.

It hasn't fallen thanks to a strange mix of luck and engineering. First, war. Pisa was fighting Genoa, Lucca and Florence almost constantly, so construction stopped for most of a century. That long pause let the soil settle under the weight, and without it the tower would almost certainly have toppled. When work resumed, builders tried to correct the lean by making the upper floors taller on one side, which is why the tower is actually slightly banana-shaped.

By 1990 the tilt had reached 5.5 degrees, and after a tower collapsed in Pavia, Pisa's was closed. Engineers couldn't touch the ground on the leaning side without risking disaster, so they did the opposite: they gently removed soil from under the high side, letting it sink back. The top moved back 45 centimetres, and the tilt fell to 3.97 degrees. Nobody wanted it straight, though: the lean is what brings the tourists. In 2008 engineers announced that the tower had stopped moving for the first time in its history.