The Leaning Tower of Pisa survived because its builders kept going to war.
Pisa's bell tower started sinking in 1178, when only two floors were built, on a foundation just three metres deep. Then construction stopped for almost a century while Pisa fought its rivals. That pause let the soil settle; without it, the tower would almost certainly have fallen.
The tower is the freestanding bell tower of Pisa Cathedral. Its foundations were laid in August 1173, and who designed it is still debated: it was long credited to Bonanno Pisano, but a 2001 study pointed to the architect Diotisalvi. The flaw was underground. The tower sat on a foundation only about three metres deep, in weak, unstable subsoil, and by 1178, with construction up to the second floor, it had begun to sink.
Then history intervened. The Republic of Pisa was almost continually at war with Genoa, Lucca and Florence, and work halted for most of a century. That gave the soil time to compress and settle under the load. When building resumed in 1272, the engineers tried to compensate by making the upper floors taller on one side, so the tower is not straight but slightly curved. The seventh floor was finished in 1319 and the bell chamber in 1372, nearly 200 years after the first stone.
The lean kept growing, and many attempts to fix it made it worse. By 1990 it had reached 5.5 degrees. The tower was closed that January, spurred by the sudden collapse of another medieval tower in Pavia. Engineers hung cables around it, added about 870 tonnes of lead counterweights, and then carefully drew out around 38 cubic metres of soil from under the raised side. The tilt fell to just under 4 degrees, back to where it had been in 1838. Nobody wanted it upright: the lean is the attraction.
It reopened in 2001, and in 2008 engineers said it had stopped moving for the first time in its history. The soft soil has one surprising benefit. A 2018 study found the combination of a tall, stiff tower and soft ground means it doesn't resonate with earthquake shaking, which helps explain how it has survived at least four strong quakes since 1280.
Source: Wikipedia — Leaning Tower of Pisa · Text summarised from Wikipedia (CC BY-SA 4.0)
