The Tower of Pisa is still standing thanks to what tilts it

The Tower of Pisa started leaning almost immediately: construction began in 1173, and by the third floor the subsoil — clay, sand and silt, soft as a sponge — had already given way on one side. It took some two hundred years to finish, with wars and long pauses in between that, incidentally, gave the ground time to settle.

You'd think a crooked tower on spongy soil is the worst possible setup in earthquake country. And the region has been hit by several strong quakes since the Middle Ages. The tower has survived every one of them.

In 2018, a team of sixteen engineers finally explained why: the combination of a tall, stiff tower and that very soft ground means the two never vibrate in step with an earthquake. The soil cushions the shaking and the tower simply doesn't resonate. It's a textbook case — perhaps the most extreme in the world — of what engineers call soil-structure interaction.

In other words: the same mud that bent it protects it. If Pisa stood on solid foundations, it might have collapsed centuries ago.