The Leaning Tower of Pisa looks like it could tip over. But it doesn’t. Even though it leans, it has survived several powerful earthquakes. How it stays standing has been a mystery for hundreds of years. Now scientists think they know the reason. Something called dynamic soil-structure interaction, or DSSI, has kept the Tower upright for all these years. DSSI is a combination of the Tower’s height, the stiffness of the marble it’s made of, and the soft soil it’s built on. Together these keep the tower from shaking during earthquakes.