Why does the Leaning Tower of Pisa lean?

Answer

It began tilting during construction in the 1170s and was later stabilized.

Context

The tower is the bell tower of Pisa's cathedral complex, and work on it began in 1173. Pisa sits on ground made of layered, water-saturated sediments, including soft clays and silts deposited by the nearby Arno River, rather than on solid rock. The foundation is shallow for a structure of this mass, so the weight pressed down unevenly on that compressible soil. The south side sank further than the north, and the tilt appeared as the early stories rose. Construction then paused for long periods, including during wars between Italian city-states, and many historians think those gaps gave the clay time to consolidate and kept the tower from toppling before it was finished.

Builders tried to compensate while the work continued. Upper stories were constructed with one side slightly taller than the other, which is why the tower has a gentle banana-like curve in addition to its overall tilt. The lean grew over the centuries, reaching roughly 5.5 degrees from vertical by the late twentieth century, which is a horizontal offset of several meters at the top. Britannica reports that corrective work reduced the lean to under 4 degrees.

A common misconception is that the architects designed the tower to lean, or that it was a deliberate show of skill. Records and engineering studies point instead to a foundation problem that nobody intended. Another misconception is that the stabilization work in the 1990s and early 2000s made the tower vertical. The aim of the project, which removed soil from beneath the north side, was to reduce the tilt and halt further movement, not to erase it, partly because a perfectly upright tower would no longer be the landmark visitors come to see. Monitoring reported afterward by news outlets suggests the structure has since become slightly more stable.