The Leaning Tower of Pisa Was Never Straight and the Builders Knew It

Most people’s mental model of the Leaning Tower of Pisa goes something like this: somebody built a tower, it went sideways, whoops, now it’s a tourist attraction. That’s the story. The actual story is that builders noticed the lean when they were only a few stories up, spent 200 years trying to fix it while continuing to build the thing, failed to fix it, built it crooked on purpose to compensate, accidentally stabilized it twice by starting wars and losing them, and then in 1990, almost watched the whole thing fall over. The Italian government had to close it, remove the bells, and spend eleven years doing emergency engineering before they could let tourists back in. Also the tower is not just tilted. It curves. Nobody tells you that part.

The Ground Underneath Pisa Is the Villain Here

The name Pisa probably comes from a Greek word meaning marshy land, which tells you something. The city sits between two rivers on the Tuscan coast, and the ground is a mix of clay, fine sand, and shells that has no interest in supporting heavy marble buildings.

What makes it specifically bad for the tower is that the soil is not even. The north side of the site has more sand. The south side has more clay. Clay compresses under load in a way that sand does not. So when builders laid a foundation on this asymmetric ground and started stacking marble on top of it, the south side sank more than the north side. This is not bad luck or poor engineering. It is just what clay does.

Construction started in 1173, part of Pisa’s grand Square of Miracles cathedral complex. The plan was an eight-story marble bell tower, about 185 feet tall, to show everyone what a wealthy and powerful city Pisa was becoming after sacking Palermo. By 1178, five years in, they had finished the third story. The tower was already visibly tilting south. Construction stopped.

Two Wars Accidentally Kept It Standing

Here is the thing about those construction stops: they saved the tower. When builders halted in 1178, Pisa was going to war with neighboring city-states. The tower sat unfinished for nearly a century. During that time, the marshy ground under the foundation kept slowly compressing under the weight of the existing structure, settling and stabilizing in a way it would never have had time to do if construction had continued straight through.

When work resumed around 1272, the lead engineer Giovanni di Simone understood the lean had to be addressed. His solution was to build the next stories slightly taller on the south side, the low side, to try to pull the axis back toward vertical. Reasonable idea. The problem was that adding extra masonry to the south side added extra weight to the south side, which was already sinking. The lean continued regardless.

In 1278, construction halted again when Pisa was defeated at the Battle of Meloria. More ground stabilization. More accidental rescue. Another century of soil consolidating. The belfry at the top was finally added in 1372. Nearly 200 years after laying the first stone, the tower was done. And it was bent.

It Is Actually Curved

This is the part that I find genuinely hard to picture until someone explains it clearly. The Leaning Tower of Pisa is not tilted in one consistent direction from bottom to top. Because each construction phase tried to correct for the previous lean by making one side taller, the axis of the tower shifts slightly with each phase.

The lower stories lean south. The middle stories, built to compensate, curve slightly back toward vertical. The belfry at the top leans in yet another slightly different direction from the floors beneath it. On the south side of the belfry, there are six steps down to the main structure. On the north side, only four. That two-step difference is centuries of builders trying to undo each other’s corrections, frozen into the stone.

If you look at the tower carefully from the right angle, you can see it is not a straight lean but a gentle banana curve. It is subtle, but it is there. The tower bends as it rises.

The Near-Collapse Nobody Talks About

When the tower finished in 1372, it had a lean of about 1.6 degrees. For the next 600 years, the clay underneath kept slowly compressing. The lean kept increasing.

By 1817, two British architects surveying the tower measured it at around 5 degrees. By 1990, it had reached 5.5 degrees, which put the top of the tower about 15 feet from where it would be if vertical. Engineers ran the numbers and concluded the tower was approaching the point at which it would topple.

Italy closed it to visitors in 1990. Removed the bells. Appointed an international committee of engineers and architects to figure out what to do. The committee’s problem was that they could not straighten it too much. The lean is the reason anyone visits Pisa. Fix it completely and you have just fixed a mediocre bell tower in Tuscany.

So they had to reduce the lean just enough to keep it standing without making it boring. They added 600 tons of lead counterweights to the north side. They drilled into the ground on the north side and slowly extracted soil, letting the north foundation sink slightly while the south side relatively rose. Over eleven years of careful, nerve-wracking adjustments, they pulled the lean back to 3.97 degrees, roughly where it stood in the early 1800s.

The tower reopened in 2001. By 2008, sensors showed it had stopped moving entirely. Engineers now estimate it will remain stable for at least 200 more years.