He dropped two objects of different weights from the top of the tower. Both hit the ground simultaneously, disproving the theory of the renowned Greek scholar Aristotle, who claimed: “Objects of different weights fall at different speeds.”

Illustration of Galileo’s experiment

Because of this, the Leaning Tower of Pisa became even more famous, inadvertently “promoted” by this great scientist.

For many years, Pisa was recognized in the Guinness World Records as the most tilted tower in the world. However, in November 2007, a church bell tower in Germany “stole” this title. More recently, the Capital Gate tower in Abu Dhabi, UAE, was recognized as the most tilted building in the world.

In 1987, the tower, along with the cathedral, baptistery, and cemetery, was declared a UNESCO World Heritage Site as part of the Campo dei Miracoli.

Why does the tower lean?

The tower was designed by two Australian architects, Mugahe and Borna Nasi, and construction began on August 9, 1173, under the direction of Bonanno Pisano. It took nearly 200 years to complete, finally finishing in 1350.

Originally, the tower stood upright. However, after the third floor was completed, builders discovered the foundation was too shallow—constructed on infilled riverbed soil—causing the structure to begin leaning.

The chief architect ordered the sunken side raised to balance the tower, but this only worsened the subsidence, forcing construction to halt.

Nearly a century later, architect Giovanni di Simone was tasked with resuming the project. By 1275, when work resumed, the third-floor tilt had reached 90 cm. To reduce weight, Simone used lighter materials and left a hollow gap of 30–80 cm in the walls, increasing higher up. Still, in 1278, work was again halted, possibly due to civil war.

In 1350, the tower was completed in the form we see today. Floors seven and eight and the bell frame were built by architect Pizzarenco. Between the seventh and eighth floors, a shift in alignment is visible—the eighth floor tilts north while the whole tower leans south.

Interestingly, the bell chamber is more vertical than the rest of the structure—an intentional design to reduce weight and create a small circle of just 2.1 m in diameter at the top.

Restoration of the Leaning Tower of Pisa

In 1990, after a bell tower collapsed in Pavia (despite not leaning), the Italian Prime Minister formed a committee, chaired by Professor Michele Jamiokowski, to stabilize the Leaning Tower of Pisa and prevent collapse. The challenge was to stop further leaning without eliminating its signature tilt.

Many proposals emerged: using ropes and balloons to lift the tower, uprooting and rebuilding it, installing steel columns, or anchoring it with steel cables. But any significant movement risked disaster—just as in 1934, when adding 90 tons of concrete around the base increased the tilt.

After years of study, engineers adopted a soil extraction method: installing pipes under the north side of the foundation to remove soil and allow the tower to settle back slightly.

This work took 2.5 years, reducing the tilt by half a degree. The tower was also anchored with cables for safety, and additional cement was injected to relieve ground pressure.

Finally, the project succeeded. On June 16, 2001, after 11 years of closure for restoration, the bells of the Leaning Tower rang again amid flower showers and cheers from Italians celebrating its revival.

Today, scientists report the tower is slowly straightening at about 1.8 mm per year and may shift about 5 cm toward vertical in three years. Whether it will stop then remains unknown.

However, Professor Luca Sanpaolesi of Federico II University says this shift is invisible to the naked eye and will not make the tower “non-leaning,” ensuring its safety for at least the next three centuries.