The Leaning Tower of Pisa is the bell tower of a cathedral complex. During construction, foundation and soil conditions caused it to lean, requiring repeated changes in building and stabilization methods.

The curve of the tower also records attempts to respond to the lean during construction. Modern engineering reduced risks; the inclination was not the designers' intended shape.

Construction

Construction of the tower occurred in three stages over 199 years. On 5 January 1172, Donna Berta di Bernardo, a widow and resident of the house of dell'Opera di Santa Maria, bequeathed sixty soldi to the Opera Campanilis petrarum Sancte Marie. The sum was then used toward the purchase of a few stones which still form the base of the bell tower. On 9 August 1173, the foundations of the tower were laid.

History following construction

Between 1589 and 1592, Galileo Galilei, who lived in Pisa at the time, is said to have dropped two cannonballs of different masses from the tower to demonstrate that their speed of descent was independent of their mass, in keeping with the scientific law of free fall. The primary source for this is the biography Racconto istorico della vita di Galileo Galilei (Historical Account of the Life of Galileo Galilei), written by Galileo's pupil and secretary Vincenzo Viviani in 1654, but published in 1717, long after his death.

During World War II, the Allies suspected that the Germans were using the tower as an observation post. Leon Weckstein, a U.S. Army sergeant sent to confirm the presence of German troops in the tower, was impressed by the beauty of the cathedral and its campanile, and thus refrained from ordering an artillery strike, sparing it from destruction.

Numerous efforts have been made to restore the tower to a vertical orientation, or at least keep it from falling over. Most of these efforts failed; some worsened the tilt. On 27 February 1964, the government of Italy requested aid in preventing the tower from toppling. It was, however, considered important to retain the current tilt, due to the role that this element played in promoting the tourism industry of Pisa.

The tower and the neighbouring cathedral, baptistery, and cemetery are included in the Piazza del Duomo UNESCO World Heritage Site, which was declared in 1987.

The tower was closed to the public on 7 January 1990, after more than two decades of stabilisation studies and spurred by the abrupt collapse of the Civic Tower of Pavia in 1989. The bells were removed to relieve some weight, and cables were cinched around the third level and anchored several hundred meters away, and residences in the path of a potential collapse were vacated. The selected method for preventing the collapse of the tower was to slightly reduce its tilt to a safer angle, by removing 38 cubic metres (1,342 cubic feet) of soil from underneath the raised end. The tower's tilt was reduced by 45 centimetres (17+1⁄2 inches), returning to its 1838 position. After a decade of corrective reconstruction and stabilization efforts, the tower was reopened to the public on 15 December 2001, and was declared stable for at least another 300 years.

In total, 70 metric tons (77 short tons) of soil were removed.

After a phase (1990–2001) of structural strengthening, the tower has been undergoing gradual surface restoration to repair visible damage, mostly corrosion and blackening. These are particularly pronounced due to the tower's age and its exposure to wind and rain. In May 2008, engineers announced that the tower had been stabilized, and had stopped moving for the first time in its history. They stated that it would be stable for at least 200 years.

A ceremony for the 850th anniversary of the laying of the foundation stone was held on 9 August 2023.