The Leaning Tower of Pisa is the third structure by time in the Cathedral Square and is situated behind the Cathedral.
The tower presently leans to the southwest at an angle of 3.97 degrees. At the beginning it intended to stand vertically but was impossible due to a poorly laid foundation and loose substrate that has allowed the foundation to shift direction. The height of the tower is 55.86 m from the ground on the lowest side and 56.70 m on the highest side. Its weight is estimated at 14,500 tones.
Two different masses of cannon balls dropped of the tower to demonstrate that their descending speed was independent of their mass. So it’s considered an apocryphal tale because the only source for it comes from Galileo’s secretary.
A work of art that performed in three stages during 177 years, the construction of the first floor of the white marble campanile began on 1173, and is surrounded by pillars, classical capitals, leaning against blind arches. In 1178 the tower began to sink after construction of the third floor, the cause was a mere three-meter foundation, set in weak, unstable subsoil.
Later the construction was subsequently halted for almost a century, the Pisans still engaged in battles with Genoa, Lucca and Florence, which allowed to the underlying soil to settle. In 1198 temporarily clocks was installed on the third floor of the unfinished construction.
In an effort to compensate the tilt, the engineers built higher floors with one side taller than the other, but the tower start to lean in other direction and actually it’s curved.
In 1284 the construction was halted again and in 1319 the seventh floor was completed and the bell-chamber was not added until 1372. Tommaso di Andrea Pisano built the bell-chamber with the Romanesque style of the tower.
There are seven bells, one for each note of the musical scale, and the largest one was installed in 1655.
Benito Mussolini ordered that the tower be returned to a vertical position in 1934, so the concrete was poured into its foundation and the catastrophic result was that the tower actually sank further into the soil.
In 1964, the government of Italy prevents the tower from toppling but was necessary considered to retain the current tilt due to the vital role in the tourism industry of Pisa. On the Azores islands a multinational task force of engineers, mathematicians and historians was assigned to discuss stabilization methods.
The reason was the stonework expanding and contracting each day due to the heat of sunlight. Many methods were proposed but only after over two decades of work on the subject, the tower was closed to the public. The solution was to slightly straighten the tower to a safer angle, by removing 38 m3 of soil from underneath the raised end.
It was straightened by 45 cm to the position that occupied in 1838, and after a decade of corrective reconstruction and stabilization efforts, the tower was reopened to the public in 2001, and it was declared stable for at least another 300 years.