Tower of Pisa

The Leaning Tower of Pisa (Italian: Torre pendente di Pisa) or simply The Tower of Pisa (La Torre di Pisa) is the campanile, or freestanding bell tower, of the cathedral of the Italian city of Pisa. It is situated behind the Cathedral and it is the third structure by time in Pisa's Piazza del Duomo (Cathedral Square).

Although intended to stand vertically, the tower began leaning to the southeast soon after the onset of construction in 1173 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 (183.27 ft) from the ground on the lowest side and 56.70 m (186.02 ft) on the highest side. The width of the walls at the base is 4.09 m (13.42 ft) and at the top 2.48 m (8.14 ft). Its weight is estimated at 14,500 tonnes. The tower has 296 or 294 steps; the seventh floor has two fewer steps on the north-facing staircase. The tower leans at an angle of 3.97 degrees^[1]. This means that the top of the tower is 3.9 meters from where it would stand if the tower were perfectly vertical.
is said to have dropped two cannon balls of different masses from the tower to demonstrate that their descending speed was independent of their mass. This is considered an apocryphal tale, and the only source for it comes from Galileo's secretary.^[7]

In 1934 Benito Mussolini ordered that the tower be returned to a vertical position, so concrete was poured into its foundation. However, the result was that the tower actually sank further into the soil.^[8]

During World War II, the Allies discovered that the Nazis were using it as an observation post. A U.S. Army sergeant was briefly entrusted with the fate of the tower. His decision not to call in an artillery strike saved the edifice.^[8]

On February 27, 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 vital role that this element played in promoting the tourism industry of Pisa. ^[9] A multinational task force of engineers, mathematicians and historians was assigned and met on the Azores islands to discuss stabilization methods. It was found that the tilt was increasing due to the stonework expanding and contracting each day due to the heat of sunlight^[10]. This was working in combination with the softer foundations on the lower side. Many methods were proposed to stabilize the tower, including the addition of 800 metric tons of lead counterweights to the raised end of the base.^[11]

On 7 January 1990, after over two decades of work on the subject, the tower was closed to the public. While the tower was closed, the bells were removed to relieve some weight, and cables were cinched around the third level and anchored several hundred meters away. Apartments and houses in the path of the tower were vacated for safety. The final solution to prevent the collapse of the tower was to slightly straighten the tower to a safer angle, by removing 38 m³ of soil from underneath the raised end. The tower was straightened by 18 inches (45 centimetres), returning to the exact position that it occupied in 1838. After a decade of corrective reconstruction and stabilization efforts, the tower was reopened to the public on December 15, 2001, and has been declared stable for at least another 300 years.^[11]

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

In May 2008, after the removal of another 70 tonnes of earth, engineers announced that the Tower had been stabilized such that it had stopped moving for the first time in its history. They stated it would be stable for at least 200 years.