Galileo Galilei is said to have dropped two cannon balls of different masses from the tower to demonstrate that their speed of descent was independent of their mass. However, this is considered an apocryphal tale, its only source being Galileo's secretary.

During World War II, the Allies discovered that the Nazis were using the tower as an observation post. A U.S. Army sergeant was briefly entrusted with the fate of the tower and his decision not to call in an artillery strike kept the tower from being destroyed.

Lead counterweightsOn 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.

A multinational task force of engineers, mathematicians and historians gathered on the Azores islands to discuss stabilization methods. It was found that the tilt was increasing in combination with the softer foundations on the lower side. Many methods were proposed to stabilize the tower, including the addition of 800 metric tonnes of lead counterweights to the raised end of the base.

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

On January 7, 1990, after over two decades of stabilization studies, the tower was closed to the public. The bells were removed to relieve some weight, and cables were cinched around the third level and anchored several hundred metres 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 cubic metres (50 cu yd) of soil from underneath the raised end. The tower was straightened by 18 inches (45 centimetres), returning to its 1838 position. After a decade of corrective reconstruction and stabilization efforts, the tower was reopened to the public on December 15, 2001, and was declared stable for at least another 300 years.

In May 2008, after the removal of another 70 metric tons (77 short tons) 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. Two German churches have challenged the tower's status as the world's most lop-sided building: the 15th-century square Leaning Tower of Suurhusen and the 14th century bell tower in the town of Bad Frankenhausen.[20] Guinness World Records measured the Pisa and Suurhusen towers, finding the former's tilt to be 3.97 degrees. In June 2010 Guinness World Records certified the Capital Gate building in Abu Dhabi, UAE as the "World's Furthest Leaning Man-made Tower". The Capital Gate tower has an 18-degree slope, almost five times more than the Pisa Tower; however that tower has been deliberately engineered to slant.