One of the world's most easily identified structures, Italy's Leaning Tower of Pisa towers seven stories high and weighs 14,500 tons. The architecture and history of the tower has long been the subject of debate among architects and historians, and many mistakes in its construction have been uncovered.
Those who began work on the tower's foundation in 1173 did not realize that the soil on which they were building was the site of former estuary. This meant the sandy soil was mixed with shells and other sediment and was unstable from the start. After completing just three stories, the workers stopped when the tower began to lean.
Centuries after the tower's completion, in 1838, architect Alessandro della Gherardesca decided to dig a walkway around the base of the tower to make the foundation visible to visitors.
In 1859, builders and architects realized that the excavation of the walkway, or catino, had contributed to an even greater lean. Since the walkway was below the water table on the south side, the digging had created a rush of water that flooded the base of the Romanesque-style tower.
Tower officials were horrified to find that della Gherardesca's excavation of the walkway caused the structure to lean almost half a degree more.
The Current State of the Tower
No real progress was made in preventing the tower's further incline until the 1990s when it was closed to tourists. The government hired architects and engineers to work on saving the structure. Some success resulted from extracting soil beneath the tower, and Italian tourism officials say it is more stable than it has ever been.