What caused the tilt of the Leaning Tower of Pisa?

The Leaning Tower of Pisa is the freestanding bell tower of the cathedral of the Italian city of Pisa. Famous for its unintended tilt, the 184 feet tower took nearly 200 years to build. Work started in 1173, and five years later, it started tilting. The tilt was noticeable as workers were making the second floor. The shift was caused by shallow foundations of 9.9 feet and unstable subsoil. Engineers working on the tower in later years built upper floors with one side shorter than the other to compensate. That means the structure is curved as well as tilted. Work was slow and sometimes abandoned for decades as Pisa was often at war with nearby Florence, Genoa, and other city-states. They finally completed the tower in 1372. Engineers believe the tower's future is now secure for at least 200 years.

Source: Institution of Civil Engineers