Famous for its unintended tilt, the 183.7-foot tower took nearly 200 years to build - work started in 1173. Five years later, it started tilting. The tilt was noticeable as workers built the second floor. The lean was caused by shallow foundations and unstable subsoil. Engineers working on the tower in later years built upper floors with one side shorter than the other to compensate. This means the structure is curved as well as tilted.

Source: Ice Group