It took 177 years to build the Leaning Tower of Pisa. The construction of the tower began in 1173 A.D., and it began to sink in 1178 A.D. The tower was closed to the public in 1990 for fear of collapsing; however, it was reopened in 2001.
The tower's lean is due in part by an unstable sub-soil and a shallow foundation; however, a restoration team was employed in 1999 that took 11 years to balance the tower and manage its dangerous tilt.
The tower's lean was corrected to a 3.99 degree angle from its previous 5.5 degree angle. The Leaning Tower of Pisa was announced as a UNESCO World Heritage site in 1987.