Construction of the tower of Pisa occurred in three stages across 177 years. The tower began to sink after construction had progressed to the second floor in 1178. This was due to a mere three-metre foundation, set in weak, unstable subsoil, a design that was flawed from the beginning.
Construction was subsequently halted for almost a century, because area was almost continually engaged in battles with Genoa, Lucca and Florence. This allowed time for the underlying soil to settle. Otherwise, the tower would almost certainly have toppled.
On January 7, 1990, the tower was closed to the public. The bells were removed to relieve some weight, and anchor cables were added. The final solution to prevent the collapse of the tower was to slightly straighten the tower to a safer angle, by removing soil from underneath the raised end. The tower was straightened by 45 cm. The tower was then reopened to the public on December 15, 2001, and was declared stable for at least another 300 years
Further in May 2008, after the removal of more ground, 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.
All of this just means we have 200 more years to mock tourists pretending to hold up the tower!
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