On December 2001, the Leaning Tower of Pisa was finally reopened to the public, after having been closed for almost a dozen years. During that time, engineers completed a 25 million-dollar renovation project designed to stabilize the tower. They removed 110 tons of dirt and reduced its famous lean by about 16 inches. Why was this necessary? Because the tower has been tilting further and further away from vertical for hundreds of years, to the point that the top of the 185-foot tower was 17 feet further south than the bottom, and Italian authorities were concerned that if nothing was done, it would soon collapse. What was the problem? Bad design? Poor workmanship? An inferior grade of marble? No. The problem was what was underneath. The sandy soil on which the city of Pisa was built was just not stable enough to support a monument of this size. The tower had no firm foundation.
Source: Alan Perkins in “The Wise and Foolish Builders”