When construction of this campanile began in 1173, Pisa was a trading center at the peak of its military might and artistic achievement. Much to the embarrassment of Pisans, however, their white marble tower began to tilt even before its third story was finished in 1274.

Perhaps engineer Bonanno Pisano failed to consider the consequences of designing a 185-foot-tall tower with a stone foundation only about ten feet thick. And this thin base rests on soft sand, rubble, and clay -- not firm underpinnings for an almost 16,000-ton tower. The structure continued to settle unevenly.

Nonetheless, construction continued. To make up for the tilt, builders made each new tier a little taller on the short side -- but the additional stone only made the tower sink more. Upon completion in 1350, the tower was leaning a full 4 feet, 7 inches from vertical.

Up in the belfry, the weight of the bells caused the structure to tilt even further. By the late 20th century the tower was leaning more t ...more