The construction of the Tower of Pisa began in August 1173, and continued for about 200 years due to the onset of a series of wars. It was designed as a circular bell tower, and is constructed of white marble. The tower has eight stories, including the chamber for the bells.
Although it was designed to be perfectly vertical, it started to lean during construction. what the architects failed to realize was that the soil was unstable, and the foundation was insufficient to support the tower.
Each year, the tower leans further a millimeter each year. By 1990, the tower had leaned more than 5 meters out of line.
Engineers have worked to stabilize the foundation of the tower, and were successful in straightening the tower slightly, to prevent the tower from leaning further and toppling over.