Why does the Leaning Tower of Pisa lean?

Construction began in 1173, and the tower started to tilt after the second floor was added, because the ground beneath is soft clay, sand and shells and the foundation is only about 3 metres deep. Wars halted building for nearly a century, which allowed the soil to settle and probably saved the tower from collapse. Builders tried to compensate by making upper floors slightly taller on the leaning side, giving it a subtle curve. By 1990 the lean had reached about 5.5 degrees, and it was closed for stabilisation work that removed soil from the high side. It now leans about 4 degrees and is considered stable for centuries.