The campanile, or freestanding bell tower, of the Duomo Pisa was built in the 12th century by Bonnano Pisano.
Although a masterpiece of architecture, it is mostly famous for its tilt. It rests on highly unstable sandy soil which gives the famous Torre de Pisa its tilt – and its nickname. In fact, you might notice that the baptistry leans the other way.
Every year it shifts another 1-2mm, although in recent history the structure has been girded with steel cables to try to halt its impending collapse. For safety reasons, the interior has been closed to the public. However, the view from outside is just as amazing.