The tower of Pisa, popularly leaning tower, is the bell tower of the cathedral of Santa Maria Assunta, in the famous Piazza del Duomo, which is the most famous monument due to the characteristic slope, symbol of the city and iconic symbols of Italy. It is a belfry standing high about 56 meters, built over two centuries, between the twelfth and the fourteenth centuries. The slope is due to a collapse of the underlying soil already in the early stages of construction. The inclination of the building measures 4.8 degrees from the vertical axis. The tower is run by the Opera of Primaziale Pisana, which manages all the monuments of the Piazza del Duomo in Pisa. It has been proposed as one of the seven wonders of the modern world. The work began on August 9, 1173. As was usual with the headlights and the buildings adjacent to the sea in general, the foundations were left to rest for a whole year. Some studies attribute the paternity of the project to the Pisa architect Diotisalvi, who was building the baptistry at the same time. The analogies between the two buildings are in fact many, starting with the kind of foundations. Others suggest Gherardi, while according to Vasari the work was started by Bonanno Pisano. Vasari's dissertation, which was found to be of no foundation, was considered valid especially after the find in the vicinity of the bell tower of a tombstone named Bonanno, walled in the atrium of the building; In the nineteenth century an epigraphic fragment of pink material was always found, probably a tile on which a metal plate was fused, which is located on the jamb of the entrance to the building. On this fragment is obviously inverted: "Pisan citizen named Bonanno". Such a shoe was probably related to the Duomo's port, destroyed during the fire of 1595. The first phase of the work was interrupted in the middle of the third floor, due to the collapse of the ground on which the base of the bell tower rises. The yield of soil, made up of soft clay that is normally consolidated, is the cause of the slope of the tower and, to a lesser extent, of all the buildings in the square. The works were restored in 1275 under the guidance of Giovanni di Simone and Giovanni Pisano, adding to the previous building three more floors. In an attempt to straighten the tower, the three planes added tend to curve in the opposite direction to the slope. The bell tower was completed in the middle of the next century, adding the bell tower. The structure of the bell tower incorporates two rooms. One at the base of the tower, known as the Fish Room, due to a bas-relief depicting a fish. This hall has no ceiling, being in fact the cable of the tower. The other is the bell ring, the seventh ring. Delimited by the walls of the upper walkway, it is also open to the open and in the center, through an opening, you can see the ground floor of the tower. There are also three stairs ramps: one uninterrupted from the base to the sixth ring, where you exit the outside; one, the smallest spiral that leads from the sixth ring to the seventh; finally an even smaller, always spiral, which leads from the seventh ring to the summit. The bells are playing before the duo and at noon by means of an electro-robotic system. In ancient times every bell was used at a time of liturgical day. For example, Pasquereccia played for Easter, the Third in the third time of the day (nine in the morning), Vespruccio's bell at vespers' hours (six in the afternoon). The bell of San Ranieri, originally called "Justice", was in the homonymous palace. He used to play for the traitors' death and, supposedly, he also played for the death of Count Ugolino. It was brought to the bell tower in the 15th century to replace the original Pasquareccia and later rebuilt in 1606.