The construction of this imposing mass was started in the year 1174 by Bonanno Pisano. When the tower had reached its third floor the works ceased because it had started sinking into the ground. The tower remained thus for 90 years. It was completed by Giovanni di Simone, Tommano Simone (son of Andreo Pisano), crowned the tower with the belfry at half of 14th century. Unfortunately, even today the great mass continues to sink very slowly. It is a question of about 1 mm. every year.