The world-famous Leaning Tower is the symbol of the city of Pisa. Its construction was begun in 1173 under the direction of Bonnano. After only a few years, when the 3 lower floors were just finished, cause a fundamental break the tower had already a slant to south-east. This is based on the underground, which deform under the weight over time. Subsequently, the building has been suspended for about 100 years. To offset the imbalance, the subsequent 4 floors were built diagonally. Then, the construction had to be interrupted again, until the belfry has been finally finished in 1372. The tower is 54 meters high and has seven bells, that could not ring for al long time because of danger. Recent discoveries show that the tower is located on the edge of a former island next to an ancient port, that has been already shipped in the construction period. Meanwhile, the imbalance of the tower already 4.43 degrees. Citizens and scientists are trying since a long time to find a solution for the problem of the monument. In 1987, the tower, together with the Baptistery and the adjacent Cathedral of the UNESCO World Heritage explained.