Construction of the Tower of Pisa began in 1173     1     Pisa was a center of power and wealth. But work was stopped several times because of wars. The eight-story tower was not completed until the late 1200s. The tower itself is made of marble (大理石) and     2     (weigh) 16,000 tons. However, the leaning of the tower was not for its weights     3     for the soil underneath. The ground is     4     (soft) on one side than on the other. The south side has sunk 6.1 feet below ground level in the past centuries. The north side has sunk 12.3 feet!