Why does the learning tower of Pisa lean?

The Leaning Tower of Pisa is a marble bell tower at Pisa, Italy. Even though the tower has been leaning since it was under construction, it has stood for hundreds of years, despite the fact that it looks as if it will fall any minute. The tower was begun over 800 years ago, in 1174, and completed in 1350. After the first three of its eight storeys were built, the ground started to sink, and the tower began to lean. It has continued leaning-a millimetre each year-and now leans 14 feet out of line. Still it does not stop to the thousands of tourists each year from climbing its 300 winding steps to the bell tower to view Pisa and the surrounding countryside.