The Leaning Tower of Pisa is the campanile, or freestanding bell tower, of the Pisa Cathedral. It's famous worldwide for its nearly four-degree lean, as the result of an unstable foundation.
The tower began to lean during construction in the 12th century, due to soft ground which could not properly support the structure's weight, and it worsened through the completion of construction in the 14th century. By 1990, the tilt had reached 51⁄2 degrees. The structure was stabilized by remedial work between 1993 and 2001, which reduced the tilt to 3.97 degrees. (Wikipedia)