Completed in 1370, the tower would have been an architectural marvel even without the lean. However, it was built on clay soil that subsided during construction so it had a tilt even when it was completed.
The tower did not move for nearly 500 years but nearby excavation work in the 1830s destabilized the base and began to lean a little more and more as the years went by. Finally, the Italian government took action in the late 1980s.
The $27 million effort consisted of sinking the ground on the high side of the tower and basically using steel cables to hold it in place while it was stabilized. The tower is now open for tours and engineers believe it will stay in place for a couple of hundred more years.