Why is the Leaning Tower of Pisa leaning and not falling over?
Mainly because of efforts that have been taken to save the structure.
The origins of Italy's famous tipping tower are a bit of a mystery. And—luckily for the person who designed it—so is the architect's name.
The bell tower was constructed over the course of 200 years, starting in 1173. It was originally intended to stand upright, but began tilting early in its construction. It has since both sunk downward and tilted, as a result of the wet soil and decay of the structure.
Various conservation efforts have been made over the centuries, some helping to stabilize the tower and others accidentally making matters worse. With the tilt increasing, the tower was closed to the public in 1990. Bands and counterweights were used temporarily, while soil was removed from underneath the raised side of the tower. By the time the tower reopened in 2001, the tower's tilt was decreased by 17 inches, putting it back to where it had been in 1838. It is now expected to be stable for another 300 years.
—The Fact Monster