Pisa’s Campanile, the “Leaning Tower of Pisa,” was commenced in 1173. By the time the third story was built, it already had begun to lean significantly. Subsequent layers of marble, built up over a substrate of too-soft ground, were compensated upward, giving the tower a slight banana shape.
Related blog: "On the road: Galileo's Italy, Day Five," by David J. Eicher
Photo credit: David J. Eicher