Any child who has stacked blocks on a soft carpet knows, though, that sooner or later you add one block too many. At Pisa, the belfry was one block too many. The tower had already sunk 10 feet into the soil, according to Burland's calculations, but the belfry caused it to sink another few inches, which quickly caused a big jump in its tilt, to about four degrees. Today that shallow layer has become the seat of the tower's problem, Burland believes. Analyzing data collected by previous committees, he has found that the tower as a whole, even as its tilt continued to increase, had stopped sinking in the 20th century, apparently because the Pancone Clay strengthened again.
Every little nudge moves the tower closer to that fate.
Give it a nudge and it does not come back, as it would if it were truly stable, like a ball at the bottom of a bowl. The tower jumped half a degree south. More recently, the gradual increase in the tilt has been caused, Burland thinks, by groundwater rising up under the base of the tower during the annual rainy season. For some reason it pushes up on the north side of the tower more than on the south. The tower ratchets in one direction, and it never comes back.