Who hasn't heard of the Leaning Tower of Pisa? Architects began building this most famous of medieval campaniles in 1173, but work stopped abruptly five years later when they first noticed a pronounced lean to the north. (It is caused, we now know, by a weak foundation of silty alluvial soil from a former estuary.) Only three stories had gone up, and good thing: If they had built all eight right away, without letting the tower settle, it would likely have collapsed.
Work resumed, and in 1250 architects began trying to adjust the lean, which gave the tower a slight banana shape by the time they reached the eighth and final story. (The campanile is more or less level at about the fifth floor.) By 1272, the tower began leaning toward the south, and before they constructed the bell tower in 1350, architects actually tried to angle the top of the tower back toward the north by adding four steps on the north side and six steps on the south side at the base of the bell tower, thus giving the banana an ever-so-slight S shape.
For centuries, many have offered solutions to how to right the tower. In 1934, engineers working for Italian leader Benito Mussolini, who considered the flawed tower antithetical to Fascist ideals, tried to right it by injecting almost 200 tons of cement into the base. The "fix" actually added a tenth of a degree to the tilt. In the 1950s, officials silenced the seven bells, the largest of which weighs three and a half tons, for fear their vibrations could trigger a collapse. And in 1990, at a time when 700,000 annual visitors were ascending the campanile and the lean increased by one-20th of an inch every year, officials closed Pisa's famous Torre Pendente to the public.
Since then, a special committee of experts have tried several methods to right or at least halt the lean of the 185-foot tower, whose top today lies 16 feet south of the base. They installed sophisticated monitors, which can discern movements at the campanile's apex to within four-ten-thousandths of an inch. These instruments pick up the tower's daily sway of about one-hundredth of an inch, which is caused by the temperature of the sun-facing south side rising by day and falling at night.
The device also picked up a frightening overnight increase in the lean, in what committee members recall as "Black September." On September 6th, 1995, after engineers had added 600 tons of lead ingots to the north side to counteract the southward tilt, the tower jumped one-sixteenth of an inch to the south. In tower terms, that's a lot, and some feared the tower would topple imminently. Within 24 hours, engineers began adding an additional 230 tons of lead, and the movement stopped.
Amazingly, a fall 1997 earthquake centered on nearby Assisi did not exacerbate the lean, but it did succeed in regalvanizing the committee to seek a lasting solution. In late 1998, they employed a method long proposed by John Burland, a soil engineer at Imperial College, London and a member of the committee. They began extracting soil from beneath the north side, hoping the tower would respond by slightly righting itself. The tower obliged, and by June 1999, the team had gained back an inch. The Leaning Tower now leans as it did roughly 30 years ago.