The tower of Pisa has been leaning so long -- nearly 840 years -- that it's natural to assume it will defy gravity forever. But the famous structure has been in danger of collapsing almost since its first brick was laid.
It began leaning shortly after construction began in 1173. Builders had only reached the third of the tower's planned eight stories when its foundation began to settle unevenly on soft soil composed of mud, sand and clay. As a result, the structure listed slightly to the north. Laborers tried to compensate by making the columns and arches of the third story on the sinking northern side slightly taller. They then proceeded to the fourth story, only to find themselves out of work when political unrest halted construction.
The tower sat unfinished for nearly 100 years, but it wasn't done moving. Soil under the foundation continued to subside unevenly, and by the time work resumed in 1272, the tower tilted to the south -- the direction it still leans today. Engineers tried to make another adjustment, this time in the fifth story, only to have their work interrupted once again in 1278 with just seven stories completed.
Unfortunately, the building continued to settle, sometimes at an alarming rate. The rate of incline was sharpest during the early part of the 14th century, although this didn't dissuade town officials or the tower designers from moving forward with construction. Finally, between 1360 and 1370, workers finished the project, once again trying to correct the lean by angling the eighth story, with its bell chamber, northward.
By the time Galileo Galilei is said to have dropped a cannonball and a musket ball from the top of the tower in the late 16th century, it had moved about 3 degrees off vertical. Careful monitoring, however, didn't begin until 1911. These measurements revealed a startling reality: The top of the tower was moving at a rate of around 1.2 millimeters (0.05 inches) a year.
In 1935, engineers became worried that excess water under the foundation would weaken the landmark and accelerate its decline. To seal the base of the tower, workers drilled a network of angled holes into the foundation and then filled them with cement grouting mixture. They only made the problem worse. The tower began to lean even more precipitously. They also caused future preservation teams to be more cautious, although several engineers and masons studied the tower, proposed solutions and tried to stabilize the monument with various types of bracing and reinforcement.
None of these measures succeeded, and slowly, over the years, the structure reached an incline of 5.5 degrees. Then, in 1989, a similarly constructed bell tower in Pavia, northern Italy, collapsed suddenly.