Although the Leaning Tower of Pisa's tilt has made it one of the most recognized structures in the world, the tower's designers never intended for it to lean at all. What's more, the tower's tilted stance, which has worsened steadily since shortly after its construction began, has for centuries threatened its stability. In 1990, officials deemed the tower unsafe and closed it to visitors.

Long before its closure, engineers recognized that something must be done to prevent the tower's collapse. Over several centuries, dozens of attempts were made to stabilize it. Unfortunately, until recently, none of these interventions had any positive effect on the tower's stability -- and some imposed greater risks on the structure than doing nothing at all.

By the late 1990s, according to some mathematical models, the tower was leaning so much that it should already have fallen! Something had to be done. In 1999, engineers began removing soil from underneath the tower's north side in an effort to correct its southward lean. Tests and experience with other towers suggested that extracting soil from the high side of the foundation would cause the structure to settle on that side, moving the entire tower back toward a more vertical stance.

Indeed, in the first few months, the soil extraction technique caused the tower to move to the north about 2.5 cm (one inch). In the ensuing months, by extracting soil at a higher rate, engineers moved the tower another 42.5 cm (17 inches).

By June of 2001, engineers declared the stabilization project complete, even though the tower continued to lean about 4 meters (13 feet) past vertical at its top. Presumably, they could have reversed the lean entirely. Instead, they reduced the lean by just 10 percent, an amount sufficient to stabilize the tower for several hundred years without taking away the building's historic charm. The still-leaning Tower of Pisa was reopened to the public in December 2001.