When Will the Leaning Tower of Pisa Fall Over?

Photograph of the Leaning Tower of Pisa taken circa 1890-1900. CREDIT: Library of Congress View full size image

Experts say the famous tower at Pisa will lean for at least another 200 years. It may even stay upright well, almost upright forever. That's all thanks to a restoration project, which brought the tower back from the brink of collapse a decade ago.

From the first moment of its construction on unstable subsurface soils in 1173, Pisa's bell tower tilted farther and farther to the south. Its early-onset lean even influenced the way it was built, as its architects tried to compensate by angling the structure northward, resulting in its being banana-shaped.

A few ill-advised construction projects accelerated the Leaning Tower's invisibly slow fall during the past couple of centuries; it tilted 5.5 degrees, its acutest angle ever, in 1990. By all calculations, the tower should have toppled at just 5.44 degrees, but fortunately it defied the predictions of computer models just long enough for engineers to come up with a fix.

Restoration work undertaken from 1999 to 2001 stabilized the tower. Engineers placed weights on the structure's north end, while at the same time extracting soil from below, causing it to slowly sink back in that direction.

The Leaning Tower of Pisa still leans south, but now it does so at just 3.99 degrees. Barring a large earthquake or other unforeseen catastrophe, engineers believe it will stay put for at least a few hundred years.

• How Were the Egyptian Pyramids Built?
• What Do the Empire State Building's Lights Mean?
• Why Are Construction Cranes So Dangerous?

Follow Life's Little Mysteries on Twitter @llmysteries, then join us on Facebook.