Is the Leaning Tower of Pisa really falling over?

The Leaning Tower of Pisa is visited by millions of tourists every year. How long will this Italian attraction last? Understanding the tower's future structural integrity requires a look at its past.

Two centuries of on-and-off building interrupted by wars began with the construction of the bell tower within the Piazza del Duomo. Builders of the tower noticed a lean from the first few floors. The main culprit? The area has a high water table.

The builders got creative instead of scrapping the attempt. They tried to correct the tilt by building each floor at an angle, only to have the tower lean even more. Gabriele Fiorentino is a research fellow in the Department of Civil Engineering at the University of Bristol in the United Kingdom.

The tower tilted at 1.6 degrees after it was completed. A hollow cylinder that rose eight stories was the finished structure. It was covered with marble, columns and vaults and was composed of rock fragments and mortar.

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The Italian government took action to protect the landmark when the tower's slant grew to 5.5 degrees. In 1990 it appointed a committee of experts to mitigate the lean, but not eliminate it.

The monument is part of the culture in Italy.

The committee put 600 tons of lead to the base of the tower in 1993 to compensate for the sinking southern side. Even after they added an extra 300 tons to the north side, the rate of tilt continued. The committee tried to remove the ground beneath the north side of the tower with long tubes and drills.

The structure began to move northward as soil was removed. The tower was reduced in lean by 10% and left at a 5-degree slant.

He said it was impossible to know how long the tower will remain. Within the next 300 years, it could tilt back to its previous position, shifting atop the soft soil again. The tower is safe for a few reasons.

The long interruptions to the tower's construction gave the structure time to settle into the soil, fortifying its structure until the next bout of building. The center of mass of the tower is lower to the ground, making it more stable. The tower has survived earthquakes despite its precarious tilt. It has a longer, less destructive natural vibration period, or the time it takes structures to vibrate back and forth, thanks to the soft soil under the tower.

The tower is monitored with instruments that measure factors such as its tilt and water table. Ancient Romans wanted to build monuments that would last.

The original was published on Live Science on July 5, 2011.