Why doesn't the Leaning Tower of Pisa fall over during its over 800 years history? To understand their situation, it's helpful to understand why the tower tilted in the first place.
In the 12th century, the wealthy maritime republic of Pisa set about turning its cathedral square into a magnificent landmark. In 1173, construction began on a free-standing bell tower. The engineers and architects of the time were masters of their craft but knew far less about the ground they stood on. Less than five years later, the tower's southern side was already underground. If workers added more weight, the pressure from upper stories would sink the structure and fatally increase the lean. But construction stopped at the fourth story for nearly a century, which allowed the soil to settle.
When construction began again in 1272, workers compensated for the tower's minor tilt by making the next few floors taller on the southern side. But the weight of the extra masonry made that side sink even deeper. By the time they completed the seventh floor and bell chamber, the angle of the tilt was 1.6 degrees.
In 1838, engineers dug a walkway around the base to examine the sunken foundation. But removing the supporting sand only worsened the tilt. In 1935, the Italian Corps of Engineers injected mortar to strengthen the base. However, the mortar wasn't evenly distributed throughout the foundation, resulting in another sudden drop and moving the tower closer to its tipping point.
With exact data and new computing technology, in 1992, the construction team drilled diagonal tunnels to remove 38 cubic meters of soil from under the tower's north end. Then, they temporarily counterbalanced the structure, with 600 tons of lead ingots before anchoring the base with steel cables. More than six centuries after its construction, the tower was finally straightened to a tilt of about four degrees.
Today the tower stands at 55– or 56– meters tall, and it should remain stable for at least 300 years as a monument to the beauty of imperfection.
