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In 1990, the Italian government [br]enlisted top engineers

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to stabilize Pisa’s famous Leaning Tower.

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There’d been many attempts to right the[br]tower during its 800 year history,

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but this team’s computer models revealed [br]the urgency of their situation.

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They projected the tower would topple if [br]it reached an angle of 5.44 degrees—

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and it was currently leaning at 5.5.

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No one knew how the tower was still [br]standing, but the crisis was clear:

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they had to solve a problem that [br]stumped centuries of engineers,

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and they needed to do it fast.

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To understand their situation,

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it’s helpful to understand why the [br]tower tilted in the first place.

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In the 12th century, the wealthy [br]maritime republic of Pisa

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set about turning its cathedral square [br]into a magnificent landmark.

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Workers embellished and enlarged [br]the existing church,

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and added a massive domed [br]baptistry to the plaza.

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In 1173, construction began on a [br]free-standing campanile, or bell tower.

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The engineers and architects of the [br]time were masters of their craft.

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But for all their engineering knowledge,

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they knew far less about the [br]ground they stood on.

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Pisa’s name comes from a [br]Greek word for “marshy land,"

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which perfectly describes the clay, mud, [br]and wet sand below the city’s surface.

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Ancient Romans counteracted similar [br]conditions with massive stone pillars

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called piles which rest on [br]Earth’s stable bedrock.

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However, the tower’s architects believed [br]a three-meter foundation would suffice

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for their relatively short structure.

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Unfortunately for them, [br]less than five years later,

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the tower’s southern side [br]was already underground.

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Such a shifting foundation would [br]normally have been a fatal flaw.

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If workers added more weight,

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the pressure from upper stories [br]would sink the structure

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and fatally increase the lean.

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But construction halted at the [br]fourth story for nearly a century

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as Pisa descended into prolonged warfare.

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This long pause allowed [br]the soil to settle,

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and when construction [br]began again in 1272,

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the foundation was on [br]slightly more stable footing.

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Under the direction of [br]architect Giovanni di Simone,

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workers compensated for [br]the tower’s minor tilt

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by making the next few floors taller [br]on the southern side.

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But the weight of the extra masonry [br]made that side sink even deeper.

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By the time they completed the seventh [br]floor and bell chamber,

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the angle of the tilt was 1.6 degrees.

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For centuries, engineers tried numerous [br]strategies to address the lean.

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In 1838, they dug a walkway around the [br]base to examine the sunken foundation.

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But removing the supporting sand only [br]worsened the tilt.

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In 1935, the Italian Corps of Engineers [br]injected mortar to strengthen the base.

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However, the mortar wasn’t evenly [br]distributed throughout the foundation,

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resulting in another sudden drop.

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All these failed attempts, along with [br]the ever-sinking foundation,

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moved the tower closer to [br]its tipping point.

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And without definitive knowledge [br]of the soil composition,

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engineers couldn’t pinpoint [br]the tower’s fatal angle

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or devise a way to stop its fall.

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In the years following WWII,

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researchers developed tests to [br]identify those missing variables.

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And in the 1970’s, engineers calculated [br]the curved tower’s center of gravity.

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With this data and new [br]computing technology,

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engineers could model how stiff [br]the soil was, the tower’s trajectory,

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and the exact amount of excavation [br]needed for the tower to remain standing.

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In 1992, the team drilled diagonal tunnels

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to remove 38 cubic meters of soil [br]from under the tower’s north end.

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Then, they temporarily counterbalanced [br]the structure with 600 tons of lead ingots

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before anchoring the base [br]with steel cables.

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More than six centuries [br]after its construction,

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the tower was finally straightened… [br]to a tilt of about four degrees.

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No one wanted the tower to fall,

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but they also didn’t want to lose the [br]landmark’s most famous feature.

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Today the tower stands at [br]55– or 56– meters tall,

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and it should remain stable [br]for at least 300 years

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as a monument to the [br]beauty of imperfection.