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In 1990, the Italian government 
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
tower during its 800 year history,

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

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They projected the tower would topple if 
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 
standing, but the crisis was clear:

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they had to solve a problem that 
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 
tower tilted in the first place.

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In the 12th century, the wealthy 
maritime republic of Pisa

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set about turning its cathedral square 
into a magnificent landmark.

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Workers embellished and enlarged 
the existing church,

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and added a massive domed 
baptistry to the plaza.

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

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The engineers and architects of the 
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 
ground they stood on.

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

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

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Ancient Romans counteracted similar 
conditions with massive stone pillars

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called piles which rest on 
Earth’s stable bedrock.

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

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for their relatively short structure.

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Unfortunately for them, 
less than five years later,

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the tower’s southern side 
was already underground.

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Such a shifting foundation would 
normally have been a fatal flaw.

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If workers added more weight,

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the pressure from upper stories 
would sink the structure

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and fatally increase the lean.

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But construction halted at the 
fourth story for nearly a century

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as Pisa descended into prolonged warfare.

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This long pause allowed 
the soil to settle,

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and when construction 
began again in 1272,

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the foundation was on 
slightly more stable footing.

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Under the direction of 
architect Giovanni di Simone,

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workers compensated for 
the tower’s minor tilt

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by making the next few floors taller 
on the southern side.

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

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By the time they completed the seventh 
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 
strategies to address the lean.

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

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But removing the supporting sand only 
worsened the tilt.

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

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However, the mortar wasn’t evenly 
distributed throughout the foundation,

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resulting in another sudden drop.

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

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moved the tower closer to 
its tipping point.

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And without definitive knowledge 
of the soil composition,

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engineers couldn’t pinpoint 
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 
identify those missing variables.

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

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With this data and new 
computing technology,

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

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and the exact amount of excavation 
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 
from under the tower’s north end.

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

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before anchoring the base 
with steel cables.

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More than six centuries 
after its construction,

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the tower was finally straightened… 
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 
landmark’s most famous feature.

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

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and it should remain stable 
for at least 300 years

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as a monument to the 
beauty of imperfection.