In 1990, the Italian government enlisted top engineers to stabilize Pisa's famous Leaning Tower.
There'd been many attempts to right the tower during its 800 year history, but this team's computer models revealed the urgency of their situation.
They projected the tower would topple if it reached an angle of 5.44 degrees — and it was currently leaning at 5.5.
No one knew how the tower was still standing, but the crisis was clear: they had to solve a problem that stumped centuries of engineers, and they needed to do it fast.
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.
Workers embellished and enlarged the existing church, and added a massive domed baptistry to the plaza.
In 1173, construction began on a free-standing campanile, or bell tower.
The engineers and architects of the time were masters of their craft.
But for all their engineering knowledge, they knew far less about the ground they stood on.
