To understand why the Leaning Tower of Pisa never toppled, you must look back at its 850 years of history involving jelly-like land, decades of war, and some radical changes of plans.

The Leaning Tower of Pisa is the 57-meter (186-feet) tall bell tower found in the Piazza del Duomo of Pisa Cathedral. 

The jaunty angle of the Leaning Tower of Pisa is no optical illusion. In the early 1990s, it leaned at an angle of 5.44 degrees, approaching the catastrophic threshold of 5.5 degrees that would have almost certainly spelled game-over for the structure. Fortunately, cunning engineering work completed in 2008 managed to crank it back to an angle of 3.99 degrees.

The lean has been a problem for centuries. Construction of the Leaning Tower – which presumably then was just called the Tower of Pisa – began in 1173 CE. After just five years of building work, the marsh-like soil of Pisa started to slowly swallow the foundations of the south side, causing a lean.

If the engineers had simply carried on and added more stories, It’s almost certain the tower would have toppled under its weight. Suddenly though, construction was halted after just a few stories were built because the Republic of Pisa had become engulfed in a flurry of battles with Genoa, Lucca, and Florence. 

It wasn’t until 1272 CE that construction resumed, by which time the soil had slightly consolidated, allowing the builders to go higher and higher once more. As the stories grew, however, architects became aware of the growing tilt and adapted the original plans to make the south side of the tower slightly taller. 

This is evidenced by the different number of steps found on the top story: six on the south, four on the north. It also means that the structure is slightly curved, although it's barely discernible to the naked eye. 

By this time, it was too late. The tower had acquired its iconic lean, causing sleepless nights for town planners for centuries to come. When the 20th century came around, engineers were keen to address the issue, but some of their attempts were more successful than others.

A low-down of some of these efforts can be seen in the wonderfully animated TED-Ed video above.