Reason why the Leaning Tower of Pisa hasn't toppled over

The Leaning Tower of Pisa is a historical structure that has captured the eyes of thousands of tourists due to its iconic wonky structure.

Standing as a 57-metre (186-feet) tall bell tower, it resides currently adjacent to the Cathedral of Pisa in the Piazza del Duomo in Pisa, Italy,

Despite its lean, the tower has defied the odds of nature and stood firm for centuries. But how?

The tower's story begins in 1173 when it was constructed on unstable, marshy soil, measuring only about 3 metres deep.

The Leaning Tower of Pisa was constructed on uneven soil / Athanasios Gioumpasis/Mondadori Portfolio/Getty Images

Five years into the construction, the unstable soil foundation caused the south side to sink, giving the tower its unique slant.

Construction was paused due to wars between Pisa and Genoa, Lucca, and Florence, allowing time for the soil to settle. This unexpected break was a blessing in disguise as adding any more height to the structure would've caused it to topple over.

When building resumed, architects cleverly adapted their plans.

They decided to construct the south side taller to compensate for the uneven weight distribution, securing the Pisa Tower's stability.

Inside the tower, there is also an uneven amount of steps to level out the weight - six on the south side and four on the north - resulting in a subtle curvature of the structure.

Although, this is barely noticeable to the naked eye.

The construction of Pisa Tower was carried out in three stages. The first stage involved the construction of the tower's base, and the second added the first three floors, arches and columns.

The third stage was the construction of the final three floors, designed with a slightly different inclination to correct the tower's lean.

Architects have compensated for the uneven weight to prevent the tower from toppling over / Athanasios Gioumpasis/Mondadori Portfolio/Getty Images

All in all, the tower took nearly 200 years to build and was finally completed in 1372.

Over the years, engineers have been monitoring the tower closely and discovered it leaned at an angle of 5.44 degrees, which was dangerously close to the breaking point of 5.5 degrees.

They believe this further leaning was due to the soil settling and becoming more compacted underneath the tower.

In recent decades, engineers have used advanced techniques to correct the tilt.

By 2008, engineers managed to pull the tower back up to 3.99 degrees with measures in place to keep it stable over time.

Sensors showed that the motion of the lean had stopped without further human efforts, and engineers expect the tower to remain stable for at least 200 years.