Why is the Leaning Tower of Pisa?

Pourquoi la Tour de Pise est penchée

Pisa is world-famous for its leaning tower. But do you know why?

The Tower of Pisa is actually the campanile of the Cathedral of Our Lady of the Assumption. Located in the Piazza dei Miracoli, it is Pisa‘s most famous and most photographed monument. No trip to Tuscany is complete without a visit to this small town and its UNESCO World Heritage square.

The tower, whose foundation stone was laid in 1173, is the main reason why tourists flock to Pisa, of which it is the emblem. Beyond its beautiful Romanesque architecture, the success of this monument is due to its astonishing inclination. Have you ever wondered why the Leaning Tower of Pisa is leaning? Here are some answers.

The construction of the tower in question

The Leaning Tower of Pisa currently leans a little over 3 degrees to the south. Although the angle of inclination has not always been the same, the monument began to lean very soon after it was erected.

Tour de Pise penchée

Photo credit: Flickr – Softeis

In fact, it was the very characteristics of the terrain that tipped the campanile over from the outset. It was built on an alluvial plain, i.e. one made up of alluvium (ancient sediments, debris), a water-rich soil that can lead to land subsidence. It would then be a sedimentary rock known as marl, sensitive to water and conducive to slope instability, which would explain why the Leaning Tower of Pisa is leaning. Other hypotheses point to a faulty foundation.

A tower that never stops moving

The Tower of Pisa was closed to the public in 1990, as it continued to slowly subside. Major work was then undertaken in 1993 to secure the building. Lead blocks on the ground, suspenders to stop the collapse, micro-excavation on the northern part of the foundations – everything was done to save the campanile, which was then thought to be doomed to fall.

Contrepoids en plomb sur la Tour de Pise

The lead counterweights on the Leaning Tower of Pisa in 1998 – Photo credit: Wikimedia – Rolf Gebhardt

The work paid off, as by 2003 the tower had recovered 50 cm of its original inclination, the same as it might have had at the end of the 18th century. More astonishingly, experts noted that between the end of the work and 2013, the campanile had recovered 2.5 cm of its own inclination. It would seem that, after hundreds of years of leaning, the Leaning Tower of Pisa has suddenly decided to straighten up!

This spontaneous movement to straighten the tower is in fact the consequence of the interventions begun in 1993. Either way, it looks as if the Tower of Pisa has not finished moving and making our heads spin!

Main photo credit: Flickr – Andy Hay