The Leaning Tower of Pisa has straightened itself by 1.6 inches

The Leaning Tower of Pisa is one of the most popular tourist attractions in Italy, having over six million visitors every year, making around €21 million (£18.5m; $22.3m) for the local economy.

A photo opportunity in front of the tower is a popular one, whether you pose as if you are resting up against it or holding it up.

But while it still may be a huge attraction for millions of tourists, the tower's tilt has been a cause for concern for engineers and historians for decades.

However, thanks to stabilisation works, the iconic tower is slowly but steadily moving upright.

But how did it get its tilt in the first place?

Massimo Dallaglio / Alamy Stock Photo

Well, when work started on building the clock tower back in 1173, just five years later it began to tilt.

Workers noticed the shift while building the second floor, which was caused by unstable subsoil.

To compensate, workers built upper floors with one side shorter than the other - meaning the structure was curved as well as tilted.

In 1990, the tower was still tilting southwards, which led to some worrying that the tower might collapse entirely - so closed it to the public.

To fix this, engineers decided to try out 'soil extraction' - which involved digging out two lorry loads of earth from under the tower's north side.

They then used steel cables to pull it upright, which was effective.

The project was completed in 2001 after straightening the tower by 15 inches, and has continued to do so ever since.

In the 21 years since, the tower has straightened itself by another 1.6 inches, as revealed by a recent survey.

Image Professionals GmbH / Alamy Stock Photo

The recent study which was funded by preservation organisation Opera Primaziale della Pisana (OPA) found that the tilt continues to reduce but still sways at an average of 0.02 inches a year.

Nunziante Squeglia, professor of geotechnics at the University of Pisa, told Italy's National Associated Press Agency: "Although what counts the most is the stability of the bell tower, which is better than expected."

An OPA spokesperson also told the press agency: "Considering it is an 850-year-old patient with a tilt of around five meters and a subsidence of over three meters, the state of health of the Leaning Tower of Pisa is excellent."