The Leaning Tower of Pisa is leaning less and less as the years go by as engineers have been desperately trying to straighten it.

“It’s still straightening,” engineer Roberto Cela said of the tower in Italy. “And many years will have to pass before it stops.”

The tower has leaned ever since 1173, when it was built on ground that was too soft for it. In 1990, the tower was closed to the public for eleven years because of safety fears due to the fact that it’s tilt reached 4.5 meters (15 feet) from the vertical, threatening to turn it into a pile of rubble.

“We installed a number of tubes underground, on the side that the Tower leans away from,” said Cela, technical director at the OPA, which takes care of Pisa’s main monuments. “We removed soil by drilling very carefully. Thanks to this system, we recovered half a degree of lean.”

Engineering lecturer Nunziante Squeglia of Pisa University, who works with the Surveillance Group that was set up after the rescue work, has spent decades studying the tower. He said that it straightened by 41 centimetres (16 inches) until 2001, and another four centimetres since then.

“The tower tends to deform and reduce its lean in the summer, when it’s hot, because the tower leans to the south, so its southern side is warmed, and the stone expands. And by expanding, the tower straightens,” Squeglia said.

“The tower was much more mysterious when I arrived, it wasn’t clear why it was leaning, and increasingly leaning,” he added. “It is a building that has been extensively studied for over 100 years but there are still so many things to know.”

Cela said that he believes the tower “will never be completely straight”.

“When they were building it, there were attempts to straighten it (by adding stone on one side), so it has a slight banana shape,” he added.