The Leaning Tower of Pisa Is Leaning a Bit Less These Days

The Leaning Tower of Pisa has long faced a most serious quandary, one even more pronounced in the Instagram Age: How does the world's most iconic cockeyed building maintain its tourist-snaring slant while simultaneously avoiding catastrophic structural failure?

The answer is, ahem, a straightforward one: carefully, patiently and with an assist from the crème de la crème of engineering.

Extensive stabilization efforts that commenced in the early 1990s — and concluded in 2001 — to correct the slouching Tuscan campanile just enough to avoid further sinkage but not so severely as to rob Pisa of its top-rated photo-op status are still holding strong. Thanks to the decade-long rehabilitation project, the tower was straightened by a total of 41 centimeters (16 inches). This may not seem like a lot, but for a structure as old and as imperiled as the Leaning Tower of Pisa, every inch counts.

And here's the kicker now generating headlines: Since reopening in 2001, the slightly less lopsided tower has continued to self-adjust to a more vertical position, shedding an additional 4 centimeters (1.5 inches) of tilt over the past 17 years without any human intervention. Today, Italy's prima architectural mishap remains open and is not at risk of toppling over any time soon, per the team of engineers tasked with monitoring the tower.

A Physics-defying Wonder

Completed in 1372 within Pisa's famed Piazza del Duomo, this freestanding octagonal bell tower built from white marble and limestone in the Romanesque style has pretty much been gravitationally challenged from the get-go.

With its regrettably thin foundation resting atop unstable soil, the 186-foot-tall tower's trademark tilt became apparent in the early stages of the construction process when the third story — of eight total — was added by builders circa 1178.

Still, builders forged ahead under the assumption that the structure would correct itself as time marched on. And time did march on — for another 200 years until the tower was finished. Yet the column-encircled tower, described as resembling a "massive wedding cake knocked precariously askew by a clumsy giant guest," never straightened out, despite the best efforts of subsequent builders.

There's little doubt that the tower's epically drawn-out construction was frustrating for inhabitants of Pisa in the Middle Ages and on — and all to complete a structure seen as so basic, so essential in medieval Europe. Yet continual and lengthy lags in construction, most of them prompted by battles involving the erstwhile maritime powerhouse known as the Republic of Pisa, ultimately benefited the off-kilter tower. With decades passing between construction phases, the soft soil beneath the structure was allowed to settle before more weight was added up top. If completed at a more brisk pace, the tower surely would have crumbled.

"No matter how many calculations we made, the tower should not have been standing at all," John Burland, a professor and expert in soil-mechanics at London's Imperial College, tells Scientific American. "The height and weight coupled with the porous soil meant it should have fallen centuries ago."

The Leaning Tower of Pisa Has Survived Earthquakes

Even more odd than the tower failing to collapse on its own is that it has also stood resilient during a slew of Italian earthquakes, including a couple of major ones. George Mylonakis, a professor in geotechnics who has studied the tower's unlikely longevity, credits a phenomenon called "dynamic soil-structure interaction" for the structure's resilience.

"Ironically, the very same soil that caused the leaning instability and brought the tower to the verge of collapse can be credited for helping it survive these seismic events," Mylonakis explained to The Washington Post earlier this year.

Reversing the Seemingly Irreversible

As years passed without incident, the residents of Pisa became accustomed to and proud of their city's curiously indestructible landmark.

Once an object of embarrassment, the tower morphed into a global tourism hot spot — an imperfect Italian icon that travelers insisted on seeing with their own two eyes, preferably with camera in hand. (Located an hour west of Florence and serving as the capital of a province of the same name, Pisa is a river-straddling treasure trove of well-preserved medieval architecture with rich culture and a top-notch culinary scene ... in other words, there's more to the city than the obvious.)

"Locals used to think of it as an architectural failure, then it was seen as a boon for the city," Gianluca De Felice, general secretary of the nonprofit Opera Primaziale Pisana, tells The New York Times.

Felice's organization is tasked with overseeing the tower along with three other UNESCO World Heritage site-listed religious monuments located at Piazza del Duomo, which is also known as Piazza dei Miracoli (Square of Miracles) and considered sacred by the Catholic Church.

Efforts to protect the tower and prevent it from sagging to the point of collapse began in earnest in the mid-20th century. Some efforts proved to be successful, others made the tower tilt even more.

By 1990, the Leaning Tower of Pisa was at its all-time leaning-est, measuring 5.5 degrees from the perpendicular. Faced with a potential public safety hazard, officials shuttered the tower and temporarily cleared the surrounding area in case everything came crashing down.

It didn't. In 2001, stabilization work on the Leaning Tower of Pisa — with a fresh new slope of 3.97 degrees — was completed. The tower reopened and engineers declared that another intervention to improve the tower's posture wouldn't need to take place for 300 years. And tourists, for the most part, couldn't even tell that the tower was leaning oh-so-slightly less — roughly the same position it had held in the early 1800s, not the 1990s.

A World-Famous Landmark Auto-Corrects

"We rejuvenated the tower by around 200 years," Italian archaeologist and art historian Salvatore Settis relays to the Times. "The good news is that the tower continues to straighten — if slightly."

As mentioned, a dedicated committee of engineers and historians led by Settis have observed that the tower has straightened itself an additional inch-and-a-half since the lean-tweaking overhaul wrapped up nearly two decades ago.

The committee recently reported that the north-leaning tower, currently clad top to bottom in hundreds of sensors measuring a range of phenomena, is in "very good" condition and will likely no longer continue to slowly self-correct.

So how did engineers not only straighten the Leaning Tower of Pisa but get it to a point where the ancient bell tower was able self-adjust to an even more vertical position over a 17-year span?

Mostly, this was achieved through digging, draining and making the 14,500-metric-ton tower less top-heavy by removing its most ponderous bells.

Efforts to Prevent Additional Leaning

Today, officials are careful not overdo it in terms of weight capacity, only permitting visitors in "controlled" groups who have booked in advance to climb the tower's 297 steps to take in the stunning views from the bell chamber.

Per the Times, of the 3 million annual visitors to Piazza del Duomo, only around 400,000 of them ascend to the top of the tower. (To be fair, snapping photos of the tower's exterior is the main event, not necessarily taking in the sweeping views of the city and Tuscan countryside beyond.)

Mostly, however, the tower saving/straightening efforts — headed by Burland of Imperial College — involved securely anchoring the tower before drilling under its raised south foundation and removing a total of 1,342 cubic feet of soil.

Explains Scientific American:

Burland's team painstakingly extracted about 20 liters of soil at a time from under the south side of the base and steadily installed a system of tunnels and wells to drain the water that was keeping the soil wet, causing the base to sink. The reparations raised the base on the north side by four meters and lifted the entire tower along with it. While digging, Burland says they found the remnants of a concrete foundation that had been built in 1828; they attached the tower to it with massive chains, creating even stronger footing.

The 1.5 inches of self-adjustment that followed was due to the soil continuing to settle. Scientific American explains that these not-totally unexpected movements ceased several years ago but the committee opted to wait until the most recent annual measurement was taken to go public. After all, they weren't entirely sure that the Leaning Tower of Pisa had even finished de-leaning.

"We knew those measures would have protracted consequences," Nunziante Squeglia, an engineering professor at the University of Pisa who serves as a consultant to the tower-monitoring committee, tells the Times.

Speaking to Scientific American, Burland explains that if the tower's foundational tilt were to ever be fully corrected through additional stabilization efforts, it would continue to lean given that its upper floors were built on a curve to compensate for its sagging base. "It is like a banana," he says. "The thing was never straight."

And even if the tower's base did somehow continue to naturally straighten itself like it did from 2001 onwards, Squeglia tells the Times that this never-going-to-happen scenario would take at least 4,000 years.

The Tower No Longer the World's Most Tilted

However, it's worth noting that the Leaning Tower of Pisa is no longer the world's most tilt-y tower.

As the Times reported in 2012, several German church towers claim to have the world's most substantial slants, including the tower attached to a church in the northern village of Suurhusen that's positioned at an angle of 5.19 degrees compared to the Pisa tower's current 3.9 degrees. A 12th century church tower in the Swiss ski village of St. Moritz, however, is believed by many to be the true record-holder with an inclination angle of 5.4 degrees. (Since the early 1980s, the sagging structure has received periodic straightening assistance courtesy of hydraulic lifts.)

A handful of modern structures lean at even more dramatic angles, although these buildings were deliberately designed to tilt. This is far from the case with the Leaning Tower of Pisa, a 646-year-old engineering blunder and aberration of the built environment that, by some not-so-small miracle, is still standing.