Venice Is Saved! Woe Is Venice.

Jason Horowitz and

Jason Horowitz and Emma Bubola reported from Venice over years of floods, visiting MOSE’s headquarters and speaking with numerous officials and experts.

In the middle of the night, as the tide rose, winds whipped and waves grew, an engineer in a command center on an artificial island on the rim of the Venice lagoon clicked an arrow on his screen reading, “Lift.”

Deep underwater, at the four mouths where the lagoon meets the sea, 78 giant walls fastened to the seafloor with hinges emptied themselves of water, filled with air and rose to the surface, where they held back the swelling sea like a defensive line of floating yellow Legos.

Over the long November night, the city’s high-water forecasters drank coffee in an office by the Rialto Bridge, watching live feeds of 20-foot waves crashing on the other side of the walls. Eventually, the sea level outside the walls reached more than five and a half feet — the third highest in more than a century of records, a level that would normally risk lives, strand Venetians and tourists, and drown the economy.

Not this time. The city was drenched with rain, but hardly a drop of seawater. Children wheeled book bags to school. Venetians read newspapers on water buses that ran smoothly in placid canals. Storekeepers put away their water pumps. MOSE, an Italian acronym for Experimental Electromechanical Module, evoking the biblical Moses, had parted the waters and saved the city.

“Without the walls, it would be a disaster,” said Alvise Papa, the director of the tide forecast center, who grew up rescuing merchandise from his father’s hat shop when high water shot up like fountains through cracks in the floor. “Instead, it’s normal life. Let’s thank the god of MOSE.”

But even as Italy now hails its against-all-odds success, MOSE’s story — 50 years in the making — and Venice’s — some 1,500 — are still being written. MOSE has already become much more than an engineering project. It came to embody Italy’s ambition and technical ingenuity, but also its political instability, bad governance, bureaucracy, corruption, debt and defeatism as delays mounted.

Now, though celebrated as the city’s sentinel, it may yet stand as a monument to the inexorable nature of climate change and the futility of man’s efforts to stop it. MOSE’s walls, costing 5 billion euros, about $5.3 billion, took so long to come together that the pace of climate change is already outstripping the projections they were built to withstand.

After all of the effort to get the barriers up, the future challenge will be finding ways to keep them down. Venice is already using MOSE more than expected, and faces the prospect of needing it much more than it had ever imagined against rising seas, so often that it would threaten to seal the city from the waters that are its lifeblood.

Its incessant deployment, experts warn, could render Venice’s lagoon a fetid swamp choked by noxious algae, turning the city’s charming canals into stinking open sewers.

Yet if the waters are not held at bay, there is little doubt that Venice will eventually be submerged and uninhabitable, its sublime palaces and churches eroded by the sea’s salt, its history essentially washed away.

Today, Venice is safe, but it is staring at a future of excruciating trade-offs, with the sea level so high so often that the city will require constant protection.

“At that point, I must decide,” Mr. Papa said. “Do I save the city or do I save the lagoon?”

Venice exists because of and in spite of the sea. Since its founding, water has both protected and threatened it. Venetians have always struggled to keep a balance between the two.

When refugees from the Italian mainland first settled on the mud flats and islets here in the fifth century, they built foundations with wooden piles in the sediment. They erected sea walls in white Istrian stone, impermeable to salt. They manipulated the lagoon to fit their needs.

Their ingenuity built the Republic of Venice into a rich and strong maritime power. Its first doges, or leaders, sailed to the spot where the lagoon meets the sea and tossed a gold ring off a ship to signify Venice’s marriage to the sea.

Over centuries, new trade routes in the Americas and the rise of Napoleon stripped Venice of its geopolitical importance. Its power ebbed. But the water did not.

Venice, once an example of man’s dominion over nature, became known as a drowning place. Venice “sinks, like a seaweed, into whence she rose,” the Romantic poet Lord Byron wrote. Thomas Mann made the city a metaphor for decay in “Death in Venice.”

In 1897, Venice began taking the measure of its enemy, establishing a reference mark for high water at the Punta della Salute entrance of the Grand Canal. In the first two decades of the 20th century, Venice had high tides above 110 centimeters, about three feet and seven inches, only six times.

But the average sea level in Venice has risen nearly a foot since 1900. In the past 20 years, tides have exceeded 110 centimeters more than 150 times.

But it is not just that the seas are rising. Venice is sinking. The tectonic plates under the city are naturally settling, a process accelerated in the 20th century by the pumping of groundwater for use in the industrial port of neighboring Marghera.

From 1950 to 1970, Venice sank nearly five inches. The pumping has long stopped, but Venice still sinks about two millimeters a year.

In November 1966, a fatal flood of more than six feet hit, the worst yet measured. Water paralyzed Venice, destroying buildings and the already fragile sense of the city as a secure place.

Italy was confronted with a terrible question: Could Venice be saved?

Acknowledging “general sea level rise,” Italy’s National Research Council held a competition in 1970 for companies to come up with proposals on how to rescue the city.

Ideally, it wanted walls that could open and close to stop high water, while also allowing ships to pass and maintaining the natural exchange of waters between the sea and the lagoon.

Riva Calzoni, the Milan firm behind the winning idea, sketched sea walls that filled with air and floated up to meet the high tides, and then filled back with water to lower again, a secure but nearly invisible defense that would cost less to maintain than a fixed, exposed structure.

But if the idea of MOSE was elegant in its simplicity, the reality was more complicated. The project would accompany Italy through the next half-century.

In 1984, the government subcontracted the building of MOSE to a consortium of major Italian companies, and estimated that the walls would be put in place by 1995. It was not until 2003 that Prime Minister Silvio Berlusconi, a proponent of big public works, laid the first stone. The estimate then was that the project would be finished by 2011.

But in November 2010, a panel of experts was still debating which metal should be used on the hinges to lock the submerged walls into the seabed.

In the Palace of the 10 Wise Men at the foot of the Rialto Bridge, officials met in the headquarters of the Magistrate of the Waters, an ancient body overseeing Venice’s life aquatic. Surrounded by portraits of past magistrates going back centuries, some experts rebelled against the political pressure they felt to give approval on technical questions.

“I don’t want to be an accomplice,” Lorenzo Fellin, an electrical engineer on one key panel, recalled saying as he stormed out of a meeting to protest what he considered bullying from the magistrate. Mr. Fellin explained that he had the clear impression that “the idea was that since they had already spent so much public money, the project needed to be completed, whether it worked or not.”

MOSE became a constant source of controversy and doubt. Once the hinges were in place, critics raised concerns about whether they were rusting underground, and whether a physical phenomenon called resonance would break the walls.

Over the years, a culture of secrecy, shady business practices and government corruption seeped into the project. In 2014, Venice prosecutors revealed a scheme to overbill the government and bribe politicians to keep the project, and public money, flowing. They arrested 35 people, including top officials — among them the magistrate.

“The last photo that was put up,” Valerio Volpe, the official who now oversees water-related public works in Venice, said as he pointed at a picture in the portrait-lined boardroom. “Because, unfortunately, he was arrested.”

Afterward, from 2014 to 2018, public financing dried up as the state, loath to enable more graft, examined expenditures with extreme caution. Many businesses involved in the scandal folded.

The project was nearly an orphan — scorned even by the engineer who designed it and was known as the father of MOSE, Alberto Scotti. “I’ve rejected my paternity,” he said over maintenance delays.

On the night of Nov. 12, 2019, a sharp drop in temperature caused what Mr. Papa, the head forecaster, described as never-before-seen “anomalous tropical cyclone.”

“The wind went crazy,” he said.

At its height, the tide hit more than six feet and flooded more than 85 percent of the city, killing two people and causing untold damage. In the five-star Gritti Palace hotel, water rose from the floor, soaking sofas, chairs and carpets. The storm swallowed a newsstand.

“It wasn’t here anymore,” said Walter Mutti, a news agent. “The water carried it all away.”

Days later, the water came again, reaching more than five feet, itself one of the worst levels in decades.

The border between the canals and the sidewalks dissolved. The lagoon spilled into the city. Wooden pikes floated up next to store windows. Long sirens whined. Schools were closed and restaurants shuttered. Tourists by the Rialto Bridge huddled around their suitcases on wooden planks, looking like climate-change refugees.

St. Mark’s Square was a vast, deep pool. Water reached the ribs of wading politicians and the lips of tall garbage cans. Standing in the square, you could feel currents. The whole city was part of the lagoon now.

“We had to escape,” said Enrico Pinzan, a mosaic restorer in St. Mark’s Basilica, who had run down to grab a precious crucifix from the crypt, where water had breached an outside wall and gushed through the windows. He and others tried to hold it back with sacks. But the water was too strong and started shooting through the bricks.

This was it, the big one that MOSE had been designed to stop. Engineers at the time said it was ready. But it stood down.

The failure to stop the great floods brought political pressure, international scrutiny and uncomfortable introspection to Venice and all Italy. A change had to be made.

In the days after the flood, Mirco Angiolin, the site manager at the sea wall’s command center, lamented that the walls were ready but that no one was in charge to say, “I take responsibility,” and to activate MOSE when it was needed.

“We need a chief,” he said.

Rome accelerated the appointment of Elisabetta Spitz, a top public-sector manager, as MOSE’s overseer.

She said she “made the decision” on Oct. 3, 2020, to lift the walls, not to answer a crisis but as a simple test. With relatively little fanfare, the walls went up as Italy’s attention was absorbed not in beating back the sea but in breaking the wave of the coronavirus contagion.

They worked. Engineers popped Prosecco.

Since then, Venice has been protected from high-water events, but the parts of the city that flood at lower levels remain precariously exposed.

The experts who had conceived MOSE estimated that the sea walls would need to be raised an average of five times a year to stop tides of about three feet, seven inches. Since MOSE began functioning about two years ago, the walls have already been raised 49 times.

On Monday, the Intergovernmental Panel on Climate Change, an authoritative body of experts convened by the United Nations, said the Earth was likely to cross a critical threshold for global warming within the next decade. According to their best-estimate, the sea level in Venice could rise by nearly two and a half feet by the end of the century, if emissions are high.

At that rate, experts say the walls would need to be up more often than they were down. Combine that with the increasingly common violent winds and record rainfalls that push more water into the lagoon, and the walls may need to be raised nearly constantly, turning the decision on its head.

“You would be talking about opening the lagoon — not closing the lagoon,” said Georg Umgiesser, a scientist based in Venice at the Marine Institute of Italy’s National Research Council.

Luigi D’Alpaos, a professor emeritus of hydraulics at the University of Padova and a staunch critic of the project, said in his office decorated with maps of the lagoon that the constantly raised walls would turn the lagoon into “the swamp of Venice.”

Fishermen and other shipping interests have already expressed concern about the walls, but officials supportive of MOSE point out that navigable locks are in the works for boats to pass when the barriers are up.

Defenders of the sea walls expressed frustration with doomsday predictions about their effect in a century’s time, especially since MOSE has now left Venice better defended than many other coastal cities.

“In 50 years, in 100 years, what do we know what the lagoon will be like?” said Mr. Scotti, the project’s head engineer. Or for that matter, he added, what technology there will be.

Already, instead of using MOSE to protect the lowest-lying parts of the city, like St. Mark’s Square and its magnificent 11th-century basilica, local and national authorities are spending millions of euros to dam and raise vulnerable areas.

Luigi Brugnaro, the mayor of Venice, has asked the government for another €1.5 billion over 10 years to help protect the city.

In November, Italy and Venice’s top officials inaugurated glass barriers — which one called a “mini MOSE” — around St. Mark’s Basilica and its shimmering gold mosaics. Venice’s patriarch splashed the transparent barriers with holy water.

For now, Italians are celebrating their success — for Venice, for their country, perhaps even for the world.

“There’s the plan of exporting MOSE as an international model,” said Ms. Spitz, the manager. Selling intellectual property to other cities threatened by rising seas, she argued, could also help pay for MOSE’s enormous upkeep, estimated at €63 million a year, in addition to the $200,000 in energy and labor costs every time the sea walls are raised.

Mr. Brugnaro, the mayor, said that New York City officials had contacted Venice because they feared similar high water and “they want to understand how we did it.”

Since MOSE started working, real estate prices for once waterlogged ground-floor apartments, which were banned as far back as in the Republic of Venice, have gone up.

They were “perfect,” Ms. Spitz said, “for a bed-and-breakfast.”

All of which raises a question: Which Venice is MOSE saving?

The city, once dense with energy, creativity and industry — and Venetians — is now largely abandoned by residents, becoming a floating and brocaded theme park. It is filled with incalculable treasures, but ever more lacking in real life. Its addiction to tourism has become emblematic of Italy’s transformation from a place that made big things to an Instagrammable paradise.

That is now a threat that Venice has the luxury to contemplate another day: It has, for now, won its survival from the rising sea. On the morning of the third highest tide level ever recorded, a palpable relief spread through the city as the water stayed out of the lagoon and in the canals.

Managers of frequently flooded cafes talked about a “transformation,” tour guides explained to Americans in ponchos that “if it were not for MOSE,” the city would be flooded, and Lucia Montan stepped off the Rialto Bridge carrying a tote bag adorned with a graphic of the yellow sea walls. “It’s a wonderful feeling,” she said. “Finally we’re safe.”

In the forecasting center, messages came in from the mayor’s spokesman mocking all the people who had protested against MOSE. Mr. Papa’s colleagues answered a hotline and listened to elderly and disabled Venetians asking whether the record-high water would flood their homes.

“The MOSE is up,” a worker, Alessandro Tosoni, responded. “There’s no problem.”

Produced by Mona Boshnaq.