CANCUN, Mexico — What's a beach resort without a beach?

It's a question that concerns people here in one of the world's biggest tourist destinations, a city locked in a 30-year-long struggle to keep its coastline from washing into the sea.

Last month, workers wrapped up the first part of a gigantic project to restore Cancun's beach after Hurricane Wilma blasted the resort last October. They sucked up sand from a site 20 miles offshore, carried it to Cancun on a ship and used pipes to pump the sand onto the shore.

But experts say it's just a matter of time before nature takes the beach away again. Engineers are trying to find a way to permanently protect the beach, perhaps with giant underwater barriers.

At stake is the largest of the famed beach resorts that ring Mexico, an international attraction that dwarfs others like Cabo San Lucas, Rocky Point and Mazatlan. Cancun accounts for one-third of all of Mexico's tourism income, more than $3 billion a year.

Cancun's beach problems are not unique, but long-term beach loss would be a catastrophe because the city also is a jobs center for tens of thousands of Mexicans. Tourism pumped about $11 billion into Mexico in 2005. Officials there already attribute a dip in tourism to beach erosion.

"They have to do something," Delfino Mora Sosa, an employee at the Flamingo Hotel, said as he gazed at the new man-made beach from the hotel's roof. "People come here for the beach. Without that, there's no reason to come."

The soft, cool sand and dazzling turquoise waters are what inspired the Mexican government to turn Cancun Island into the country's first planned resort in 1974.

Back then, Cancun was little more than a sandbar near the town of Puerto Juarez, on the eastern side of the Yucatan Peninsula. The government built causeways, water-treatment plants and a modern airport. Soon, hotels were popping up all along the island.

Cancun now attracts 4.6 million tourists a year, easily outdrawing by more than 3 million visitors the Baja resort area of Los Cabos, which includes Cabo San Lucas.

But Cancun has a special weakness. Unlike resorts like Acapulco or Ixtapa, which are on the mainland protected by large bays, Cancun's resorts and hotels are on a barrier island, a straight line of sand completely exposed to the Caribbean Sea. Waves keep the sand constantly moving.

Cancun's location puts it in the Caribbean hurricane corridor, and it has been hit several times in the past few decades.

After Hurricane Gilbert raked the island in 1988, a few hotels built concrete walls to try to protect their stretches of the shore. But the walls just amplified the crashing of the waves, making the sand erode even faster, a study by the Mexican Committee for Ocean Resource Engineering said.

By 2005, Cancun's beach had shrunk from a high of 178 feet in 1973 to 15 feet in some places, making it the worst-eroded beach in Mexico, said Ruben Berrios, an adviser for the Infrastructure Department of Quintana Roo state.

"In some areas, there were nothing but rocks left," he said.

Then Hurricane Wilma struck Cancun with 145-mph winds. When it was over, some hotels were hanging over the water. State and local officials hired a Belgian company, Jan de Nul Dredging, to repair the beach at a cost of $20 million. With its pipe-and-ship system, the company promised to restore about 9,200 cubic meters of sand a day, enough to fill 90 semis.

Engineers say the beach will shrink quickly to an average width of 92 feet, a huge improvement but still only about half of that in the early 1970s.

"What we need is a permanent form of protection and constant maintenance," said Carlos Sanchez, director of oceanography for the Federal Electricity Commission, which led the engineering for the repair project.

Environmentalists are worried that all this earth-moving could affect sea life. But in Cancun, people who depend on the tourist industry are unapologetic. Mexico's tourism industry ranks only behind oil sales and remittances from immigrants as a source of foreign income.

The resort lost about 354,000 visitors last year because of the hurricane, and the city is trying to rebuild as quickly as it can.

"Cancun really had no choice," Mora Sosa said. "You can't have Cancun without a beach."