But this spring, things appear to be on the up. Travellers have been returning to Cancun by the chartered planeload, driven by cut-price packages available even in the smartest resorts, and the sands are once again there to welcome them. The beaches, trumpets the Cancun Convention and Visitors Bureau (CVB), are back and they’re better than ever. The coast of Cancun has undergone a complex, highly political and remarkably long-winded beach recovery project costing $71m (£46.7m). From my vantage point on the boardwalk of Playa Delfines, a rare undeveloped patch of beach on the western tip of Cancun’s tightly packed hotel strip, things certainly looked a million dollars: surf-like champagne, water of an impossible, iridescent turquoise and a fine powder of white calcium carbonate that seems reduced in brilliance to go merely by the name sand.

I hadn’t been in Cancun for 15 years. Downtown, previously no more than a couple of blocks of largely residential buildings, is now a sprawling administrative centre, while the “Zona Hotelera” has an almost uninterrupted terrace of high-rise tourist addresses along its beachfront. But the beach itself, all 6.8 miles of it, looked just as idyllic as I remembered. “A Japanese tourist asked me if we dyed the water,” said Erika Mitzunaga Magana from Cancun CVB, as we stood surveying the beach, newly loaded with eight million cubic metres of sand, sloping gently down to that impossibly blue water. A fantastical idea? Not when you consider that the resort of Cancun is an entirely preplanned and man-made destination. The water is without embellishment, as is (or was) the sand. The rest? Legend has it that Cancun was selected for development by computer, then planned and executed equally as systematically. Almost, but not quite.

In the late 1960s, a government-backed consortium of Mexican bankers was looking for a big tourism investment. They picked a spot, a narrow spit of Caribbean coast so remote it hadn’t been properly named. Some maps called it Kankun (“nest of snakes” in Maya), others “Kan Kun,” or “Can Cún” (the Spanish form). There were barely any human settlements, just marshes, mangroves and, the incredible dune-backed beaches, separated from the mainland by two narrow canals that opened on to a huge emerald lagoon system. The first hotels opened in 1974 and the international airport was inaugurated with 2,600m of runway and capacity for wide-body jets. And in they came.

Like any good boy band, Cancun was fabricated but is no less gorgeous for that, and three decades later, it and the adjoining Riviera Maya that runs 80 miles down to the Mayan ruins at Tulum bring in about 50 per cent of Mexico’s tourism revenue. Cancun alone has 27,000 hotel rooms (unofficial figures suggest it’s more like 38,000), so the erosion of its beaches is of national interest. Last month, Felipe Calderon, the president of Mexico, no less, came to declare the beaches open – ironically, swiftly decamping after the ribbon cutting to the more low-key charms of nearby Cozumel island for celebratory drinks. Mr Calderon’s helicopter cavalcade could be seen buzzing back to Mexico City along the Riviera Maya shortly after that. All fixed and home in time for tea? It very much depends on whom you talk to.

The plumped-up beaches naturally come as a relief to the tourist industry; less so the Mexican taxpayer who has partly forked out for the project. And less still the environmental non-governmental organisations that are lining up to defend this eco-rich stretch of coastline.

“There’s been no consistent policy on how to conserve the beaches,” said Gonzalo Merediz from Amigos de Sian Ka’an, one of the area’s largest NGOs, when I took a tour of his Cancun office. Here, walls were plastered with the countless accolades awarded to Sian Ka’an, a biosphere reserve and Unesco World Heritage listed area, 80 miles east of Cancun, rich with tropical forests, mangroves and marshes. The reserve also encompasses part of the Mesoamerican Reef, second in size only to Australia’s Great Barrier. Here, tourism development is restricted but with Cancun’s sprawl heading further east, the group is monitoring coastal development and erosion closely.

Cancun, a young resort, lost its beachy charms early in life thanks to the removal of its dunes and grasses, which play an integral role in keeping the sand from drifting into the ocean. Add to this the weight of hotel buildings on what is a very narrow sand bar, structures that additionally compound the problem by blocking natural wind flow from sea to lagoon, and you have a complex recipe for beaches that leave with the tide – especially during hurricane season. “Each hotel has its own idea of how to keep sand in front of its property,” explained Merediz. “Artificial reefs, sea walls, re-establishing the dunes. But after hurricane Wilma, $20m was spent on a recovery project – the sand quickly eroded – and now $90m. And the same amount or more will be needed again without proper planning.”

The science of why the beaches have eroded is not nearly as complex as the politics attached to their recovery. Over the past decade, the responsibility for this has skipped back and forth from federal to state levels. After hurricanes Gilbert and Wilma hit the coast, interest jumped to a national level and costly research into how, where and why the sands were vanishing was contested, scrapped and done all over again. Experts from such beach-beleaguered countries as Cuba, Dubai and Spain have been called in to give advice. This time the answer has been to bring in more than 1,000 tons of hi-tech machinery to dredge the ocean floor (which went down like dynamite fishing with local conservation groups) and build a huge sea wall at Cancun’s eastern tip to catch the constantly shifting sands.

Will the wonder wall work? Even if it does, there’s no doubt from any camp that the beach recovery will need repeating at regular intervals. “It’s not sustainable to rebuild the beach every five years,” said Marisol Venegas from local consultancy Redes Toursimo. In the 1970s, long before tourism came with an eco prefix and footprints were seen in the sand, not in carbon, Cancun’s development was comparatively humanitarian and cutting edge. “There was a plan,” Marisol explained. “A great, detailed plan but …” Marisol has spent the past 23 years working in the tourist industry in Cancun, playing integral roles in the development of the destination. We met for a coffee in Toks, an American-style diner far from the tourist strip amid the malls and offices of Cancun’s municipal sprawl.

Cancun, Marisol explained, was to have been one of the world’s great tourism projects, a resort where not only every tourist avenue was thought out but also each municipal park plotted and residential zone catered for. “But it grew too fast,” she said. “Municipal facilities, services, sewage and such; it’s all vastly overtaxed. Sand is the least of our problems.” For Marisol, Cancun is a perfect example of why mass tourism, investing it all on one honey-pot destination, doesn’t work. “It’s not the invisible hand of the market or the weather that’s the enemy – it’s bad planning.”

We drove 20 minutes north into Cancun’s forgotten residential zones, squatted land, where electricity is siphoned from the main grid by tela de araña (spider webs) of wires, and churches – lean-tos of corrugated iron – are more plentiful than shops. Hammocks were slung between doorways of tumbledown houses, a place of sleep and, if gruesome local paper reports are to be believed, a place of final rest. Cancun has one of Mexico’s highest rates of alcoholism, depression and suicide. Here, the phrase hanging in your hammock has a decidedly sinister meaning: it’s the preferred method of suicide. But Marisol, like many I spoke to, still loves and is loyal to Cancun. Alongside her tourism work, she has founded a non-profit high school. Everyone she is surrounded by seems to be similarly motivated, leading everything from community choirs to environmental pressure groups.

Puerto Morelos, 20 minutes west of Cancun is a living example of local activism. Here, children ride horses bareback across grass-carpeted plazas and fishermen still sell their daily catch – fleshy octopus and prized langoustines – from the scruffy little marina. Tourism comprises a few low-rise, brightly painted guest houses run by 1960s dropouts and the beach, a rarity for this region, has easy public access. Puerto Morelos, known locally as Muerto (“dead”) Morelos for its lack of development, is a last stand on the Cancun-Tulum mass tourism corridor, but even this town’s sleepy days could be numbered. There are plans to expand the community from its current population of fewer than 10,000 residents to more than 150,000 people by building housing for hotel industry workers and removing mangroves in the process.

Mangroves not only form a buffer zone protecting coastlines from erosion and flooding but also form a vital sinkhole for carbon. In theory it’s illegal to develop on mangroves in Mexico, and with 33 per cent of land within this state protected, compared with a national average of about 11 per cent, Puerto Morelos should be safe. Yet with few resources to police protected areas, things aren’t as sound as they seem. “Tourism is a threat but also an opportunity,” said Alfredo Arellano, regional director of the Mexican government’s Protected Areas Commission. “Cancun originated as a mass tourism destination but there are opportunities to create new models for development.”

Along with a budding range of eco-tourism initiatives within reserves like Sian Ka’an, Arellano noted the latest generation of hotels along the Riviera Maya, such as the new Banyan Tree and Fairmont, have greener construction and operating practices. “They are still few in number but they are creating models that are successful and sustainable.” And this coast – reef, mangroves, dunes and all – is worth sustaining. I end my trip on the beach in Tulum, which along with Cancun regularly gets voted the best stretch of sand in Mexico, but unlike Cancun, hotels here are still low rise and locally run. Yet 15 years ago the only accommodation here had been tented. With an international airport in the pipeline, could Tulum become another Cancun? If so, let’s just hope plans are not made on shifting sands.