The shimmering surface is down to an agricultural gold rush that has turned one of Spain's poorest corners into Europe's largest greenhouse. An area so arid and dusty that it provided the backdrop for spaghetti westerns, Almería has made a fortune by covering itself with a canopy of transparent plastic. Above all, it is a monument to the way we now grow our food. Almería, and the area around it, is Europe's winter market garden, spread across 135 square miles.

Symbols of hastily acquired wealth abound. Farmers glint with gold jewellery. New shopping malls rise above the plastic. Immigrants from as far off as Mali, Colombia or Ukraine offer their toil and their sweat. Instead of trying to sell cars or banks, billboards advertise seeds.

Antonio Moreno, one of thousands of smallholders who have built this plastic jungle, knows how to put fresh tomatoes on British tables in January or courgettes at Christmas. He grows crops that have no direct contact with nature beyond sun, air and water. "You really should wear shorts in here," he says in the 45C (113F) heat as he points to tubes from which tomato plants sprout.

Mr Moreno's plants will never touch soil - they grow from bags filled with oven-puffed grains of white perlite stone. Chemical fertilisers are drip-fed to each plant from four large, computer-controlled vats in a nearby room. He talks proudly of his vats. They hold, he says, potassium nitrate, magnesium and potassium sulphate, calcium nitrate and phosphoric acid. "The plants get exactly what they need, nothing more and nothing less," he says. "There is no waste."

Swamped

He will crop tomatoes continuously from October to July. The greenhouses are so successful that they have swamped the plain of Dalías, where people such as Mr Moreno's father used money earned in French car factories or Swiss restaurants to buy small plots. Now the sheeting is moving up the valleys of the nearby Alpujarra hills, one of Spain's most bucolic, unspoiled areas. Diggers are also gouging terraces in nearby Granada province.

"They block up dry riverbeds and destroy mountainsides but nobody does anything, however much we complain," says environmentalist Juan Antonio Martínez, surveying the scarred hills at Albuñol. "If there is a serious storm, much of this will be washed away."

A chemical tang hangs in the air along the dried-out bed of the Albuñol, where empty pesticide containers bearing toxic warnings lie among the plastic litter. On the coast at El Pozuelo plastic waste is piled calf-high. "The worst thing is that much of this is done with European Union grants. Money is handed out to young farmers via local authorities who simply ignore the requirement that the environment be respected," he says.

The Alpujarras are the adopted home of many northern Europeans offering esoteric treats such as Buddhist retreats or hikes into the snow-capped Sierra Nevadas. Some in the small, white-painted villages are said to be selling up. "We denounce things to the police but nothing happens. It is chaos," says Gari Amtmann, a long-time German resident in Válor village. "For many people this is all they can do to make money."

There is growing evidence, too, of more serious damage. In his laboratories at San Cecilio University hospital in Granada, Professor Nicolás Olea has detected a link between some pesticides and increased risk of breast cancer in women and testicular problems in boys.

Although Prof Olea has not proved a link, he says the signs for those who work in or live near the greenhouses are too strong to ignore. He points at up to 40kg of pesticide applied per hectare (88lbs per 2.5 acres). "Every time we test the hypothesis, the results point the same way." Pesticide-related residues are now present in umbilical cord blood and placenta. Last month he exposed an increased risk of cryptorchidism (undescended testicles) among boys.

But it may take 20 years to prove cause and effect. That, he says, would be too late. "We do not know what will happen, but we have reasonable doubt."

Supermarkets

British supermarkets are secretive about how much produce comes from Almería. Tesco calls this "commercially sensitive information". But Rafael Losilla, editor of a local farming magazine, names Tesco, Waitrose and Sainsbury's as valued customers. "Britain is the third-biggest export market after France and Germany," he says.

Supermarkets said rigorous tests and standards imposed on farmers were in place to prevent goods with excess pesticide residues reaching the shelves. But Prof Olea fears potential dangers are being ignored. "Something may have 10 substances in it that are all at legal levels, but what does the mixture mean? ... Why not measure the combined effect of the cocktail?"

However, he recognises UK consumer power has been an unexpected force for good. In the 1990s, British supermarkets became the first to demand rigorous controls - and pay extra for them. "The British are tough to work with," says farmer Juan Segura, who helps run a farmers' cooperative in San Isidro.

On the packing line here, small boxes of cherry tomatoes bearing Sainsbury's "sweet and juicy" stickers have been kept back for checks. Quality certificates awarded on behalf of Tesco or by Oxfordshire-based CMi certification company are pulled out. "We do not want to lose them as clients. Co-op members have been thrown out for breaking the rules," says Mr Segura. One packer was sacked after her chewing gum was found on an avocado at a French supermarket.

Mr Segura says farmers look for ways to limit pesticide use. "Biological controls" - or getting "good" insects to eat "bad" ones - is the latest solution. But in the driest corner of a country struck by severe drought farmers fret more about water. Drip irrigation may cut waste but aquifers are still drying up. Some are so full of intruding seawater that some crops can no longer be grown.

As problems arise farmers seek scientific fixes. Soon some will not even need rainwater. A desalination plant, turning Mediterranean seawater into freshwater, is set to rescue them. That will leave only the sun and air untouched by the human hand, or machinery, before reaching their plants.