The Ancient Romans called this region Campania felix, "happy Campania", and you can still just about see why. Once it was an earthly paradise: the aquamarine Tyrrhenian Sea full of fish, the bulk of Vesuvius to the south, which menaced destruction but was also responsible for the immense fertility of the soil.

Here in the broad plain east of Naples, towns sprang up which, thanks to that fertility, were still prospering 1,000 years later. They had become important seats of culture: in the 16th century one of them, Nola, produced the restless genius of Giordano Bruno, one of the first people to reach the conclusion that the Earth went around the sun rather than the other way around, and that the stars were also suns with planets of their own, and that they were infinitely numerous and would exist for ever. For these outrages against received wisdom the Church burned him at the stake in Rome.

The man who shows me around Bruno's town and takes me to admire the view at the medieval convent of Sant'Angelo in Palco, raised high above the plain like a great pulpit, is a Nolan like Bruno, and hugely proud of his town, its history and heritage. And like Bruno, it has fallen to him to spell out truths that the authorities do not wish to hear.

The name of cardiologist Dr Alfredo Mazza, today a still-boyish 40, became known across Italy and beyond seven years ago when, in a report published by The Lancet Oncology, the British cancer journal, he came up with the term "Triangle of Death" to describe the zone bounded at its eastern extreme by his home town and to the west by Marigliano and Acerra, 8km and 17km away respectively.

His research revealed that in this area the incidence of some types of cancer is massively higher than elsewhere in Italy. Across Italy, on average, 14 males in every 100,000 die of liver cancer; here, it was 35.9. The incidence of bladder cancer was nearly twice as high, of leukaemia 30 per cent higher. And though he could not prove it, he believed there was an explanation. "Two hundred and fifty thousand people in the region have been exposed to toxic pollutants for decades," he said. "Pollutants in the air, water and in produce from the area are well above regulation levels."

Pollution from cars and lorries in Campania probably rivals anything to be found in the industrial heartland in the north of the country. But for Dr Mazza, the density of the traffic has a different meaning. "This area is significant as it is the most important crossroads of autostrade in southern Italy," he tells me. In other words, Nola, Marigliano and Acerra are very easy to reach. If the modern development of Italy had taken place in a more balanced way, with the south enjoying levels of investment similar to the north, that ease of access might by now have transformed Dr Mazza's "Triangle" into something like the densely industrialised area between Milan and Bergamo. But despite Naples' fabulous port, that never happened. What happened instead was far, far worse: this hinterland of Naples, thanks to the motorways, has become the pattumiera, the poisoned dustbin, of the country. And thanks to a Neapolitan godfather-turned-state witness called Nunzio Perrella, in the early 1990s it began to emerge that the Camorra, the Naples Mafia, had discovered a lucrative new trade.

As the thousands of factories, refineries and other industrial plants of the north prospered through the boom years of the 1980s and 1990s, some unacknowledged boss – a gangster, an influential businessman, perhaps a powerful politician – hit on a cunning way to give the industries of Italy a unique edge over competition anywhere else on the continent. Instead of paying exorbitantly to have their toxic waste disposed of correctly by specialist companies, they would pay organised crime to truck it away and simply "lose" it. The gangs would take charge of the whole thing: their well-educated, white-collar members would iron out the bureaucratic issues, faking the paperwork, paying off any official busybodies, paying off also the owners of the land where the toxic waste ended up. The manufacturers, refineries and the rest paid the gangs only a fraction of what it would have cost to get the job done safely and legally.

The Camorra took delivery of the waste and brought it down to their own backyard – not to the densely populated streets of Naples but to the agricultural hinterland, "Campania felix". They dumped it anywhere and everywhere: in the fields, in old wells, in worked-out quarries, in or around canals, in caves. Sometimes they simply buried the loaded trailers or containers underground. Sometimes they mixed the waste with soil and scattered it on the fields. This went on for years and because the k Italian state, especially in the south, is notoriously lax, for a long time no one was any the wiser.

Yet the coming and going of lorries all night could surely not be ignored. Then there were the strange manifestations, which have the tinge of urban legends: smoke pouring from the ground in particular weather, as if the earth were volcanic; foul stenches coming from nowhere; canal water or soil that was a sickly shade of blue. And then, as years turned into decades, the young people started to fall sick.

Carolina Capasso, who lives in Marigliano, lost her 21-year-old son Andrea to sarcoma of the lungs, one of the cancers which, according to Dr Mazza, is most likely to be caused by toxic waste. "Gradually it became clear that more and more people, especially young people, were having health problems," she says, recalling the slow dawning of local awareness of the problem. "They had allergies, leukaemia, various tumours. And as they grew older one would die of cancer, one of leukaemia, and gradually we began to realise that there was something wrong. In 2009, my son started to feel bad and we found out he had cancer: a kid of 21."

She blames in particular a warehouse full of agricultural chemicals near her home where there had been an explosion and fire years before, the aftermath of which, she says, had never been properly dealt with – but, as in the rest of the Triangle, linking cause with effect and trying to hold anyone to account is a hopeless task. "I am convinced my son fell ill because of these substances, the disgusting stuff there is in Marigliano," she goes on. "Gradually we discovered nobody was doing anything about it. Andrea was ill with this cancer for seven months [before he died]. Other kids died, too. What can I tell you? Marigliano is a town of the living dead."

A few months after Andrea died, Antonella di Francesco followed him, contracting tongue cancer and dying at the age of 35. Their families lived near to each other in the same forlorn and ramshackle modern estate in Marigliano.

I go to see Gennaro di Francesco, Antonella's father. He has lost his wife, too, dying in her fifties, so now he lives alone with his 11-year-old granddaughter, Teresa, Antonella's daughter. Their first-floor flat is bare and comfortless. It is a few days before Christmas when I visit and there is a large Christmas tree in one corner, hung about with clumps of cotton wool. Gennaro, a metal worker by trade, submits to my questions as one might to a blood test, his large grey eyes wide and blank. Teresa, dark of complexion and with a sweet gypsy smile, makes me a tiny cup of coffee.

It was two years from Antonella's diagnosis to her death, Gennaro says. "She was in hospital in Naples for a month; they gave her radiation and chemotherapy and she began to get better, but then she got worse again. Then I took her to a cancer hospital in Milan where they did an operation to remove part of her jaw, then to another hospital in Turin for another operation." None of it did any good. By the end he was having to feed her through a tube in her stomach.

"Lots of young people have been dying around here," he recalls. "Ten or 20 that I heard about, and it's still going on today; every now and then you hear about another." The toxic waste is an equally persistent presence. "Everyone knows it's a problem but they don't admit it and they don't do anything about it. Because it's a big business. The politicians say they are going to clean it up but they don't."

So where, I ask, is the problem concentrated? What is the source of the poison? "Go to Boscofangone," he says. "Beyond Faibano. That's where they dump it all."

Place names such as Boscofangone, literally "muddy wood", and Pantano, "quagmire", take one back to the remote past, when this area was marshy and subject to frequent flooding. In the 17th century the Spanish Bourbon rulers of Naples took the problem in hand, building a 55km canal, the Regi Lagni, from Nola to the sea with another 210km of secondary canals feeding into it, "producing an image of a naked fish vertebra with the spines running off it", as a local historian puts it. It was a magnificent feat of engineering and it kept the plain well drained for centuries. It was only with the onset of the boom years after the Second World War that things began to go awry.

I drive from Marigliano to Polvica, looking for the elusive Boscofangone, through terrain that is neither city nor country. Plastic bags of rubbish dot the side of the road. Industrial buildings, including a shiny new rubbish recycling centre, alternate with orchards and vegetable fields; I am halted at one point by a flock of dirty-brown, long-eared sheep crossing the road to pasture in a field of dandelions. This area has not switched from agriculture to industry – the two continue to co-exist – but it's as if they live in different dimensions, each oblivious to the other.

I stop at a bar in the town of Polvica, up against the Partenio mountains, much-scarred by quarrying, to ask advice. The vivid, paunchy barista, Massimo Bernardo by name, with a face like Gene Wilder, tells me where to go. "Turn left by the Esso station, drive as far as the little round medieval church," he says. "The Boscofangone canal starts there."

Mr Bernardo knows all about the toxic-waste problem but has convinced himself it is all in the past. "Yes, there were lorries running along the canal all night, dumping their loads," he remembers. "But they've cleaned it all up. This is the best soil in Italy! We produce the best tomatoes, the best potatoes, the best oranges... Why do they import all that stuff from abroad when we have the best here?"

I follow his directions. At the entrance to the canal's towpath, barred by a swing gate, there is an official notice, describing "Interventi di straordinaria manutenzione per l'adeguamento funzionante" ("Operations of extraordinary maintenance for the satisfactory functioning") of the Regi Lagni canal. Bulldozer tracks along the path indicate that the "bonifica", the clean-up, which was promised to start on 26 September 2011 and go on for 180 days, has indeed got under way: the canal is no longer frothing with rubbish as in one amazing video available on YouTube; its sides are no longer clogged with old fridges, washing machines, plastic bags and oil drums. But they have not disappeared: after walking for half an hour I discover that a fresh load of items, including those listed above, have been dumped right into the canal, blocking its flow.

Contrary to the venomous views of local activists, work to deal with the degradation of the area has been undertaken. The problem is that it was a one-off. Once cleaned, the canal would need to be monitored, protected, guarded. It would need to be reintegrated into the planner's view of the region's future. Schemes drawn up by the local authority envisage kilometres of tree-lined canals running through recreational areas and archaeological parks – but these are pipe dreams. Instead, the canal is a relic of a past that few local people seem to know or care about; and, unless adequate protections are put in place, the dumpers will merely bide their time to start dumping all over again.

Meanwhile, there is the question of what effect the many years of illegal dumping have had on the region's water table and food chain. Emblematic of that massive public-health headache are two large, neatly squared-off mounds of God-knows-what a couple of hundred metres from the canal, covered with heavy black polythene tarpaulins: heaps of toxic rubbish that have been seized and impounded here. The tarpaulin stops the emissions of whatever is inside from getting into the air, but does nothing to stop them seeping into the earth, the water table and thence the k food chain. That, far more than the visible scarring of what remains of the countryside, is the heart of the problem.

In Italy it is always difficult to separate overheated conspiracy theory from reality. Even an observer as astute and well-informed as Dr Mazza appears to have a weakness for malign, broad-brush explanations of events. "This problem of toxic waste did not come about by chance," he tells me soon after we meet. "It is the result of a compact between organised crime, the strong powers of the state, the secret services and perhaps the Freemasons, a pact to save the nation's industry." The destruction of this region's environment, according to this theory, is regarded as an acceptable price to pay.

It is an appealing explanation, but like most such theories it is short of proof: I have seen no useful evidence that Campania's disaster was the result of a diabolical plot. Undoubtedly northern Italy has used this region as a vast unlicensed landfill site. Undoubtedly those responsible for the dumping have been Camorra gangsters or people in their pay. Equally beyond doubt are the shockingly high rates for certain cancers and genetic malformations. Beyond those facts, however, it is impossible to claim with any conviction that there has been a terrible plan at work – impossible but also unnecessary. The economic logic of what the gangs have been doing is plain to see.

While dumped toxic waste is one particular problem with terrifying implications for public health, it is part of a far larger and apparently insoluble crisis in this region involving the disposal of rubbish of every sort. The lingering image of Naples in the outside world is no longer of the city's great, sweeping bay with Vesuvius behind but of streets lined with mountains of uncollected domestic trash. This revolting phenomenon comes and goes – I am fortunate to visit when it is at a low ebb – but like the toxic-waste issue, it is never really solved.

Twenty or more years ago the Camorra succeeded in obtaining a near-monopoly in the disposal of waste of every type in Campania. They continue to use this power as a blackmail tool every time a new mayor or other official threatens to crack the whip, to cut out the gangs by enforcing the differential collection of rubbish (which scarcely exists here) or to take other decisive steps to solve the problem permanently. In this, the local political forces of the left play into the gangs' hands by orchestrating hostility to new incinerators. And the easygoing, simpatico nature of the local character – people such as Massimo Bernardo, with his blithe assurance that the toxic-waste problem has been solved – doesn't help much, either.

Piera Mucerino, a local woman who has campaigned about the toxic-waste issue for years, says the problem is that people become resigned. She is haunted by the results of an experiment she once read about. "They put a dog in a cage," she explains. "They sent electric shocks down the right-hand side of the cage; the dog moved to the left. Then they sent the shocks down the left-hand side and the dog moved to the right. Then they put shocks through the whole cage: the dog gave up and stayed where it was. Then, with the shocks still running back and forth through the whole cage, they opened the door. The dog stayed where it was.

"We're like that," she concludes. "Resignation. No matter what happens, in the end we don't move. We sit there and take it. People react to the bad news, but after a time they forget about it and get on with their lives. And when people die of cancer they just hope it won't happen to them, or they pray to God. Instead of doing the big battle for everyone, people say, 'I will do the little battle for me.'"