I do. I was a young man, a hippie but with short hair working as a business executive in Arizona, and it provoked more anger in me, more rage than I could ever convey with written words.

Indra Sinha, who was Booker-nominated for his book on the Bhopal disaster, explains why the gas leak that killed 20,000 people 25 years ago – and continues to create health problems for countless more – is still a national scandal.

"Wake up people of Bhopal, you are on the edge of a volcano!"

In September 1982, Bhopali journalist Raj Keswani wrote a terrifying story, the first of a series of articles, for the city's Jansatta daily. Bhopal was about to be annihilated. "It will take just an hour, at most an hour-and-a-half, for every one of us to die."

Keswani's information came from worried staff at the Union Carbide factory, where a worker, Ashraf Khan, had just been killed in a phosgene spill. The first world war gas was used in the production of MIC (methyl-isocyanate), a substance 500 times deadlier than hydrogen cyanide, and so volatile that unless kept in spotless conditions, refrigerated to 0 Celsius, it can even react explosively with itself. Cooling it slows reactions, buys time, but MIC is so dangerous that chemical engineers recommend not storing it at all unless absolutely necessary and then only in the tiniest quantities. In Bhopal it was kept in a huge tank, the size of a steam locomotive.

Far from the shining cathedral of science depicted in Union Carbide adverts, the Bhopal factory more closely resembled a farmyard. Built in the 70s to make pesticides for India's "green revolution", a series of bad monsoons and crop failures had left it haemorrhaging money.

Union Carbide bosses hoped to dismantle and ship the plant to Indonesia or Brazil, but finding no buyers, went instead on a cost-cutting spree.

Between 1980 and 1984 the workforce was halved. The crew of the MIC unit was cut from 12 to six, its maintenance staff from six to two. In the control room a single operator had to monitor 70-odd panels, indicators and controls, all old and faulty. Safety training was reduced from six months to two weeks – reduced in effect to slogans – but as the slogans were in English, the workers couldn't understand them.

By the time Keswani began his articles, the huge, highly dangerous plant was being operated by men who had next to no training, who spoke no English, but were expected to use English manuals. Morale was low but safety fears were ignored by management. Minor accidents happened routinely but were covered up. There were so many small leaks that the alarm siren was turned off to avoid inconveniencing the neighbours. A Union Carbide memo boasted of having saved $1.25m, but said that "future savings would not be so easy". There was nothing left to cut. Then bosses remembered the huge tank of MIC. They turned off its refrigeration to save freon gas worth $37 a day.

A 1982 safety audit by US engineers had noted the filthy, neglected condition of the plant, identified 61 hazards, 30 critical, of which 11 were in the dangerous MIC/phosgene units. The audit warned of the danger of a major toxic release.

Safety was duly improved at Union Carbide's other MIC plant in West Virginia. In Bhopal, where six serious accidents had occurred – one fatal, and three involving gas leaks – nothing was done.

If safety was ignored inside the plant, Union Carbide had no plan at all for the surrounding densely packed neighbourhoods. As the situation worsened, factory staff, fearing for their own lives and those living nearby, put up posters warning of a terrible danger. Keswani wrote begging the chief minister of Madhya Pradesh to investigate the factory before Bhopal "turns into Hitler's gas chamber". His sensational style, perhaps, caused him to be ignored. His final article, "We are all about to be annihilated", appeared just weeks before the gas disaster.

As night fell on 2 December 1984, none of the factory's safety systems was working. The vent gas scrubber lay in pieces. The flare tower was undersized. The siren stayed silent. Years later – too late for the thousands who would now die in unimaginably hideous ways – a prosecuting attorney would say that Union Carbide had demonstrated a "depraved indifference to human life".

'That night'

3 December 1984, just after midnight. Death came out of a clear sky. From Union Carbide's factory, a thin plume of white vapour began streaming from a high structure. Caught by the wind, it became a haze and blew downwards to mingle with smoke coming from somewhere nearer the ground. A dense fog formed. Nudged by the wind, it rolled across the road and into the alleys on the other side. Here houses were packed close, shoddily built, with ill-fitting doors and windows. Those within woke coughing, their eyes and mouths on fire. Across the city countless women were saying, "Hush darling, it's only someone burning chillies. Go back to sleep."

Survivors' leader Champa Devi Shukla says, "We woke with eyes crying, noses watering. The pain was unbearable. We were writhing, coughing and slobbering froth. People just got up and ran in whatever clothes they were wearing. Some were in their underclothes, others wore nothing at all. It was complete panic. Among the crowd of people, dogs, and even cows were running and trying to save their lives and crushing people as they ran. All climbed and scrambled over each other to save their lives."

In the stampedes through narrow alleys many were trampled to death. Some went into convulsions and dropped dead. Most, struggling to breathe as the gas ripped their lungs apart, drowned in their own body fluids.

Aziza Sultan had two young children and was pregnant with her third. When the panic began, her entire family ran out of their house. They were in night clothes and it was bitterly cold, but nothing mattered except to run. Outside in the lane, it appeared that a large number of people had passed that way. Shoes, slippers and shawls were strewn about. A thick gas cloud enveloped everything, reducing the streetlights to brown pinpoints.

"In the panic," Aziza recalls, "lots and lots of people were running, screaming for help, vomiting, falling down unconscious. Children were wrenched from their parents' grasp. Their cries were heartbreaking. I was terrified of losing my children. I was carrying my baby son Mohsin. My daughter Ruby was holding on to my kurta, she did not once let go. We had gone about 500 metres when my father-in law spotted a truck and told us to climb aboard. We couldn't, but he was tall and strong so he got in. In the confusion, instead of lifting up his grandson, he grabbed another little boy who was running around on his own. My mother-in-law was vomiting. She was a heart patient and Hamidia hospital was still two kilometres away, much of it uphill. Soon Mohsin was being sick on me. Ruby was also vomiting. We all fell on the ground. I had a miscarriage right there in the middle of the street, my body was covered with blood."

At least 8,000 people died on "that night". Half a million were injured. In the years since, as more people died of their injuries and illnesses caused by inhaling the gas, the death toll has risen above 20,000.

The long-predicted gas leak at Union Carbide was, and remains, the worst industrial disaster in history.

The aftermath

Light came to city streets full of corpses sprawled in the agonised poses in which death had found them. They lay in heaps, limbs twisted, faces contorted.

In some places the dead were so many that it was impossible to walk without stepping on them. These were scenes from an apocalypse. The sun came up on choking, blinded people making their way to the hospitals. Some, desperate to relieve the agony in their eyes, were washing them in sewage water from the open drains.

The hospitals were full of the dying and doctors did not know how to treat them because they did not know which gas or gases had leaked, and Union Carbide would not release the information, claiming it was a "trade secret".

A quarter of a century later, Union Carbide and its owner, the Dow Chemical Company, which acquired it in 2001, still refuse to publish the results of studies into the effects of MIC. With or without these studies, 25 years of suffering prove that mass exposure to MIC destroys bodies, minds, families and a whole society.

Abdul Mansuri speaks for thousands. "My breathing problems started after the gas and got worse and worse. I can truthfully say that I have never had a day's health, or a day without pain, since 'that night'." For some the pain, physical, mental, emotional, has been too much.

Kailash Pawar was a young man. "My body is the support of my life," he said. "When my breathing is normal I feel like living. But when it becomes heavy, thinking stops and absolute pain takes over. I have become worthless." He was still in his 20s when he doused himself in kerosene and struck a match.

Today in Bhopal, more than 100,000 people remain chronically ill.

The compensation paid by Union Carbide, meant to last the rest of their lives, averaged some £300 a head: taken over 25 years that works out at around 7p a day, enough perhaps for a cup of tea.

Over the years the survivors have received little medical help. Being mostly very poor, they were often treated rudely. Government doctors would refuse to touch them. They were theoretically entitled to free treatment but were prescribed expensive drugs they did not need and which in some cases actually harmed them. In 1994 the Indian government, eager to put the gas leak behind it, shut down all research studies into the effects of the gas, just as new epidemics of cancers, diabetes, eye defects and crippling menstrual disorders were beginning to appear.

Abandoned by all who had a duty of care, the survivors decided to open their own clinic. In 1994, an advertisement appeared in the Guardian, launching the Bhopal Medical Appeal. The generous response of this newspaper's readers and others enabled the survivors to buy a building, hire medical staff and begin training. In 1996 the Sambhavna Clinic opened its doors, offering survivors a combination of modern medicine, ayurvedic herbal treatments, yoga and massage. Consultations, treatments, therapies, medicines and post-treatment monitoring are all absolutely free.

The water poisonning

After the night of horror, the factory was locked up. Thousands of tonnes of pesticides and waste remained inside. Union Carbide never bothered to clean it. The chemicals were abandoned in warehouses open to wind and rain.

Twenty-four monsoons have rusted and rotted the death factory. The rains wash the poisons deep into the soil. They enter the groundwater and seep into wells and bore pipes. They gush from taps and enter people's bodies. They burn stomachs, corrode skin, damage organs and flow into wombs where they go to work on the unborn. If babies make it into the world alive, the poisons are waiting in their mothers' milk.

Atal Ayub Nagar is a slim strip of housing sandwiched between Union Carbide's factory wall and the railway line. It used to have no handpumps and fetching water meant a trek to a well in Shakti Nagar, half a mile to the south. People clubbed together to install two handpumps. At first the water seemed OK, but then oily globules began appearing. The water acquired a chemical smell, which grew gradually worse.

A private Union Carbide memo, obtained via a US court case, reveals that as far back as 1989 the company had tested soil and water inside the factory. Fish introduced to the samples died instantly. The danger to drinking water supplies was obvious, but Carbide issued no warnings. Its bosses in India and the US watched silently as families already ruined by their gases drank, and bathed their kids in poisoned water.

In Atal Ayub Nagar, many damaged babies were being born. The situation did not improve after the state government took possession of the site in 1998. The following year, when Greenpeace was testing soil and water around the factory, it visited this place and found carbon tetrachloride in one of the handpumps at levels 682 times higher than US Environmental Protection Agency (EPA) limits. People drank this water, washed their clothes and bathed in it.

In August 2009, a sample of water from the same handpump was analysed by a Greenpeace laboratory in the UK. Carbon tetrachloride was found at 4,880 times the EPA limit. In the last decade, the water has become seven times more poisonous.

Rehana is a nine-year-old from Atal Ayub Nagar. She was born without a left thumb, her growth is retarded, her mind is weak and she hasn't the strength to go to school. Rehana's vision is not good, she's plagued by rashes and is constantly breathless.

Her father sadly asks, "Why was fate so cruel to our poor child?"

Why was fate so cruel

Long before "that night" there had been troubling rumours, mysterious deaths of cattle grazing near the factory. Babulal Gaur, a Bhopal lawyer, mediated a settlement between Union Carbide and the aggrieved farmers.

In 2004, Gaur became a minister in the local BJP government and to him fell the duty of caring for the city's gas survivors. He told the Christian Science Monitor that the Union Carbide factory had contaminated the groundwater, and complained that the previous Congress government had tried to hush the matter up.

In May 2004, India's Supreme Court ordered the state to supply clean water to the poisoned communities. Gaur's government ignored this order.

A year passed and a group of women and children went to the government offices to ask why nothing had been done. They were savagely beaten, punched and kicked by police. Weeks later Gaur, by now promoted to chief minister of Madhya Pradesh, announced an ambitious £120m plan to beautify the city with ornamental fountains and badminton courts.

To mark the 25th anniversary of the gas leak, Gaur, demoted to Bhopal gas tragedy relief and rehabilitation minister, announced that he would open the derelict factory site to the public. There was no water contamination, he said, echoing Indian environment minister Jairam Ramesh, who, with curious naivety, told journalists that he had handled some waste and not become ill.

A cynic remarked that this was like touching a cigarette and saying, "Look, I haven't got lung cancer."

Denying that contamination exists clearly serves the company's interests. No doubt it is mere coincidence that the Dow Chemical Company, has made at least one donation to Gaur's party, the BJP.

This sordid little tale is itself an echo of the bigger machinations at the centre, where Dow has been trying to twist the arm of Manmohan Singh's Congress government into letting it off the Bhopal hook in return for a billion-dollar investment in India.

When people ask, "Why is the disaster continuing? Why has the factory not been cleaned? Why have Union Carbide and Dow not faced justice?", the answer is this: Union Carbide's victims are still dying in Bhopal because India itself is dying under the corrupt and self-serving rule of rotten leaders.

Indra Sinha is the author of Animal's People, a novel based on the Bhopal disaster. For more on the Bhopal Medical Appeal: bhopal.org