When safari tourists drive to the Serengeti national park in Tanzania, few realise they are passing one of the world’s most contentious goldmines.

From the escarpment above the plain, the North Mara facility is so large that it at first resembles a bare hillside. But look closer and the artificial mound is made up of tiers of reddish brown earth, from which a thin grey plume of smoke drifts up to the sky.

Nearer still, you find a vast tailings reservoir filled with contaminated wastewater. Locals live in huts under the shadow of its mud and rock banks.

Welcome to North Mara, one of the biggest mines in Tanzania, which since 2006 has been operated by London-listed Acacia Mining and predominantly owned by the world’s biggest goldmining company, Barrick, a Toronto-based firm that holds a 63.9% stake.

For the past two decades, this mine has been a place of danger, extreme violence and allegations of environmental contamination.

Facebook Twitter Pinterest Villagers search for pieces of gold contained in discarded waste rock from the North Mara mine. Photograph: Trevor Snapp/Bloomberg

Although Tanzania is nominally at peace, over the years police and security guards have been accused of killing dozens – possibly hundreds – of local people, injuring many more and raping countless women.

There have also been reports of contamination from mining chemicals, but journalists and human rights activists who have tried to investigate these cases have sometimes found themselves the subject of intimidation, harassment and even threats of deportation from police and state authorities. Acacia says it is not involved in any crackdown on the media and it promotes transparency.

Since a legal challenge in 2015, the company has worked with authorities to improve the human rights situation. It erected walls in some areas, enhanced staff training, and put in place a grievance system.

But an investigation by the Guardian and its partners in the Forbidden Stories journalism collective has been told violence continues – albeit at a lower level – while the health problems associated with possible chemical pollution remain a concern.

The details have been slow to emerge and remain disputed. Acacia denies any wrongdoing.

It is running a vast and remote mine that is a major contributor to the national economy. The disparity of power with local villagers could hardly be greater. Most locals here are from the Kuria indigenous community and many are illiterate.

Facebook Twitter Pinterest Trespassers around the North Mara goldmine. Photograph: Forbidden Stories

Acacia bought the mine in 2006. It has so far produced 2m ounces of gold, worth $2.6bn at today’s prices, with almost double that amount still underground.

But it also inherited violence and widespread resentment.

‘Police dump the bodies outside the homes’

The original owner, Afrika Mashariki Gold Mines, acquired the land in the mid-1990s, and was accused of evicting residents without compensation.

Nearby villagers were forbidden from artisanal mining, which had been an important source of income before the mining company arrived. Locals – sometimes armed with machetes – intrude inside the mine to look for granules of gold among the waste rock and on the edge of the tailings pond. The situation is often volatile.

On some days, the guards accept bribes and turn a blind eye.

On others, they are ordered by their bosses to crack down. The worst period was around 2010-14.

“I’ve seen a lot of people get shot, some beside me. We would enter in a group and then run if they see us. We would hear the next day who had died. Police dump the bodies outside the homes,” said one local man who asked to remain anonymous, referring to conflicts at that time. He said tensions remained.

“It happened many times. The villagers get very angry. Why are they treating us like animals?”

Facebook Twitter Pinterest Children near the goldmine. Photograph: Forbidden Stories

The nearest general hospital in Tarime was treating five to eight cases of gunshot wounds from the mine every week from around 2010 to 2014, according to Dr Mark Nega, a former district medical officer.

“I saw so many people shot and killed. Some had gunshot wounds in the back. I think they were trying to run away but they were shot from behind.”

Such killings were initially played down or denied. Journalists who tried to investigate found themselves harassed by police, or believed their stories had been spiked following pressure from state authorities.

After pressure from activists and lawyers, Acacia acknowledged 32 “trespasser-related” fatalities between 2014 and 2017. Of these, six died in confrontations with police at the mine.

International watchdog groups say at least 22 were killings by guards and police during the same period. Tanzanian opposition politicians have claimed 300 people have been killed since 1999.

“For such a high number of violations to have occurred outside a conflict zone in a business context is shocking and exceptional,” said Anneke van Woudenberg, the executive director of Raid, a UK corporate watchdog.

The owners blame police. “There have been many, many investigations on various allegations, and you can’t hold me accountable for the state authority,” said the Barrick chief executive, Mark Bristow, when asked about the killings.

But the authorities work under a memorandum of understanding with the mine whereby local police work at the service of the mine in return for fuel, food, accommodation and daily stipends.

After a lawsuit against the company in 2015 by plaintiffs represented by the UK law firm Leigh Day was settled out of court with no recognition of liability, the number of shootings has declined and no rapes have been reported.

There is a new wall around operational areas. The mine and local authorities say they are educating guards and police and punishing those who break the law.

Facebook Twitter Pinterest The North Mara goldmine. Photograph: Forbidden Stories

Security arrangements have been revised and efforts have been made to improve community relations. The number of conflicts has fallen considerably.

But the problem seems far from solved.

On a recent trip to the area, the Guardian and Forbidden Stories heard of several new cases. On 3 or 4 August 2017, Daniel Chacha Range was reportedly shot and killed at the Nyabigena waste dump, where he was looking for gold.

The mine reportedly paid 72m Tanzanian shillings (£25,000) to the family last March.

On 1 December last year, a young student – Amos Chacha – was injured by a bullet from security personnel. Locals said he was accidentally hit by a volley that was fired to scare villagers near the mine.

In both cases, Acacia said it could not comment on individuals, but its guards did not use lethal weapons and it dealt with human rights abuses by other parties, such as the police, through its grievance mechanism.

‘It would never happen anywhere in Europe’

Then there are environmental problems that may prove to be Acacia’s enduring legacy long after the gold is mined out.

There can be few places in the world where so many people live so close to a vast toxic tailings dam, waste rock dump and chemical processing facility. About 70,000 people live close to the mine, many of them drawn by the prospect of jobs or gold.

Walls erected in recent years have improved the situation in some areas, but elsewhere, it is hard to tell where the border is between the village and the mine. Acacia says it has been unable to acquire all the land it needs for a 200-metre buffer zone, but is still trying to do so.

“It has been going on in Tanzania for years,” said Tundu Lissu, a lawyer and activist.

“People live side by side with the piles of waste from the mine and that is completely illegal in Tanzanian law. It would never happen anywhere in Europe because people simply would not accept that kind of arrangement.”

The Earthworks NGO estimates the average gold ring generates 20 tonnes of waste. Heavy metals and other toxins that were previously buried are released into the air and water.

Unless carefully managed, this can seep into the soil and rivers with consequences that can last for decades.

Locals report frequent spills from the tailings reservoir, including one the week before we arrived. In the past this has caused problems.

A doctor, who worked in the general hospital in Tarime for five years until 2010, said he had treated at least 200 people with skin rashes, including 50 children. He also diagnosed an unusually high number of cervical cancer cases even among women without HIV who had never given birth.

The doctor, who asked to remain anonymous, said there were three types of skin problems – a loss of pigmentation, toad-like scales and scurvy-like dryness.

He believed the likely cause was chemicals in the mine – either pollution that leaked into rivers from the tailings pond, or mercury that artisanal miners used with their hands to separate the gold from the rocks.

A second doctor said he had dealt with 10 cases of severe skin rashes up until 2014 when he left the area, and many more cases were likely to have been treated by his colleagues.

Acacia says previous cases have been thoroughly investigated by the authorities, but it will consider new cases through its grievance mechanism.

Villagers appear to have been unaware of the possible risks. Chaina Mwita Bhoke thought she was fortunate when fish started leaping out of the Tigithe River and flapping on the ground in front of her. She cooked some for her family, but soon after her children ate them, she said they were screaming that their skin was on fire. She also suffered.

A doctor’s certificate from Nyamongo hospital at that time in 2009 diagnosed skin lesions on her lower limbs caused by “contamination by poisonous water from North Mara mine”.

Others lost their livelihoods. John Nyamboge Ntara, of Matongo village, said that up until September 2017, 168 of his cows had died after grazing on land close to the tailing reservoir. “We worry for our kids,” he said. Acacia said it had investigated these claims and found no evidence supporting the allegations.

Facebook Twitter Pinterest Cattle in the Tigite River, near North Mara goldmine. Photograph: Forbidden Stories

But pollution concerns have been backed by several studies. In 2012, scientists found arsenic levels were “an order of magnitude” higher than the drinking water recommendations by the World Health Organization. Four years later, a study of Mara river fish found significantly higher concentrations of chromium, nickel, copper and selenium downstream of the mine than upstream.

The co-author of a 2009 report, Åsgeir Almås, said the levels of arsenic and other heavy metals had dramatic consequences for the immediate environment. Although he was not allowed inside the mine, he said he had little doubt about where the contamination came from. “This was likely caused by a spill from the acid-tailings dam,” he said. “At that time, they had little control of their leaching. I think the dams were not perfectly constructed.” He said that elsewhere exposure to these chemicals had been found to lead to a skin problem called keratosis, miscarriages and cancer.

Acacia says many previous cases were related to a major spill in 2009. Since then, it says, it has put in place remedial measures and found no evidence that livestock deaths were related to pollution.

More research is under way by the government’s National Environment Management Council. Earlier this year, Acacia was hit by a $2.4m fine for alleged pollution at North Mara. The Tanzanian environment minister echoed what locals had long been saying: the tailing storage facility had been releasing contaminated water into the wider area for 10 years. He claimed the dam was not built properly and instructed the company to build an alternative tailings reservoir.

Bristow, the South African CEO of Barrick, acknowledged there were problems, but insisted nobody had been contaminated because of the mine.

“The environment is an issue and so we are very disturbed about the environment; so is the government of Tanzania. They have engaged with Acacia; I am engaging with Acacia,” he told Forbidden Stories.

In a detailed reply to questions raised by the Guardian, Acacia said it took allegations seriously and would assess claims through its grievance systems. Even before the government’s order, the company said, it was looking into the construction of a new tailings dam.

It warns that the dispute threatens the economic future of the Tanzanian people. “As a company, we are and always have been committed to acting responsibly towards the people of Tanzania, their environment and their communities,” reads a statement on the company’s homepage.

The mine and its owners are under increasing pressure. Since John Magufuli became president of Tanzania in 2015, he and his ministers have accused Acacia of a raft of irregularities, including environmental breaches. Domestic critics say these measures are a ploy to pressurise the company to contribute more funds to public coffers.

Lawsuits are under way in the UK, where Acacia is listed. In 2017, the London law firm Deighton Pierce Glynn filed a second suit on behalf of 10 plaintiffs related to incidents at North Mara between 2013 and 2016. They allege Acacia’s in-house grievance process is flawed and compensation payouts are inadequate. Lawyers from a Cardiff-based firm, Hugh James, are in Tanzania meeting another group of potential claimants for a third case.

At the annual shareholders’ meeting last week, human rights campaigners called for independent and transparent assessment of grievance claims and an end to the memorandum of understanding with police. The company counters that these measures have helped to alleviate human rights problems in the past two years.

‘I think they were trying to silence us’

Rape victims are also speaking out about their experiences and the waivers they were encouraged to sign in return for modest sums of compensation.

The first was Lucia Marembela Mwita, who was caught by guards in 2009. “They took me to the airstrip in a car. One raped me. The other kept watch. It became a routine for any women they caught.”

Facebook Twitter Pinterest Lucia Marembela Mwita. Photograph: Forbidden Stories

Another woman, Nyamhanga Kichele Mwita, was ordered to lift up a large rock and told she would be beaten if she dropped it. As she clasped it, they stripped and raped her. She later found she had HIV.

Many of the victims say they kept quiet because they were ashamed to tell their husbands.

Boke Makolele still has trouble walking because the guards struck her knees, ankles and the small of her back with a kilungo, or wooden baton. “They beat me a lot because I didn’t want to do it. After they dumped me, I went to hospital in tears. It hurt so much,” she said. “I never told anyone. I was afraid.”

Eventually, more than a dozen women complained and after the case was picked up by lawyers and international NGOs, they were paid off by the company, though it made no admission of liability.

Looking back, the women believe they were deprived of their legal rights. “They called us and said sign here. We didn’t even get to take the document home to read. We weren’t aware what was written on the paper,” said one. “I think they were trying to silence us.”

Facebook Twitter Pinterest Villagers near the goldmine. Photograph: Forbidden Stories

Acacia said it did not recognise this accusation and its grievance mechanisms were in line with international standards. It no longer requires claimants to waive their legal rights.

“The tension will continue,” said Lissu, who was shot 16 times, he believes, because he criticised President Magufuli’s mining policies. He sees more hope in the exhaustion of the mine, though it will leave its mark on the land.

“When they finish the gold and go, maybe peace will return to North Mara,” he said. “Once they finish the gold, they will go and they will leave the poison behind and these mountains of waste rock.”