From a 15-foot industrial ladder, in the dim light of the warehouse, the bankers boxes resemble coffins — 28,455 stacked in rows up to 12 high, four wide, 70 deep, boxes containing 93 million pages of paper — in a mausoleum to cigarette smoking.

Here, in the Minnesota Tobacco Document Depository, lie the remains of 27 years of legal cases against Big Tobacco. There are trial transcripts, exhibits, images of the Marlboro Man and Joe Camel, a diseased lung in preserving liquid, stories of smokers' deaths, and secrets that, once revealed, helped to end the tobacco industry's dominance in the cultural landscape of the United States.

The warehouse, open to the public for 23 years, will close on Tuesday, ending an unprecedented court-ordered, industry-funded central collection of the legacy of a product that, according to the Surgeon-General, has killed more than 20 million Americans and continues to kill more than 400,000 a year.

Most of the documents have been put online by the University of California at San Francisco, which used software to lift them from company websites, and the physical copies will be destroyed. I spent years researching some of the history that is stored there, looking for the Marlboro Man models and Western landscapes used in one of the most successful advertising campaigns in history for a book I'm writing. So I thought it was important to visit the archive before it closed. I wanted to look behind the billboards. I wanted to see the evidence that it was more than the cowboy that made the brand a legend. I wanted to understand why, in their first 50 years, Marlboro cigarettes killed 2.4 million Americans.

What I found in the warehouse was a quite different Marlboro Country, scattered across a million pages. Here was Marlboro’s real legacy, buried in an impressive hard-copy testament to the power of courts to regulate corporations. The documents here informed more than 500 peer-reviewed studies and hundreds of subsequent trials, and they continue to influence tobacco policy worldwide.

In these boxes, Minnesota lawyers found evidence that tobacco companies had known for decades that smoking caused cancer, that nicotine was addictive and could be manipulated, and that filter and “light” cigarettes were not safer. The files revealed that tobacco companies targeted children and conspired to hide damaging evidence in ways that a federal court declared to be racketeering. Although the original documents dated to Minnesota’s 1994 lawsuit, they were released to the public in 1998 under a settlement between the state and five tobacco companies just before the case went to a jury.

The archive helped change the course of medical and legal history.

“I can tell you that juries, when they see this stuff laid out, there’s no doubt there was a conspiracy intended to defraud the American public,” said Michael Cummings, a professor at the Medical University of South Carolina, who has testified in more than 100 trials. Smokers win about 75 per cent of them, he said.

The documents bolstered the 1998 Master Settlement Agreement between 46 states and four tobacco companies; it ended most advertising and produces an annual stream of billions of dollars to the states. They were also key to the Justice Department’s 1999 lawsuit against the industry, decided in 2006.

“They were adjudicated as racketeers — held in the same regard as gangsters,” said Sharon Eubanks, the former US Attorney who led the case. “It never would have happened if we didn’t have those documents.”

The files gutted a half-century of Big Tobacco’s defences.

A spokesman for Altria, the largest US tobacco company and maker of Marlboro, told me by e-mail that the documents and the MSA “fundamentally changed how tobacco companies advertise, market and sell tobacco products in the United States. ... Today, we are focused on Moving Beyond Smoking, seeking to transition adult smokers to potentially less harmful smoke-free alternatives in an FDA-regulated environment”. A spokesman for Philip Morris International wrote that the company is also trying to switch to smoke-free products to avoid harmful chemicals in the smoke produced by burning tobacco: “In many countries, we believe we can stop selling cigarettes within ten to 15 years.”

Without the litigation that generated the archive here, and the additional lawsuits the archive helped bring about, that would have been inconceivable.

The repository’s roots date to 1994, the year seven tobacco CEOs stood before a congressional committee and said they did not believe that nicotine was addictive. Up to then, hundreds of attempts to sue on behalf of sick or dead smokers had failed, largely because juries believed that smokers chose to smoke. But that year, Minnesota and Mississippi — and, eventually, every state — tried a new tack, suing the industry for the medical costs of treating tobacco-related illnesses.

Minnesota’s then-attorney general, Hubert Humphrey III, son of the former vice-president and a student of history, directed his outside legal team to focus first on documents. By then, a few thousand damning tobacco documents had been released in court cases or leaked by whistle-blowers. The team, led by Michael Ciresi and Roberta Walburn, suspected that more could be unearthed in discovery, the legal process to obtain relevant documents from the other side in a lawsuit.

The industry fought tooth and nail, up to the Supreme Court twice, in 18 appeals, but eventually the first of 35 million pages began arriving in Minneapolis. Walburn said the papers, without indexes, and with stray documents on such topics as tobacco beetles, looked as if someone had taken them to the top of Minneapolis’s tallest office building, “threw them down and then scooped them up”.

Six lawyers began reading the pages, one by one. “They went into the depository pretty much every day for several years,” Walburn said. “Every Friday afternoon, our team got together in a conference room, and we had a document-of-the-week contest. We put in a dollar for each entry, and you would try and sell your document that you found. And we would have these intense arguments over which was best and why ... then we took the money and went to a bar and drank beers.”

One of the first revealing documents, found in a British American Tobacco repository outside London, was an account of a 1958 visit by BAT scientists to North American laboratories, universities, and the US headquarters of Philip Morris, Liggett Myers and American Tobacco. They wanted to know “the extent to which it is accepted that cigarette smoke ‛causes’ lung cancer.”

“With one exception [a physician at Yale], the individuals whom we met believed that smoking causes lung cancer, if by ‛causation’ we mean any chain of events which leads finally to lung cancer and which involves smoking as an indispensable link.”

That document became Trial Exhibit 2241.

Another box contained a thick study, stamped “Secret”, in which R.J. Reynolds Tobacco scientists used reverse-engineering to explain why Marlboro was about to overtake Reynolds’s own Winston as the bestselling cigarette in the 1970s. It wasn’t just the Marlboro Man, it turned out. The Reynolds scientists discovered that by adding ammonia to the tobacco blend, Marlboro’s maker raised alkalinity and created “free” nicotine, which is rapidly absorbed and instantly perceived as a nicotine kick: “All evidence indicates that the relatively high smoke pH ... is deliberate and controlled.”

“In essence,” Reynolds’s scientists reported, “a cigarette is a system for delivery of nicotine to the smoker in attractive, useful form.”

That document became Trial Exhibit 3309.

Richard Hurt, a now-retired Mayo Clinic addiction expert who testified for the state of Minnesota, remembers shouting profanities that startled his wife as he read the documents at home. “We knew that nicotine was addicting,” he told me. “But we did not realise that this cigarette was the most sophisticated drug delivery device that’s ever been invented to get nicotine to the brain within five heartbeats ... faster than treating it intravenously. This thing looks so harmless, the little white thing that you put in your mouth and puff. ... It was criminal what they had done to my patients, who, unbeknown to them, had become addicted to a product that was specially designed to do nothing more than to get them addicted and to kill them.”

Less than 2 per cent of the documents, those damning enough to win the case, were introduced in the trial. The rest were put in the depository. Years before the documents went online, researchers such as Monique Muggli, a lawyer with the non-profit Campaign for Tobacco-Free Kids, laboured in the warehouse, searching keywords, asking staff to retrieve boxes and producing dozens of peer-reviewed articles and reports. After Walburn helped the World Health Organisation to draft the 2003 Framework Convention on Tobacco Control — ratified by 181 countries, although not the United States — Muggli was part of a team that researched the Minnesota documents for the WHO and discovered extensive efforts by the tobacco industry to undermine the health organisation.

The WHO reports that global smoking rates among males over 15 years old have dropped from 50 per cent at the time of the Minnesota settlement to 37.5 per cent today. But it still counts 1.3 billion smokers and eight million tobacco-related deaths a year, and says the tobacco industry “continues to aggressively market its products worldwide”, including “electronic nicotine delivery systems” that “renormalise smoking” and “help to hook new consumers”.

In the United States, the smoking rate among adults dropped from 25.5 per cent in 1994 to 13.7 per cent in 2018. Youth rates fell from 34.8 per cent in 1995 to 8.8 per cent in 2017, according to the Centres for Disease Control and Prevention. The total number of smokers stands at 39 million.

Many factors produced that decline — the end of advertising, health warnings, taxes, smoke-free policies — but nothing has reverberated louder than the truth. Perhaps the most powerful, lasting legacy of Minnesota’s mountain of paper was summed up by Eubanks, the former federal litigator, after the US Government’s lawsuit against Big Tobacco: “The documents told the story,” she said. “Nobody could ever believe anything that they said from that point forward.”

• Jim Carrier, an author in Burlington, Vermont, is writing a book, In Search of the Marlboro Man