'Hello, this is John Doe. Interested in data?'

There is a key with a deceptively small yellow label - the key to The Guardian's secure room, which had housed the team that in 2013 worked through data leaked by Edward Snowden to expose unchecked surveillance by British and American spy agencies.

Now, it was to be home to a small group of journalists gathered from all corners of the newsroom to work on a project code-named Prometheus. This is the story of The Guardian's investigation into the murky world of tax havens, underpinned by the biggest leak in history, which would eventually surface eight months later with the publication of the Panama Papers.

The story began back in February 2015, with an article in Süddeutsche Zeitung that revealed the German newspaper had a cache of secret files about offshore companies on the books of the Panamanian law firm Mossack Fonseca. Some 80 gigabytes loaded on to a USB stick, the information related largely to the firm's Luxembourg customers.

Within weeks the paper's investigative reporters, Bastian Obermayer and Frederik Obermaier, had been approached by a second source who had read their article and wanted to share more. ''Hello, this is John Doe. Interested in data?'' The source demanded absolute anonymity. To this day, his or her identity remains unknown to the Panama Papers reporters. ''My life is in danger, we will only chat over encrypted files. No meeting ever.''

The Guardian's involvement formally began in September 2015, when Katharine Viner, the paper's editor-in-chief, and Paul Johnson, deputy editor, flew to Munich to secure participation in the consortium of journalists around the world collaborating on the story.

Back in London, the team began poring over the archives of Mossack Fonseca. These were being gradually uploaded to servers managed by the International Consortium of Investigative Journalists (ICIJ) in Washington. The database would eventually include 11.5 million emails, passport scans, contracts, share registers and even sound recordings.

It quickly became apparent that offshore agents such as Mossack Fonseca often have no special knowledge about their customers. Tax havens' lack of transparency may allow the keeping of secrets, but these secrets are often kept elsewhere.

As a result, the database needed to be approached from multiple angles, whacked with many sticks, until it could be cracked.

The Guardian brought together its specialists, ranging from the paper's foremost Russia expert to its chief sports writer

The team worked hard to keep the information safe. With investigative reporters looking into a $2 billion Russian money-laundering scheme linked to Vladimir Putin, the threat of a hack by the FSB, Russia's security agency, was very real.

More than 140 high-ranking politicians and heads of state were uncovered in the data, leaving partners in less democratic countries at risk of government retaliation.

Since the Panama Papers were published Ecuador's president, Rafael Correa, has used Twitter to name and admonish local journalists for not handing over all the data. In Venezuela, the journalist Ahiana Figueroa has been fired from her news organisation for being part of the Panama Papers team. In Tunisia, the website Inkyfada has come under suspected cyber attack after naming a former presidential adviser.

The five reporters in the core team spent long days over the winter interrogating the data, plugged into headphones and staring at screens.

As they worked, more information was being leaked. New data was uploaded at regular intervals, with the last set dating from December 2015. In February, they began to approach those we intended to name, and their research began to collide with events in the outside world. In places far away from Britain, the impact of the Panama Papers was beginning to be felt in local and apparently isolated political scandals.

In late January, two Mossack Fonseca representatives in Brazil were arrested, and later released, by authorities investigating the Petrobras affair. Two others fled the country. A prosecutor in the case described the firm as a ''money-washing machine''.

On 4 March, the law firm received a letter from the ICIJ and Süddeutsche Zeitung stating that they had seen information concerning thousands of its companies.

On 11 March, TV crews from the consortium descended on Panama City to film the firm's offices and ask for interviews. Later that day, founding partner Ramón Fonseca resigned from his position as an official adviser to the Panamanian president, Juan Carlos Varela. In a parting shot, he tweeted, ''The pen is a powerful tool. It's sad when used for evil.''

On the same day, Iceland's prime minister, Sigmundur Davíđ Gunnlaugsson, walked out of a television interview with two Panama Papers reporters, the independent documentary maker Jóhannes Kristjánsson and Sven Bergman from Swedish TV. They had, on camera, named Wintris, the undeclared offshore company he had once owned with his wife, and which his wife still owned.

The film would not be released until 7pm GMT on Sunday 3 April, the date that all 110 news outlets brought together by the ICIJ had agreed to release their stories simultaneously.

But by the Monday before publication, the Kremlin was beginning to spin. Putin's spokesman held a press conference to warn that a number of journalists were planning an ''information attack'' on the Russian president, led by the ICIJ. Key names, including those of the banker Yuri Kovalchuk and the cellist Sergei Roldugin, were released.

The atmosphere inside the secure room at The Guardian was tense. They had by now been joined by the paper's investigations head, Nick Hopkins, picture researchers and a crack team of subeditors who moulded a mass of 33 articles into publishable form.

Would the story hold? Should they bring the publication date forward? Requests for information began to flood into the ICIJ. Like a general holding back his troops before the charge, the consortium's director, Gerard Ryle, hit the phones, talking editors into holding their nerve.

 By the Friday, two days before The Guardian published, there were calls for a vote of no confidence in the Icelandic parliament. But the key data relating to Gunnlaugsson's offshore adventure remained unpublished, and so the embargo agreed months beforehand stayed in place.

After months working on its own, on Sunday 3 April, The Guardian's Panama Papers reporting team emerged from the locked office to gather around a desk in the centre of the newsroom. There were 11 items ready to be published. Viner and Johnson stood ready to give the signal.

At 6.48pm, Edward Snowden sent a Twitter message to his 2 million followers, ''The biggest leak in the history of data journalism just went live, and it's about corruption.'' His tweet linked to a Süddeutsche Zeitung article headlined A Storm is Coming.

Within minutes, the Panama Papers whirlwind had struck. In 70 countries around the world, the reverberations are still being felt.