Marina Walker remembered when she heard a source claimed to have possession of leaked data "bigger than anything you have seen before."

Could it really be that big? WikiLeaks had seen dumps that encompassed millions of documents. Walker, the deputy director of the the International Consortium of Investigative Journalists (ICIJ), already worked on one of the biggest leaks in recent history, having co-managed an offshore leaks project.

Yes, it's that big.

The numbers are staggering: 2.6 terabytes of data in 11.5 million documents tracking billions of dollars over almost 40 years.

Now called the Panama Papers, it is already recognized as maybe the biggest leak in the history of journalism. World leaders, athletes and celebrities around the globe have been implicated -- and there's more to come.

Such a historic document dump required a matching journalistic effort, one that would end up as among the broadest and most technologically challenging journalistic endeavors ever.

At the center of it all is the ICIJ, which helped coordinate the project. 

But first, they had to get it into good enough shape to be read and shared. 

"We quickly did two things. We recruited a worldwide team of investigative journalists who could mine the data for months," Walker said. "We also faced a technical challenge of making the data readable and sharing it securely. It took us many months to clean and filter and prepare the data and then load the data to the platform."

In all, more than 370 journalists from more than 100 media outlets in almost 80 countries around the world worked on the massive document, which has shed light on the movement of money through shell companies in a variety of countries.

It all started in late 2014 when a source that referred to itself as "John Doe" reached out to Bastian Obermayer, an investigative journalist for German newspaper Süddeutsche Zeitung.

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That data turned out to be just about every document generated by Panamanian law firm Mossack, Fonseca & Co. It's a firm once only known among the global elite --it's the fourth-largest law firm for offshore tax havens --  but it's now at the forefront of one of the most expansive investigative journalism efforts in history.

Sheila Coronel, a veteran investigative journalist and professor at the Columbia Journalism School, said the Panama Papers project has set a new bar for cooperation.

"I've never seen a collaboration of this nature in terms of the number of journalists and news organizations involved and in terms of the countries involved, and in terms of the independence and autonomy that was given to each of these entities to mine this very rich material to find stories that are important and relevant to their own audiences," Coronel said.

Global effort

The project tapped a variety of major international news outlets including The Guardian, the BBC, France's Le Monde, Argentina's La Nación, German broadcasters NDR and WDR, as well as journalists from Swiss newspaper Sonntagszeitung, Austrian weekly paper Falter and Austrian broadcaster ORF.

News organizations from every continent except Antartica pitched in on the project. As for U.S. publications, Univision, Fusion, McClatchy, the Charlotte Observer, and the Miami Herald are among the few.

The project comes at a difficult time for investigative journalism and the media industry as a whole, plagued by upheaval. The number of reporting jobs, at least in the U.S., has been on a steady decline. Journalists have turned to other jobs, particularly working for corporations in public relations. Two 2015 Pulitzer Prize winners had left to work in PR. 

The need to engage such a large and diverse group of media outlets came from the leak itself. With so many countries, companies and people implicated, it was necessary to look to journalists with local knowledge. 

"If you wanted to look into the Brazilian documents, you could find a Brazilian reporter," ICIJ director Gerard Ryle told Fortune. 

Not everyone was invited 

The project is also notable for which press outlets appear to have been excluded, particularly the largest news outlets in America:The New York Times, The Wall Street Journal and Washington Post, all of which do not appear to have worked on the papers and did follow-up stories. 

The Intercept, the investigative journalism outfit started by Glenn Greenwald and Laura Poitras, who were instrumental in reporting on the leaks of Edward Snowden, also appears to not have been involved.

As for why those organizations aren't a part of the project, Walker said that openness to collaboration was essential to the project. Each partner was required to share any relevant or important discovery among all the other outlets, she added.

Some media organizations are more comfortable with this kind of sharing that others, which Walker noted can be understandable based on each newsroom's own privacy concerns.

That being said, she noted that the door is open for more partners.

"That does not mean that we can't work with the Washington Post or the New York Times on this data," she said.

The secrecy of the project is particularly impressive given the size and scope of the effort. Most recently, some signs emerged that a major story was brewing, particularly after the Kremlin attempted to get ahead of the implications that Russian President Vladimir Putin was involved.

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Before that, there were few inklings of just what was about to drop. The ICIJ built a search engine available online to its partner organizations that included a chat system and two-factor authentication, according to Walker.

Paired with old school investigative journalism, that tech provided the backbone for the project.

"It's old and new reporting here," Coronel said. "What journalists are able to do now, as this latest story demonstrates, is that we are able to communicate and collaborate and report together securely across different countries, sharing basically the same set of information."

The initial salvo of stories was published on Sunday in the early afternoon Eastern Standard Time. Edward Snowden was among the first people to tweet out the link to the SZstory.

He called it the biggest leak in the history of data journalism.

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In waves

The media approach to the Panama Papers has differed from other recent massive leaks.

The most high-profile leaks have come through WikiLeaks, an organization that has a stated dedication to openness and transparency. As such, WikiLeaks has put its documents online for anyone to read.

The Panama Papers are not online, and it doesn't sound like they will be any time soon.

Le Monde noted that the decision had been made not to make the documents public -- let alone searchable -- as many of them contain sensitive and private information. 

Walker said that some structured data -- companies and names of owners -- will be made available online. Other data will be made available on request.

"We believe that information about who owns the companies should be public," Walker said.

That doesn't mean this is over. In fact, it sounds like the stories released so far are just the beginning.

The Organized Crime & Corruption Reporting Project, which also pitched in on the effort, teased that there are many more stories on the way.

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Updated 11:08 P.M. EST April 4, 2016: This piece was updated to reflect that Univision is a U.S. company. 

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