Twenty-five miles due south of Salt Lake City, a massive construction project is nearing completion.  The heavily secured site belongs to the National Security Agency.

"The spy center" -- that's what some of the locals like Jasmine Widmer, who works at Bluffdale's sandwich shop, told our Fox News team as part of an eight month investigation into data collection and privacy rights that will be broadcast Sunday at 9 p.m. ET called "Fox News Reporting: Your Secrets Out.”

The NSA says the Utah Data Center is a facility for the intelligence community that will have a major focus on cyber security. The agency will neither confirm nor deny specifics. Some published reports suggest it could hold 5 zettabytes of data. (Just one zettabyte is the equivalent of about 62 billion stacked iPhones 5's-- that stretches past the moon.

One man we hoped would answer our questions, the current director of the NSA General Keith Alexander, declined Fox News's requests to sit down for an interview, so we stopped by the offices of a Washington think tank, where Alexander was speaking at a cyber security event last year.

Asked if the Utah Data Center would hold the data of American citizens,  Alexander said, "No...we don't hold data on U.S. citizens," adding that the NSA staff "take protecting your civil liberties and privacy as the most important thing that they do, and securing this nation."

But critics, including former NSA employees, say the data center is front and center in the debate over liberty, security and privacy.

"[It] raises the most serious questions about the vast amount of data that could be kept in one place for many, many different sources,"  Thomas Drake told Fox News.  

Drake -- who worked at the NSA from Aug. 2001 to Aug. 2008 and was unsuccessfully prosecuted on espionage charges -- says Americans should be concerned about letting the government go too far in the name of security.

"It's in secret so you don't really know," Drake explained. "It's benign, right. If I haven't -- and if I haven't done anything wrong it doesn't matter. The only way you can have perfect security is have a perfect surveillance state. That's George Orwell. That's 1984. That's what that would look like."

Fellow NSA whistleblower Bill Binney, who worked at the NSA for nearly four decades, says it's about the possibility that the government's stunning new capacity to collect, store and analyze data could be abused.

"It's really a-- turnkey situation, where it could be turned quickly and become a totalitarian state pretty quickly," he said. "The capacities to do that is being set up. Now it's a question of if we get the wrong person in office, or if certain people set up their network internally in government, they could make that happen quickly."

According to NSA's chief compliance officer John Delong, whose job is to make sure the laws and policies designed to protect the privacy of U.S. persons is being enforced, part of the frustration is that the rules are specific and secret.

"I think that's sort of the collision, is you have classified rules," DeLong explained during an hour long meeting with Fox News at the NSA. "You now have a somewhat more public data center,"  

"These aren't just, like, general policy pronouncements of 'You shall protect privacy.'" he said.  

DeLong added that another misconception is that there is only internal oversight, when he says there is "a tremendous amount of external oversight" from the Justice Department, the Office of the Director of National Intelligence and others.

In an email, Vanee' Vines, a public information officer for the NSA, said that the Utah Data Center will be "a state-of-the-art facility designed to support the Intelligence Community’s efforts to further strengthen and protect the nation. NSA is the executive agent for the Office of the Director of National Intelligence, and will be the lead agency at the center.”

Because the Utah Data Center is a "secure facility" and you cannot go inside without the needed security clearances, Fox News rented a helicopter and took to the skies, where the depth and breadth of the Utah Center were stunning.

The aerial video footage is exclusive to the Fox News investigation and posted here.  Two weeks after our filming, the helicopter pilot reported to our Fox News team that he had been visited by the FBI on a "national security matter."

The pilot said, according to the FBI agents, that the NSA had taken photos of the helicopter once it made several flyovers.  These photos allowed the NSA to identify the make and manufacturer of the helicopter in California who, in turn, told the NSA who operates it in the Salt Lake City area.

The FBI wanted to know if we had the proper air space clearances to flyover the site, which the Fox News team did.   Satisfied that the pilot was not flying "terrorists" over the site, the questioning concluded.  While the pilot passed along the Fox News contact information, there was no further inquiries.

Binney said the helicopter incident "showed the capability of the U.S. government to use information to trace people, their relationship to others and to raise suspicions about their activities and intentions."

Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent.

The 5 zettabytes figure is absurd; it is outright impossible, in fact.  For the center to hold 5 zettabytes of data, it would require them to have 1.25 billion 4TB hard drives, the largest size currently available from any manufacturer.  No one has even made a working prototype larger than that as of yet.  On top of that, only roughly 600 million hard drives are even produced worldwide in a given year, *let alone* the 4TB variety in particular.  And on top of *that*, even figuring extremely conservatively for the cost-per-drive, *just* the cost of the drives and none of the enclosures or server architecture or electrical bills or anything else, you'd end up at $100 billion, or roughly 10 times the entire annual budget of the NSA.  

It's simply not possible.  5 petabytes? Easily.  5 exabytes? Maybe. That would still be pretty enormous in cost; $100 million just for the drives. Maybe even enough shipments to cause a shortage and price spikes on the 4TB models.  But 5 *zettabytes*? No way.  Not possible today. It would be outlandish even in 2020, when Seagate says (optimistically) that they might be able to produce 20TB drives.