Outside experts disagree on the centre's potential. Some say it will just store data; others envisage a capacity to analyse and break codes, potentially enabling technicians to snoop on the entire population.

William Binney, a mathematician who worked at the NSA for almost 40 years and helped automate its worldwide eavesdropping, said the computers could store data at the rate of 20 terabytes - the equivalent of the US Library of Congress - per minute. "Technically it's not that complicated," he said. "You just need to work out an indexing scheme."

Binney, who left the agency in 2001 and blew the whistle on its domestic spying, said the centre could absorb and store data for hundreds of years and allow agencies such as the FBI to use the information retroactively.

He said the centre would probably have spare capacity for "brute force attacks"- using speed and data hoards to detect patterns and break encrypted messages in the so-called deep web where governments, corporations and other organisations keep secrets. There would be no distinction between domestic and foreign targets. "It makes no difference any more to them," Binney said.

James Bamford, author of , said the public had yet to grasp data-mining's significance. "It's basically a hard drive. It's also a cloud, a warehouse. It'll be storing not just text and audio but pictures and video," he said. "There's a lackadaisical attitude to this. People pay no attention until it's too late."

Brewster Kahle, a co-founder of the Internet Archive, a non-profit organisation which sucks up knowledge in a digital equivalent of the Library of Alexandria, said technology facilitated near-ubiquitous snooping. "If one had the opportunity to collect all the voice traffic in the US it would cost less than the Pentagon spends on paperclips," he said. "Storage these days is trivial, it's not a problem."

Some, however, dispute the image of Utah as an insatiable data monster. It would have no "operational function" - no eavesdropping or codebreaking - and would struggle to store data exploding from ever-expanding use of the internet, smartphones and tablets, said Matthew Aid, an intelligence historian.

"The intelligence people I've spoken to are warning of data crunch - a polite way of saying they're drowning. They say they don't have enough capacity and will be back to Congress looking for more money to expand," Aid said. If so, the site can do so. "It's designed to be modular, you can add clip-ons. There is plenty of land."

Revelations about surveillance programmes such as Prism and Boundless Informant caused a small backlash this week. Several dozen people rallied outside the state capitol in Salt Lake City, complaining of constitutional violations. "It's just plain creepy!" one placard read.

Others, however, have seen opportunity in the NSA's arrival. The University of Utah has created a data-management course for students in consultation with the agency, which has promised internships at the data centre.

Officials in Bluffdale shrugged off privacy concerns. "They're not a very open entity but if someone reads my emails they'll be pretty bored," said Mark Reid, the city manager. A deal to extend utilities to the centre, and to recycle water, helped open up scrubland for commercial development. "It's win-win," he said.

The NSA rejected a request from county mayors to tour the perimeter last month but Bluffdale mayor Derk Timothy was not offended: "I've never been inside the fence but it's kind of to be expected with a facility like that."

Revelations about surveillance did not prove abuse of power, he said. "I don't think they crossed the line. They've been good partners to us."