The most suspicious building in America that people believe is the home of the National Security Agency

The old saying goes that hiding in plain sight is one of the best forms of disguise.

By that rationale, adding another towering skyscraper to Manhattan’s skyline would be one of the best ways to hide a building in the city.

But the purpose of the hulking 550ft structure at 33 Thomas Street in New York seems to have been rumbled – mainly because its windowless.

Everyone from Tom Hanks to followers of the web’s darkest conspiracy theory corners are convinced the brutalist tower is actually the National Security Agency’s secret base.

The fortress-like structure is also rumoured to have been built to withstand a nuclear blast and contains its own power and water supply, as well as enough supplies to shelter and feeds its inhabitants for two weeks after a terror attack or disaster.

Tom Hanks recently tweeted about the concrete and granite monstrosity: “This is the scariest building I’ve ever seen! WTF goes on inside?? Hanx.”

He was flooded with replies sharing information about the purposes of the structure, constructed and owned by AT&T, and known officially as the ‘Long Lines Building’.

So, let's breakdown the facts about one of America’s most mysterious constructions.

It was first suggested to be an NSA base after documents leaked by Edward Snowden came to light. They said the structure, codenamed Titanpointe, was spying on phone calls, fax messages and Internet data.

First developed under the name ‘Project X’, it opened in 1974 about 14 blocks north of Wall Street.

One employee at US telecommunications titan AT&T told The Intercept it housed a global 'gateway switch' that would allow workers to snoop on virtually every call and web communication coming in and out of America.

The program has allegedly been used to spy on communications to and from the UN, the International Monetary Fund and the World Bank – with the NSA having apparently spied on around 40 countries.

The FBI field office is two blocks from the Long Lines Building, and it’s said Long Lines operatives can pick up 'cover vehicles' from the agency.

Anonymous whistleblowers also say spies have codenamed AT&T ‘Lithium’.

A satellite dish on the skyscraper is believed to link to the Skidrowe program, which intercepts satellite data including emails, Skype calls, passwords, and web browsing histories.

Papers leaked by Snowden also link the building to the Blarney program, developed in the early 1970s to mine communications data.

Among the other string of codenames for Long Lines is 'AWM' – as the building has parking spaces marked with the initials, understood to denote shadowy spy agencies.

Designed by architects John Carl Warnecke & Associates, their grand vision was to create a communication nerve centre that resembled a '20th century fortress, with spears and arrows replaced by protons and neutrons laying quiet siege to an army of machines within’, according to Atlas Of All Places.

Top-secret drawings dated April 2012 show within the high-security building there is 'NSA controlled' equipment linked to the routers of its 'access partner' – referring to AT&T.

Fletcher Cook, an AT&T spokesperson, told The Intercept the company does not 'allow any government agency to connect directly to or otherwise control our network to obtain our customers’ information'.

He added: “Rather, we simply respond to government requests for information pursuant to court orders or other mandatory process and, in rare cases, on a legal and voluntary basis when a person’s life is in danger and time is of the essence, like in a kidnapping situation.”

But Long Lines continues to operate under virtual mystery, with its activities allowed to go on under the nose of New Yorkers despite outrage from human rights and privacy campaigners who claim the NSA are more snoops than anti-terror operatives.

Mogens Lykketoft, former president of the UN’s general assembly, once blasting: “Such spying activities are totally unacceptable breaches of trust in international cooperation.”

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