Sat among the myriad glittering skyscrapers of New York City is an imposing concrete building that looks nothing like its neighbours. It's not illuminated, has no windows and there's no real indication of what its purpose is.

The mysterious tower has long been an eerie curiosity lurking on the Manhattan skyline, however its true purpose may have been revealed as a top secret NSA spy base.

The 550ft skyscraper situated on 33 Thomas Street – a block away from the FBI's New York office – is commonly known as the Long Lines building, but it also has a codename, Titanpointe. the secretive structure has a dystopian, faceless facade like something out of a Judge Dredd comic, adds to the clandestine nature of what's supposedly going on inside.

An investigation by The Intercept, which is based on a short film called Project X, aims to blow open the doors of this Men In Black-style building in Manhattan as an undercover NSA spy hub. The report claims that documents obtained from NSA whistleblower Edward Snowden, together with public records and architectural plans, provides "compelling evidence that 33 Thomas Street has served as an NSA surveillance site, codenamed Titanpointe".

Built in 1974 during the Cold War, the 29-floor building – labelled as 'Project X' in the original architectural drawings – was intended to be a fortress to "house long lines telephone equipment and to protect it and its operating personnel in the event of atomic attack". Its solid structure is designed to withstand a blast, and reportedly would have been able to turn into a "self-contained city" for two weeks providing food, water and living space for occupants in the event of emergency.

When it was built, its purpose was to be a nerve centre for the New York Telephone Company to process phone calls, and today it is still in use by telecoms provider AT&T. But it seems they are not the only tenants.

NSA secret skyscraper New York
The structure is built to keep out an atomic blast and keep in all of its secrets Wikimedia Commons

The report reveals that the name 'Titanpointe' is referenced on numerous occasions in classified NSA documents regarding surveillance operations, and secret NSA travel guides for employees visiting New York City all but confirm the building's location at 33 Thomas Street.

For those visiting the building, the documents claim that employees are told to hire a cover vehicle in order to keep their identity secret and to not wear anything that would identify them as being affiliated with the NSA. Further evidence of Titanpointe's link to the NSA is seen from the parking bays outside the building, which are marked with a traffic code reserved for federal agencies.

The investigation claims supposed covert operations being carried out at Titanpointe include tapping into "phone calls, faxes and internet data", and its Blarney programme had snooped in on the communications of the United Nations, World Bank and "at least 38 countries, including close US allies such as Germany, Japan, and France".

The report also claims the NSA has been conducting a mass-surveillance programme called Skidrowe that collected data on public emails, Skype calls and internet history.

While documents point to NSA operations in the building, it doesn't outright link AT&T as working with the agency. Based on the high secrecy of their work, it's likely that AT&T employees are unaware of what's going on in the halls around them. One AT&T employee who worked at Titanpointe between 1981 and 1990 told the Intercept: "I wasn't aware of any NSA presence when I was there, but I had a creepy feeling about the building because I knew about AT&T's close collaboration with the Pentagon, going way back."

The NSA has kept silent on Titanpointe and the story, but an AT&T spokesperson was quoted as saying it does not "allow any government agency to connect directly to or otherwise control our network to obtain our customers' information", but went on to say it may bow to government requests on occasion.

Is the NSA really hidden away inside the Long Lines building? The report does point to rather convincing evidence, but the secrecy of the agency may well keep us in the dark for the time being. You could always try ringing the door bell – but don't expect an answer, unless you've got the magic password.