Inside mysterious 29-storey windowless New York building with 'dark secret'

In New York City there is a skyscraper which stands as a gigantic block of concrete, a windowless block in the skyline that stands out all the more for its lack of features.

Almost every skyscraper in the world has windows, but 33 Thomas Street in New York doesn't and that quite literal lack of transparency has stoked an awful lot of interest in the place.

If there's no way to see what's going on inside then it must be something secretive, surely?

Well, perhaps not, as the official name for the place is the AT&T Long Lines building and its stated purpose is to serve as a phone exchange or wire centre.

Basically, back in the days before everything went digital the phone lines had to be physically connected to their destination on the other end.

The brutalist building was finished in 1974 and houses communication technology (Roy Rochlin/Getty Images)

In the really olden days this required an operator to physically connect you but by the time 33 Thomas Street was being built in 1969 it had long since been done automatically.

That needs automatic infrastructure to do it and that stuff has to go somewhere, while technology marches on and these days the concrete monolith houses data and communications infrastructure with AT&T having moved out in 1999.

However, the secretive nature of being windowless has sparked all manner of speculation over what really goes on inside.

A man who said he'd been inside the building told the Daily Mail he'd worked on the place, and while inside there had been certain rooms he'd been forbidden to enter as well as some parts of the building that were entirely off-limits.

He also claimed he once found confidential papers in the building detailing what to do in the event of a radiation attack.

There are no windows in this building since machines don't need to look outside, it also helps protect the concrete slab against attack (Keyur Khamar/Bloomberg via Getty Images)

The windowless skyscraper has 29 floors and three basement levels, and according to The Intercept it was built to survive an atomic blast and safeguard the communications technology housed within.

There have also been claims that 33 Thomas Street also has enough supplies to keep 1,500 people alive for two weeks in the event of a disaster, but the building's security is there to keep the machines going rather than shelter life.

A major communications hub like it would be a prime target in the event of a conflict so it makes sense for it to be sturdy enough to survive a massive attack.

It's since been suggested that the site could even be used as a National Security Agency (NSA) surveillance site, with the likes of non-profit organisation The Intercept claiming that there was enough evidence provided by several sources, such as NSA whistleblower Edward Snowden.

A former AT&T engineer even claimed that there is a 'major gateway switch' which connects calls from the US to the rest of the world but in reality, we'll probably never know.