If you take a trip to New York City, you might find yourself passing right by a monstrous monolith of concrete known as 33 Thomas Street.

Besides the ground floor entrance, there are absolutely no windows on this skyscraper, resulting in this brutalist behemoth holding an air of mystery.

If you can’t see inside the windowless skyscraper, then they could be doing anything in there – perhaps this is where the US government has been stashing those aliens all along while we’ve all been thinking about Area 51.

One man who has been in there spoke to the Daily Mail on condition of anonymity about what he saw when he was hired to install something in the building in the early 2000s.The man and his son said they weren’t allowed into certain parts of the windowless skyscraper, which was built in 1974, and there were bits of the building that were off-limits to any and all visitors.

This is 33 Thomas Street, a mysterious windowless skyscraper in New York City. (Roy Rochlin/Getty Images)

So far, so reasonable. There are plenty of buildings where you’re not just allowed to wander in and have a poke around at whatever bits you like.

Plus, just because you’ve been hired to do some work on a place doesn’t mean you’ll have the run of the entire building – though feel free to unleash your imagination on the question of what they might have to hide.

Slightly more suspicious is that the man and his son ‘couldn’t ask what’s inside or why we couldn’t go in’ the bits of the building they were forbidden from attempting to access.

It’s not just what he saw, but also what he wasn’t allowed to see.

So obviously it’s aliens, or maybe just stuff they don’t want to talk about and let strangers have a fiddle round with.

Besides, when you have a windowless skyscraper, you might want to cultivate an aura of mystery just for the hell of it.

Besides the entrance there are no windows at all on this building (Roy Rochlin/Getty Images)

What they did stumble upon and see were documents detailing what to do with certain machines in the building in the event of nuclear fallout.

When it comes to 33 Thomas Street, however, this baby can apparently be self-sufficient for two weeks in the event of a nuclear catastrophe with enough supplies to keep 1,500 people cared for, which might be important considering what’s officially in there.

The official story is that 33 Thomas Street was built to house phone switching equipment, and it was formerly called the AT&T Long Lines Building.

Basically, it was a place with equipment that connected long distance phone calls – however, it doesn’t mean it hasn’t been safe from a few conspiracy theories.

A few years back, the New York Times reported that this windowless skyscraper could also be a supposed listening post for the National Security Agency (NSA) codenamed ‘Titanpointe’.

They also pointed towards a documentary called Project X and a 2016 report from The Intercept which claimed that the building had some parts of it dedicated to surveillance.

LADbible has contacted AT&T for comment.