Although skyscrapers and apartment blocks usually have windows (after all, people do live there) there are numerous cities around the world that are home to mysterious windowless skyscrapers.
The buildings have caused a lot of debate online, with many wondering why they have no windows, while others are desperate to know what goes on inside.
Even Tom Hanks has asked the question. In a tweet back in 2017, he shared a snap of 33 Thomas Street in New York, and asked: "This is the scariest building I've ever seen! WTF goes on inside?"
The building has no windows (Roy Rochlin/Getty Images)
Well, one man has answered the question.
Eric Guidry, who goes by @e.guidry on TikTok, shared what the buildings were really used for.
Turns out it's not exactly the supernatural conspiracy some people thought.
"So that building in New York, this windowless building in San Francisco, this one in Chicago, this one in Austin and hundreds more buildings placed conspicuously across the country, in very populous locations," Guidry said.
It turns out that the buildings are all part of AT&T's long lines network.
"Before we had digital telephony, the lines on your phones physically connected to the person on the other end's phone.
"That meant we need these massive switching hubs. Most of those buildings I showed were built in the '70s [or earlier].
"That means they didn't require actual operators.
"And because this machinery didn't particularly care if there was light or not, they just decided, why add windows?"
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33 Thomas Street is located in New York (Roy Rochlin/Getty Images)
Giving more information on the specific AT&T building in New York featured in the original TikTok video, Eric said: "That building is 33 Thomas Street, which is known to be one of the most secure towers in the world.
"It has enough gas, water and electricity to be able to continue running independently for up to two weeks and is supposed to be able to actually withstand a nuclear blast... within reason.
"Although most of the equipment has been digitalised and you don't need it for phones, it's still critical for network and internet infrastructure.
"In fact, if you're watching this in the US, this video probably went through one of those buildings."
However, several conspiracy theorists believe that the building has been used as a National Security Agency (NSA) surveillance site.
And in 2016, non-profit organisation The Intercept claimed that there was 'compelling evidence', including leaked documents from NSA whistleblower Edward Snowden, that pointed towards the building having been used as a surveillance site, code-named TITANPOINTE.
According to their 2016 investigation, a former AT&T engineer said there is 'a major gateway switch' that 'routes all calls between the US and other countries around the world'.
Other theories are a bit more far-fetched - ranging from vampires to 'lizard headquarters'.
