A bunch of windowless buildings across the US have sparked the curiosity of millions - including many on TikTok.
Thankfully, there is somebody on hand to explain the history of these unusual buildings. You can find out more in the vid below:
]Eric Guidry, who goes by @e.guidry on the video-sharing platform, responded to a clip from someone who was curious about the presence of the buildings.
He said: "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."
It's understandable that people would be curious. Even Hollywood stars like Tom Hanks have pondered what the buildings are used for.
Sharing a picture of 33 Thomas Street in New York, the Forrest Gump star tweeted in 2017: "This is the scariest building I've ever seen! WTF goes on inside?"
This is the scariest building I've ever seen! WTF goes on inside?? Hanx. pic.twitter.com/nXeUI64rXm— Tom Hanks (@tomhanks) June 2, 2017
It turns out the buildings are all part of AT&T's long lines network.
Eric explained: "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?"
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."
Plenty of social media users have bee spooked by the buildings.
One person replied to Hanks' tweet: "Anything & everything that goes out via technology is stored here. You can delete stuff but it's all stored digitally forever.
"You can run & hide but the past will eventually find you no matter what you did or where you go."
Another simply added: "Spies."
I don't know about you, but I think I'll stick with Eric's explanation.