One dog owner had a terrifying experience while taking their pooch out late at night, when a hair-raising noise made them both race back inside to safety.
Footage of the spooky sound was captured on a doorbell camera, which shows the pair standing by a pool in their backyard before sprinting indoors after hearing a shriek.
The unknown noise, which was shared by X account @CreepyOrg, is eerie enough to send shivers down your spine - you'll want to check it out for yourself:
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But what is it?
It could very well be an Aztec Death Whistle… or at least a modern replica of one.
The death whistle was first discovered in 1999, when a ceramic skull shaped whistle was found by archaeologists while excavating an Aztec temple in Mexico City, the Daily Mail reports. It's believed to have been used as a ceremonial way to mark death.
The whistle was reportedly located in the arms of a skeleton, which was holding skull-shaped instruments in each hand - possibly a symbol of Mictlantecuhtli, the Aztec god of the underworld.
One theory is that warriors would use the whistles during battles, but based on its location at the base of a temple, its death-related imagery, and the lack of whistles ever discovered at battle sites or in warrior graves, others believe it is likely they were used for ceremonial or religious practices rather than for warfare.
According to Tomás de Torquemada, a Spanish friar, an Aztec general would carry ‘a drum on his shoulders which he played at the start of a battle while others blew large shell trumpets’.
But music archaeologist Arnd Adje Both, who investigated the whistles, told HowStuffWorks that theory is just a 'possibility'.
He added: “Up to this point, we haven't excavated an individual classified as an Aztec warrior with such an instrument around their neck. For now, it seems to be more of a ritual instrument.”
Meanwhile, scientist James J. Orgill tested a 3D printed whistle and made a YouTube video saying that archaeologists first thought the instrument could be a toy.
He said: “It wasn't until 15 years later for some reason a scientist blew into the hole in the top of it and this is the sound that came out.”
So, a modern-day version of the Aztec Death Whistle could be the answer to the mysterious sound picked up by the camera late at night.
Of course, there could be a much simpler explanation to all of this – some have suggested the noise could have come from an animal, like a mountain lion or a fox. But that's a whole lot less fun, isn't it?