When you're out in the dark late at night, the last thing you want to hear is a blood-curdling screech that sends you scrambling back into the house.
As a certain Game of Thrones character once made clear, the night is dark and full of terrors, and that's even the case on board the safety of a cruise ship, as one TikTok user found out in a terrifying video.
The human body is capable of some pretty scary sounds, with the so-called death rattle letting people know that they usually have less than 24 hours to live, but even that doesn't come close to the nightmarish sound produced by the Aztec death whistle.
What is the Aztec death whistle?
Dating back to around 1250 to 1521 AD, the Aztec death whistle, or skull whistle, is a scary looking musical instrument that was found in ancient Aztec graves.
Although it initially left researchers baffled, one decided to blow on it and probably regretted it as it produced a piercing screech, similar to a human's scream.
Modern researchers have been able to recreate the death whistle using 3D printing technology, and welcomed a group of brave volunteers as part of a research study to see what impact the noise had on their brain.
Professor Sascha Frühholz, lead author of the study, said to the MailOnline: "The sound is rough and high-pitched, and as listeners, you usually do not like such sounds."
He added: "Since the Aztec skull whistle sounds close to a human scream, we wanted to investigate if humans have the same negative and aversive response to Aztec death whistle sounds."
Where and why was an Aztec death whistle used
Experts think that the whistles could have been historically used during battle or to scare victims during human sacrifice rituals, making the horrible noise the last thing someone heard before death.
Archaeologists remain unsure but after the study from Frühholz found that volunteers' brains were left stunned and confused, they seem to be in agreement that it could have been used as part of ritualistic sacrifices conducted by the Aztec civilisation - who thrived in central Mexico for nearly 200 years between the 14th and 16th century.
Chilling footage
So, it's pretty clear at this point that this sound isn't one you'd like to hear when you're out and about doing your daily business.
For those of you who aren't brave enough to listen to this scarring footage then not to fear as I've done it for you, and to me it sounds like a human shrieking mixed in with a kettle or steam train.
If you do want to listen to it, maybe turn the volume down if you're on public transport, unless you want to be accused of 'bare beating'.
In the footage, which was captured on a doorbell camera, a dog owner and his pet are innocently standing by a pool before a sound - believed to have been produced by an Aztec death whistle - can be heard in the background, and they quickly run back inside at full speed.
You can't blame them, as the last thing you'd want is to end up being sacrificed.