Gifs are short, silent animations. Unlike videos, they carry no sound files within them. Which is why it's extremely odd that a lot of people online claim to be able to hear one.
It's a gif you've probably seen before as it resurfaces every few months, always with a similar caption: Someone asking "why can I hear this", usually accompanied by a few crying faces to show how distressing they find the experience.
The gif, created by Twitter user Happy Toast, has resurfaced again after a scientist put out an appeal for help understanding why people hear a noise.
Can you hear it? You're not alone.
Dr Lisa Debruine, a researcher at the University of Glasgow, also included a poll to see how many people could hear the gif. So far, 75 percent have said that they could hear a thudding noise.
A further 4 percent have said they hear "something else" when watching the gif.
So what is going on?
Well, first off, it's definitely not just this gif. Other gifs have been posted all over the Internet that people have claimed they can hear, such as this one where you can hear elephants on a see-saw...
... and this gif you can't look at without hearing the Queen classic We Will Rock You.
We also know that our perception of sound can be influenced by visual information in other ways, it's not limited to soundless gifs. The McGurk effect, shown in this video from the BBC's Horizon program, shows how your brain can be tricked into hearing different things based on the visual information you are perceiving at the time.
In this case, you won't be able to tell whether you're hearing "baa" or "faa" because of the way the man's mouth is moving. What you see can override what you hear.
But is it possible for visual stimulus alone to cause people to hear sound? Short answer: Yes.
A study earlier this year found that 22 percent of participants could "hear" faint sounds when they were shown a flash of light, even though no sound occurred.
We already knew that some people in the population (around 5 percent) have synesthesia, a phenomenon where information received from one sense (e.g. sound) is perceived by another (e.g. taste) automatically and involuntarily. However, this study showed that a lot more of the population "heard motion" – hearing sounds in response to visual stimulus – than was previously thought. It's not an effect limited to synesthetes.
So if it's possible to trigger an auditory response using a simple flash of light, this gif of the power pylons skipping may just be a particularly good example of how a stimulus can cause this effect, hence why so many people seem to "hear" it.
People online have suggested the gif may be especially good at producing this phenomenon because of the camera shake, adding to the illusion that if it's so big it's causing the ground to shake, you should be hearing it.
Something which Lisa would like to test herself.
However, it could also partly be down to the power of suggestion. I.e. you are hearing the sound because the caption above the gif is implying that you should.
So what do you think? Could you hear the gif? And if you did, were you only hearing it because we told you you should?