Over the past few months there's been growing concern over a bizarre YouTube phenomenon.
Dubbed Elsagate, the world's biggest video-sharing site has been flooded with videos featuring child favourites like Spider-Man, Peppa Pig and Elsa from Disney's Frozen, combined with violent, sexual and disturbing content.
Children are reportedly stumbling across the videos as they pop up as suggestions in apps like YouTube Kids. At first they seem like normal kids' videos, but soon devolve into highly inappropriate material.
A blog post earlier this month titled 'Something is wrong on the internet' brought the issue to wider attention, detailing many of worst Elsagate trends such as characters - and occasionally even real children - being forced to consume faeces and urine, enduring injections, suicides, drownings, being buried alive, murders, rapes, swapping heads, cannibalism, babies crying and extreme violence.
"Someone or something or some combination of people and things is using YouTube to systematically frighten, traumatise, and abuse children, automatically and at scale," wrote James Bridle, whose article has now been read by hundreds of thousands of people.
It appears to have sparked YouTube into action. On Wednesday, the Google-owned site announced a crackdown on content that "attempts to pass as family-friendly, but is clearly not". Many of the channels and videos Mr Bridle mentioned have since been taken down, but many thousands remain.
The term Elsagate appears to have been coined in early July, according to Google Trends data. Tens of thousands of people have joined the Elsagate community on popular web forum Reddit to discuss the phenomenon.
But it remains a mystery just why thousands of accounts are posting these videos, some of which have had tens of millions of views. Is it all about making money? Or is something more sinister going on?
Here are the three main theories that have been suggested.
It's been claimed the videos are a sly way of getting young children accustomed to sexual and disturbing content so they'll be more easily groomed by paedophiles.
The regular use of needles is said to be to normalise injections used to sedate victims; pooping, to get them used to removing their clothes; abortions, to normalise pregnancy and terminations; violence, to show them what will happen if they don't comply; and crying babies,to get them used to being in a constant state of worry and terror.
Some think the videos contain coded messages to other paedophiles about where to find illegal content - both online and in real-life. Many comments appear to be gibberish, but it's claimed when they are typed into a keyboard using other alphabets - such as Arabic or Thai - and translated back into English, they suddenly make sense.
A third suggestion is the videos aren't meant for kids at all - they're for people with child and/or superhero fetishes. Some of the live action videos take place in large, pricey-looking homes with professional lighting rigs.
For as long as the internet has existed, people have been posting awful content on it. Older web users will never forget unwittingly stumbling upon Goatse or Tubgirl, for example. (Note: Do not google those images if you are at work, are underage or are offended easily. You have been warned!)
Notorious free-for-all web forum 4chan is often blamed.
The sheer volume of bizarre and disturbing videos perhaps suggests there's something else going on, however.
The most obvious explanation is that it's all about money. Many of the videos rack up tens of millions of views, with some channels' numbers running into the billions. All of these views of the cheaply-made clips adds up to big profits.
But why is the content so disturbing? Aren't there easier ways to get kids clicking?
One explanation could be the content of the videos isn't dreamt up by humans - it could be driven by algorithms. Clips featuring Elsa and Spider-Man would be popular for obvious reasons, so it doesn't take a genius to realise putting them together would equal clicks. From there, it was perhaps only a matter of time before crying babies and violent content joined them.
That might be how YouTube got populated with videos with names like 'Bad Baby with Tantrum and Crying for Lollipops Little Babies Learn Colors Finger Family', 'Rapunzel & Minnie vs Natural Disaster!' and 'TONGUE BIT OFF! Pregnant Mommy Bit Off Tongue Eating Watermelon Doc McStuffins Checkup Farting Mommy'.
"Online kids' content is one of the few alternative ways of making money from 3D animation because the aesthetic standards are lower and independent production can profit through scale," explains Mr Bridle.
"It uses existing and easily available content (such as character models and motion-capture libraries) and it can be repeated and revised endlessly and mostly meaninglessly because the algorithms don't discriminate… and neither do the kids."
In a video on Elsagate uploaded by popular YouTuber Tim Pool, it's claimed many of the videos use exactly the same animations and storylines - if they can be called that - but with different characters, so are likely created entirely by computers with little human input.
"There is a computer programme that simply has the characters and the thumbnails ready to go as wireframes, then changes the cels - the characters' animations - then automatically uploads them with the most popular tags that are on YouTube. This is why we Donald Trump and Hitler fighting. This is why we see Hitler using the toilet... this is what happens when an artificial intelligence tries to give us what we want."
Toddlers can't type what they want into the search bar - they click on whatever YouTube suggests. Just as news websites nowadays tailor their headlines to rank highly on Google, producers are mashing together disparate topics to get their clips high on YouTube.