A chilling life-saving moment was documented by a deep sea diver exploring a Mexican cave.
The scuba diver, who goes by TomCat686 on YouTube, was diving in an extremely dark part of the cenote cave.
Along with other divers in his party, he came equipped with a torch and shone the light around the cave when he came across something unexpected.
His fellow diver was the first to go over to a small picket spot the sign and, as Tom swam closer to it, its message became clear. Take a look:
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Alongside an image of the Grim Reaper, it read the words 'STOP' in large, bold lettering.
The sign continued: "PREVENT YOUR DEATH! GO NO FARTHER."
It proceeded to list some facts about the dangers of deep sea diving as a further deterrent.
"FACT: More than 300 divers, including open water instructors, have died in caves just like this one," the first one read.
"FACT: You needed training to dive, you NEED cave training and cave equipment to cave dive.
"FACT: Without cave training and cave equipment Divers CAN die here.
"FACT: It CAN happen to YOU!!!"
It concluded that there was 'nothing in this cave worth dying for' as it doubled down on point that you shouldn't dive past that point.
The video was shared back in May 2012 and has since generated almost 3.5 million views.
People have gone on to applaud the unknown person for putting the sign there and probably saving dozens of lives.
"Props to the folk who put the sign there, literally saving lives for the curious souls," one person said.
"Huge props to that guy who saved numerous scuba divers' lives by putting up that sign," echoed a second.
Another added: "We must commend the soul who went down there and posted that sign, never realized how dangerous cave diving was until just now."
While the sign issued a very important message, others described it as 'eerie'.
One individual penned: "This sign is one of the eeriest things I’ve ever seen. It definitely hits hard. I’m getting chills and I’m just standing in my house, miles away from the nearest coastline."
"Must send chills down your spine to read that down there no matter how experienced you are," agreed another.
Meanwhile, another YouTube user compared the 'surreal' warning to 'like reaching the edge of a map in a video game'.
While it's pretty eerie, it's definitely not a sign you'd want to ignore.
Niamh Shackleton