The Underwater Lakes That Shouldn't Exist

A mile below the surface of the Red Sea is something that, by all measures, shouldn't exist. A phenomenon so strange it's rewriting what we know about the origins of life on Earth, and perhaps even outside of it. In 2020, the Ocean Explorer research ship launched a submersible into the depths of the Red Sea. After eight hours of scouring the abyssal plain, they had found nothing. Just a lifeless desert in the perpetual darkness. But moments before they were about to give up and return to the surface, the submersible's headlights caught something in the distance. A murky shape, about 20 metres wide, which looked like a dark mirror laying flat on the seabed. Squinting, they made out what appeared to be a thick fog hanging over it. And as they got closer, they realised what they were seeing should have been impossible. The surface of that giant mirror wasn't glass, or rock, or even sand. It was water. How could that be? They were already underwater. But it was right there in front of their very eyes. What appeared to be an underwater lake at the bottom of the ocean, encircled by a beach, with gentle, slow-motion waves lapping against the shore. Only a handful of humans have ever seen this sight in person. And we now know something that makes it even more eerie. Any creature who dares to venture below the surface of that strange water never comes back. And yet, down here in the deathly darkness, there is something unthinkable. A strange, primitive form of life, existing in a way completely alien to what we ever thought possible on Earth. But now, in 2025, thanks to a series of remarkable discoveries, this surreal underwater environment is helping to revolutionise our understanding of how life can exist on Earth. And maybe, beyond. So where did this mysterious underwater lake come from? What could possibly live here? And whatever it is, could it be the key to understanding how life began? I'm James Stewart, and you're watching Astrum Earth. Join me in this video as we journey beneath the surface and come face to face with whatever is hiding down there. Now, I should start off by saying, despite making giant leaps in our understanding of underwater lakes over the last few years, Ocean Explorer was not the first to find one. The earliest reference to an underwater lake dates back to the 1st of March 1964, and it's a tiny article barely taking up a quarter of a page buried deep in a scientific journal. The author, a humble English oceanographer called Henry Sharnock, had found what he called anomalous bottom water, two kilometres under the Red Sea. I like to imagine the wonder, and perhaps even the terror, that Henry felt when research indicated it was possible for a lake to exist at the bottom of the ocean. What it was, or why it existed, he had no idea. All he knew was that it was there, and that it should have been impossible. Without knowing it, Henry had stumbled on something that would shape how we understand life on our planet forever. While he could only guess at the water's origin, Henry spotted one intriguing detail. It was incredibly salty. Far saltier than the water above it. This seemingly minor observation gives us the first clue to finding out what these bizarre formations actually are. It seems impossible. How does a lake in the middle of an ocean exist? One thing I am not salty about is my choice of skincare. We've once again partnered with our friends at Geology to bring you skincare that doesn't just float, it makes waves. If I'm honest, my old skincare routine was pretty washed up, but since using Geology, it's gone from brine to fine. Geology is a 41-time award-winning brand, separated from its competitors like brine, separated from seawater. Their salicylic acid face wash, total power wash for your face, washes away impurities like a riptide without leaving you high and dry. Retinol night cream, smooth saline to reduce wrinkles, and if you're tired of stripping your scalp like a tide pool in low tide, their co-wash keeps your hair anchored and healthy. Take their 60-second quiz today and get a personalized routine shipped and delivered right to your door. And the best part is you can soak up 70% off your personalized trial set. So what are you waiting for? Wade on in. And unlike fish wandering into a brine pool, you'll definitely come back to Geology. See, the more salt dissolves in the volume of water, the more dense it gets. That's why you might be able to float on your back in the sea, but not in a swimming pool. The higher salt content in the seawater makes it more dense, so relatively speaking, you become less dense and rise to the surface. It's the same with an underwater lake. That incredibly salty water is more dense than the seawater around it, so it sits at the very bottom of the ocean, collecting in the natural depressions on the seafloor. In fact, the water here is so salty and so dense that a submersible literally bounces off the surface like a trampoline. Theoretically, if you were able to breathe and survive the pressure down there, the surface of the lake would feel like a giant waterbed. Scientists call these underwater lakes brine pools, and they can stretch up to 20km in length, forming majestic lakes, rivers, and even underwater waterfalls. But even more curiously, they can reach hundreds of metres in depth. So you might be wondering, what stops the brine from just spreading out and mixing with the surrounding seawater? How is there such a surface line? After all, if you were to add a splash of brine to a glass of fresh water, it wouldn't sink and collect at the bottom of the glass, would it? It would just mix in with the water, right? So what stops this from happening at the bottom of the ocean? Well, the answer comes down to two distinct natural processes working together in a unique way. The first has the complex name of double diffusive convection. And once again, Henry's observations gave us the first clue of what was happening here. That anomalous water he noticed wasn't just unusually salty, but unusually warm. So imagine, at the very bottom of the sea, you have this anomalous water that is both warm and salty. Directly above it, you have water that is cooler and less salty. This means that both salt and heat in the bottom water want to spread upwards to the top water. In theory, this should mean that the extra salty water just dissolves away and gets lost in the rest of the ocean. But here's the key thing. Heat spreads 100 times faster than salt, so naturally, it gets there first. This means that the water directly above the surface of the lake is warmer than usual, which causes it to rise into the water above. Meanwhile, on the other side, the water directly below the surface of the lake gets cooler, which makes it sink down into the lake. As a result, the top layer of the lake wants to move further down into the lake, rather than up out of it. And the seawater directly above it wants to move up and away from the lake, rather than down into it. This double diffusive convection is what creates that sharp boundary between brine and seawater, which causes a clear tide line to form, resulting in the surreal appearance of an underwater lake. But if you're especially eagle-eyed, you might have noticed something wrong with that model. Even if there is a clear-cut threshold between brine and regular water, there is still salt going across that line. So effectively, the lake is constantly losing brine to the water above. In theory, then, the lake should vanish, like a rock pool evaporating over time. And that's where the second natural process comes in. See, underwater lakes aren't just puddles of salty water. They're constantly in flux. Just as brine is continually vanishing into the water above, it's also being replenished from underneath. The lake exists only because of the delicate natural balance between these two opposing processes, the first trying to shrink the lake and the other to grow it. So where is it being replenished from, and what is this seemingly endless source of warm salty water coming from deep in the earth? You may have heard of plate tectonics, but to answer this question, we need to understand salt tectonics. Scientists scratched their heads about this for decades, until eventually the answer began to reveal itself. Across the entire world, we found brine pools in just three places. And these three places all have one thing in common, a weird geological feature which allows these pools to form. The first we've mentioned already, the Red Sea. The other two are the Gulf of Mexico and the Mediterranean Sea. So what's the underlying feature they have in common? They're all bodies of water that aren't giant oceans perhaps? They're all partly enclosed? Well, that's part of the answer. But here's the key thing, at some point in their history, all three of these water bodies were cut off from the ocean. During the Jurassic period, the Gulf of Mexico was sealed off from the Atlantic Ocean by a narrow strip of land. More than 100 million years later, so was the Mediterranean. And by this point, the Red Sea had also been shut off from the waters of the Indian Ocean. When these seas got closed off from the ocean, they essentially became giant basins. With no new water flowing in, the remaining water in those basins evaporated over time, leaving behind something unusual, a thick residue of salt. Over the following millions of years, that salt residue was buried by layers of sediment which pushed it deeper and deeper into the earth. So when those dried up basins eventually rejoined the oceans and filled with seawater again, they created a unique set of geological conditions. Above you have a sea, and below you have an enormous pile of salt. It's only because of the sediment layer in between that the salt doesn't come into contact with the seawater. But you might be able to guess what happens next. As the sediment keeps piling up over time, and the salt gets buried deeper into the earth's crust, something extraordinary happens. The sediment on top gets so heavy that it begins to squash the salt underneath, which forces it to shift and move around. This process, known as salt tectonics, causes warping and cracking in the sediment layer, which allows seawater to seep down over time and mix with the ancient salt below. This forms an ultra-concentrated brine deep down in the earth. As the sediment continues to move around, that deep underground brine, warmed by the earth's heat, gets squeezed up into the ocean and settles on the ocean floor. Now at last, finally, we've got ourselves an underwater lake. But brine doesn't just sit in lakes, waiting for its next victim to enter its domain. Brine can also flow underwater too. During a dive in 2018, the remotely operated vehicle that was part of a NOAA expedition to the Gulf of Mexico peered over a low rise in the sea floor, gazing towards the sediment-line depression that lay below. It spotted a small pool, filled with the dark blue water we're accustomed to seeing with brinefalls, accompanied by the graveyard of dead mussels and bright white shells that had fallen victim to it. But as the camera panned up, away from the pool, it captured something even more dramatic than the brinefall itself. Described by NOAA as long blue-grey-black streaks of stained sediment, juxtaposed against the tan coloured slope above the brinefall. Tracing the sediment path upwards, brine appeared to be flowing from above, down the slope and into the pool below. What they had found was an undersea brine waterfall. It's possible this particular waterfall began to form over 180 million years ago, during the Jurassic period. Subsequently, it has cut upwards towards the sea floor in columns and ridges known as diapirs. The concentrated brine fluid, much denser than salt water as we've discussed, can flow onto the sea floor. The diapirs act like channels, directing the brine towards the low-lying areas it occupies, and creating the waterfall. But what manner of creature could possibly survive down here, if indeed anything can survive at all? Phew, okay, so now we know what these underwater lakes are and where they come from. Let's dive into the real question. What is lurking down there, and what does it mean for us? At a first glance, the conditions don't exactly seem ideal for life. Unsettlingly, scientists found the shorelines littered with the corpses of fish, crabs, lobsters and eels. What they didn't know is that some of those corpses had been there for decades, preserved in the super salty water, like an underwater mass grave of mummified sea creatures. But they soon realised what was happening. Whenever a fish or eel passed by, which admittedly was quite a rare sight on the deep abyssal plain, scientists noticed they mostly avoided the briny water. And when they did wander below the surface, they rarely came back. Within a few seconds of diving below the lake, they were struck by violent convulsions. Fish writhed uncontrollably, and eels wrenched themselves in knots. This toxic shock is caused by the extreme level of salt, and it's the fate of any unlucky creature that ventures too deep into the lake. Once it strikes, their only hope is to rise above the surface into the familiar ocean water above. Most, however, are not so lucky. Because of these extreme levels of salt, underwater lakes are also anoxic, meaning there is no oxygen for life to survive. At least, not as we know it. And if that weren't inhospitable enough, the salty anoxic water is also loaded with toxic methane, which should surely snuff out any remaining chance of life. You would really think this is the worst place on earth to live, wouldn't you? A toxic hot tub of death, miles below the ocean's surface. No oxygen, no light, alone in the perpetual darkness of the deep sea abyss. And yet there is life. A host of fantastical creatures living in one of the most extreme habitable environments on earth. Vast colonies of bright green tube worms stretch for hundreds of meters in all directions. Giant hundred-year-old mussels pack tightly around the shores of the lake. This mussel beach is home to a carnival of shrimps, snails, weird squat lobsters, and jelly-like crabs. Even larger predators like fish, octopuses, and cutthroat eels come to the lake in search of food, like a watering hole on the African savannah. But most successfully of all, hagfish. These slimy eel-shaped fish lurk on the water's edge, waiting for unsuspecting prey to wander into the brine. The moment that toxic shock takes hold, the hagfish swoops in and snatches their body for food. These hagfish are ancient creatures, even older than dinosaurs. Often called living fossils, they're strikingly similar to the very first vertebrates to appear on earth hundreds of millions of years ago. But recently scientists noticed something even more curious about them. They are the only animals that seem to be immune to the toxic effects of the lake. While all others die in seconds, hagfish come and go as they please, diving into the lake to catch their food. This, among other things, is a mystery yet to be solved. Even more staggering than how our hagfish thrives down here is how the ecosystem itself does, because it involves a toxic gas. So how on earth does this alien ecosystem actually survive? Not only is in fact the submersible's headlight is the first light that has shone here since the ocean was formed. Before deep sea ecosystems like these were discovered, scientists thought all life was powered by the sun. Green plants soak up the sun's energy and turn it into food through photosynthesis. These plants, which are then eaten by animals, form the base of the food chain. Oddly enough, this applies even in the deep ocean where the sun doesn't reach. Life here still survives on morsels of food that drift down from the surface, a marine snow made of dead plankton, sea plants and other food, all of which derive their energy from the sun. But when researchers first stumbled upon brine pool environments, it was clear that this was something different, an oasis of life completely independent from the sun. Instead, these creatures seem to have their own mysterious source of life-supporting energy. So if it's not the sun, what do they live on? To find the answer, we need to look inside the mussels that cluster around the shores of the lake. Under a microscope, scientists found that the interiors of their shells were packed with bacteria. Now these weren't ordinary bacteria like the kind we see every day. These were specialised bacteria with one extraordinary ability, which holds the key to the survival of the entire ecosystem. They can turn chemicals into food. The specific chemical is methane, and fortunately for them, brine pools are loaded with it. This is called chemosynthesis. Just as green plants turn sunlight into food through photosynthesis, these bacteria can do the same thing with methane spewing out from deep in the earth. So unlikely as it may seem, the toxic soup, which kills almost any animal that touches it, is actually the very thing that keeps them alive. Just as plant photosynthesis is the basis of life for all animals in the sun, these chemosynthetic bacteria turn the toxic methane in the pools into the lifeblood of an entire ecosystem. The bacteria themselves live deep in the gills of the mussels, both depending on the other for survival. In return for supplying them with energy, the mussels protect the bacteria from grazers and filter feeders. At the same time, they also provide a shelter to the shrimps, lobsters and crabs that live and feed amongst them. Such an abundance of life in the middle of a vast and otherwise barren abyssal plain, totally independent of the sun's energy, astounded the scientists who first found it. In fact, it seemed to them that these ancient lifeforms were a window into how life might have first begun. And now, in 2025, scientists increasingly wonder whether nearly 4 billion years ago, the first life on earth might have appeared in an environment just like this. Previously, it was a very different story. In a scrawled letter from 1871, Charles Darwin suggested that life first appeared in, as he puts it, some warm little pond with all sorts of ammonia and phosphoric salts. While it might seem so at a glance, Darwin's little pond was certainly not an underwater one, but rather a shallow tidal pool with heat, light and electricity present. This gave rise to the popular primordial soup theory, which imagined life was born in a warm, nutrient-rich rock pool, spectacularly flashed into being by a bolt of lightning. This idea persisted for almost a century. But in the late 1900s, when we discovered deep-sea chemosynthetic environments like brine pools, everything we thought we knew about life's origins was turned on its head. If life could clearly survive without the sun, then could it have started in a similar way? The first of these chemosynthetic environments to be found wasn't actually a brine pool, but something called a hydrothermal vent. Hydrothermal vents are enormous smoking chimneys at the bottom of the deep ocean, spewing out torrents of superheated chemical soup from deep in the earth. Much like brine pools, around these vents scientists found an abundance of life, surviving not from the sun's energy, but from the energy in that chemical soup, harvested by colonies of chemosynthetic bacteria. The discovery was groundbreaking. Even today, more than 50 years after they were first discovered, the conventional view says that hydrothermal vents are the most likely site for the very first life on earth. But now there's another contender. Before brine pools were discovered, it was inconceivable that life could survive in such extremely salty conditions. But in recent years, the uncovering of rich bacterial and complex life there widens the goalposts for how and where life can survive. And there's one even more tantalising piece of evidence that points towards a brine pool-like environment as the potential cradle of life. We know life almost certainly appeared somewhere in the ocean between 3.5 and 4 billion years ago. But here's the interesting thing. Scientists think earth's primordial oceans at the time were number one, poor in oxygen, number two, rich in methane, and number three, most intriguingly of all, extremely salty. What else is poor in oxygen, rich in methane, and extremely salty? A brine pool. What this means is the brine pool environment is remarkably similar to the conditions in earth's ancient oceans at the exact time life began. That's why scientists who study them think they might be the best analogue we have of the place where life first began. And to take this a step further, could these alien environments guide the ongoing search for life beyond earth? While this, after all, is a channel about earth, not space, let's just take a moment to imagine. If some environments similar to brine pools were to exist elsewhere in the solar system, or even beyond that, it would be as good a place as any to find life. Two of the most likely contenders are Europa, a moon of Jupiter, and Enceladus, a moon of Saturn. This is because beneath their icy crust, they both have vast underground oceans of liquid water. What's more, these oceans are likely as salty as the oceans on early earth, potentially creating similar conditions to those where life began on earth. Before the discovery of deep sea chemosynthetic environments like brine pools, these subterranean oceans would have been written off as unsuitable for life. This is because the layer of crust above blocks out the sunlight, previously thought to be essential to support living beings. But now, it's a far more exciting story. Deep in the pitch black oceans of these two distant moons, could there be briny oases of life hidden beneath the ice? It's certainly possible. Either way, we'll have to wait at least until the year 2030 to find out, when NASA's dedicated Clipper spacecraft will finally arrive at Europa and begin its search for life. Of course, we're leaning towards speculation here. We simply don't know enough about brine pool environments to make concrete claims that life began under similar conditions, or that they can be found beneath the ice of Europa and Enceladus. What we do know is they completely changed our understanding of how life can exist. And on top of that, there's one thing we can say with no doubt at all. As we continue to unravel the mysteries of these surreal underwater environments, the potential for incredible discoveries only grows. After all, who knows what wealth of secrets might still be lurking below the surface, just waiting to be found.