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
Charnuk, had found what he called
anomalous bottom water, 2 km 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?
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 seaater around it. So, it sits
at the very bottom of the ocean,
collecting in the natural depressions on
the sea floor. 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 water bed.
Scientists call these underwater lakes
brine pools. And they can stretch up to
20 km in length,
forming majestic lakes, rivers, and even
underwater waterfalls.
But even more curiously, they can reach
hundreds of meters 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 clear 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 a
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 rockpool 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 a 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
rejoin 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 salty
domain. Brine can also flow underwater,
too.
During a dive in 2018, the remotely
operated vehicle that was part of the
Noah 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 muscles 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 brine pool itself.
Described by Noah as long blue, gray
black streaks of stained sediment,
juxtaposed against the tan colored slope
above the brine poolool.
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 diapers.
The concentrated brine fluid, much
denser than salt water, as we've
discussed, can flow onto the sea floor.
The diapers act like channels directing
the brine towards the lowlying areas it
occupies and creating the waterfall.
But what manner of creature could
possibly survive down here? If indeed
anything can survive at all?
Few. Okay, so now we know what these
underwater legs 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 have been there for decades,
preserved in the super salty water, like
an underwater mass grave of mummified
sea creatures. But they soon realized
what was happening. Whenever a fish or
eel passed by, which admittedly was
quite a rare sight on the deep abyssal
plane, scientists noticed they mostly
avoided the briney 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 wrench
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
stretched for hundreds of meters in all
directions. Giant 100-year-old muscles
packed tightly around the shores of the
lake.
This muscle beach is home to a carnival
of shrimps, snails, weird squat
lobsters, and jellyike 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
there no oxygen, extreme salinity, and a
cocktail of toxic gases, there is also
no sun. 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 supplies 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
derived their energy from the sun.
But when researchers first stumbled upon
brinepole 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 muscles that clustered 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 everyday. These
were specialized 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, briles 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 muscles, both depending on
the other for survival.
In return for supplying them with
energy, the muscles protect the bacteria
from grazes 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 plane,
totally independent of the sun's energy.
astounded the scientists who first found
it. In fact, it seemed to them that
these ancient life forms were a window
into how life might have first begun.
[Music]
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 scrolled 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 imagine
life was born in a warm, nutrient-rich
rockpool, 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 brile, 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 briles, 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 pores 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 goalpost
for how and where life can survive. And
there's one even more tantalizing piece
of evidence that points towards a
brine-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
brinepole. What this means is the
Brianpole 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 analog
we have of the place where life first
began?
[Music]
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 environment
similar to Brian Falls 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 oasises 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 Brian Paul 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?
[Music]