37 meters down off the coast of southern France, there’s a tunnel with no light in it.
It runs back into solid limestone for almost the length of two football fields, roughly 175 m before the rock finally opens into a hidden dry chamber.
This place had remained sealed away for 30,000 years.
No human has ever stood inside.
No animal has ever entered this cave and no sunlight has ever reached those walls.
But then in 1985, a recreational diver from the coast of France accidentally stepped inside the cave.
And what he found there wasn’t the ancient remains or tools of an extinct civilization.
It was something far more complicated and shocking than you can ever imagine.
My name is Edmund and in this video we’re going underwater into a cave most people have never heard of to discover what scientists have found there and why almost nobody’s allowed to enter it today.
Let’s find out what was inside the discovery.
In 1985, Onory Kosare was exploring the underwater caves along the Kalank coastline near Marseilles, France.
He wasn’t searching for anything.
After years of diving in the area, he knew the coastline well.
Then, around 37 meters below the surface, he noticed something unusual.
A dark opening in the rock.
It didn’t look like much.
Just a cavity disappearing into the cliff.
But Kusker decided to investigate.
The opening led into a narrow tunnel filled completely with seawater.
It stretched approximately 175 m into the rock with the passage rising as it continued farther inside.
Kosker had no idea where the tunnel ended.
So over the following years, he explored the passage, extending his journey farther each time than before.
However, during one of the dives, something terrible happened.
His lamp failed.
The darkness became absolute.
He couldn’t see a thing.
With no light to guide him, Costco had to turn around and make his way back through the flooded passage.
He survived the dive, but he realized that that the tunnel had become too dangerous to explore alone.
So in 1990, he decided he needed other divers.
He brought in two Belgian cave diving brothers, Bernard and Mark Van Espen, to help him extend the safety guideline deeper into the passage.
The cave floor was covered with extremely fine silt.
A careless movement could stir it up and turn the water into a cloud of sediment, leaving the divers with almost no visibility.
So, they advanced slowly, extending their line as they went.
Then, the passage opened into a huge chamber, and what they found there wasn’t what they had anticipated, an underground lake.
And above that lake was something even stranger.
A pocket of trapped air, which meant there was another hidden chamber beyond that lake.
However, that is when they ran into a problem.
Their guideline ran out.
Without that safety line, they couldn’t safely go any farther.
So, they had to turn around without ever seeing what was waiting inside that chamber.
However, that wasn’t the end of their expedition.
A year later, they would return and push beyond the flooded passage, finally discovering what was actually waiting on the other side of the cave, the hidden chamber.
In June 1991, Mark Van Espen returned to the cave.
He laid the final stretch of line needed to reach the dry section safely.
The dive was short, but it confirmed what Costco had suspected for years.
There really was a hidden dry chamber beyond the flooded passage.
Then came July 9th, 1991.
Costscer returned with his niece, Sundrine, and friends.
Together, they pushed farther into the dry chambers than anyone had gone before.
The dry sections opened into at least two connected chambers.
In places, the ceilings rose more than 30 meters overhead, and the first people inside needed a moment to take in the size.
Stallctites hung from the rock just as expected inside a limestone cave.
But this cave was different.
It had been sealed from the outside world by rising sea water for roughly 9,000 years.
And what they found left them shocked.
Entire sections were covered with paintings of animals.
The walls also carried mysterious hand stencils and geometric symbols.
Some were painted while others were carved directly into the rocket.
All of this had remained inside a cave sealed away for thousands of years.
Costs first reaction wasn’t excitement.
It was disbelief.
He suspected someone had entered the cave before them and spray painted graffiti on the walls.
However, as they looked closer, they realized something much more remarkable.
These were ancient paintings and engravings preserved inside a chamber isolated for tens of thousands of years.
Soon, word of the discovery spread through the local diving community.
That’s when three divers from Grenobal, a major city in southeastern France, decided to explore the underwater passage themselves.
The tunnel was narrow, dark, and difficult to navigate.
Fine silk could quickly turn the water opaque.
The three divers lost their way inside.
They never found their route back out.
All three died inside the tunnel.
Kosker and his friend Yan Guyeran later helped recover their bodies.
After that tragedy, keeping the cave secret was no longer an option.
On September 3rd, 1991, Coster formally reported the site to France’s maritime affairs department in Marseilles.
The discovery was passed to regional archaeological authorities under France’s Ministry of Culture.
And now they were going to send archaeologists deep inside that chamber to find out whether those paintings were real or a hoax.
The archaeological investigation.
The paintings were hidden 37 m underwater at the end of a narrow passage filled with silt.
Anyone trying to reach them needed to be both an archaeologist and a capable cave diver.
And very few researchers could do both.
Eventually, authorities turned to John Cortean, a specialist in the Neolithic period and one of the few researchers capable of making the dangerous dive.
On September 18th, 1991, Corten joined an expedition aboard the research vessel Aronaut.
With him was John Clots, a leading specialist in prehistoric cave art.
But there was one problem.
Clots couldn’t dive, at least not yet.
However, after seeing photographs of the inside of the cave, he became so fascinated that he eventually learned to scuba dive so he could study it himself.
Over the next several days, the researchers carefully examined the walls, paintings, and engravings.
Then, on September 20th, 1991, Corton reached an important conclusion.
The artwork was genuine.
This wasn’t a modern prank or graffiti painted inside an inaccessible underwater cave.
The paintings and engravings were genuinely prehistoric.
But how had Ice Age artwork ended up inside a cave that was now 37 meters beneath the Mediterranean?
The location made the discovery even stranger.
Before Kusker Cave, every known decorated ice age cave in France had been found west of the Ron River.
Kusker Cave was different.
It was in Provence, east of the Ron, where nothing comparable had been found before.
But were these paintings really as old as everyone expected?
Researchers needed credible evidence, so they turned to radiocarbon dating, and the results revealed something surprising.
The dates didn’t point to one visit.
They pointed to two separate periods of human activity.
Five different samples from the wall art fell between roughly 18,000 and 19,200 years ago, but another sample was much older.
A hand stencil was dated to roughly 2700 years ago.
That meant people hadn’t arrived at the cave just once.
They had visited this place during two distinct periods separated by thousands of years.
Now, this is where all this started to make sense.
The paintings really were ancient.
The cave really had been visited by prehistoric people.
And those visits happened during two very different periods.
But there was still something researchers needed to understand.
What was it about these paintings that made the cave so unusual?
To answer that, they needed to take a much closer look at the artwork itself, and what they found on those walls would make the story even stranger.
The unusual animals.
The first thing they noticed on the walls was the animals.
Researchers have identified more than 170 animal figures representing 11 different species.
Horses were everywhere.
There were 63 of them, followed by 28 ibecks, 17 deer, 10 bison, and seven oroxs.
The bison were gathered around a crack in the rock known as the file de bison or bison fisher.
But then the researchers noticed something that didn’t make sense at all.
Seals and strange birds that looked almost like penguins.
But penguins had never lived anywhere near this part of the world.
So, what were the artists actually drawing?
Researchers believe the birds may have been great ox, large flightless seabirds that once lived around the North Atlantic.
Their black and white bodies made them look surprisingly similar to penguins.
The great o eventually went extinct in the 1800s, so these paintings may show an animal that no longer exists.
And the strange marine imagery didn’t stop there.
Some markings seem to show jellyfish or octopuses, although researchers still aren’t certain what the artists were actually drawing, and the artists didn’t use one method to create these paintings.
Some images had been carefully engraved into the rock using flint tools.
Others were created simply by dragging fingers through a soft mineral coating.
Around 50 images were drawn in charcoal, while roughly 12 combined different techniques.
But the animals weren’t the only things researchers found inside the cave.
As they looked deeper into the chambers, they found something that didn’t make sense at all.
Something that looked surprisingly human, the human figures.
Among the oldest markings inside the cave are mysterious handprints.
But how did ancient people actually create them?
The process was surprisingly simple.
Someone placed a hand flat against the rock and blew pigment around it.
When they pulled their hand away, all that remained was a ghostly outline of where it had been.
Researchers have identified about 65 hand stencils inside the cave.
Around 44 were made with black pigment, while 21 were made with red.
Radiocarbon dating places these markings in the Gravian period, roughly 27,000 years ago.
But there is something unusual about many of these hands.
Look closely, and some of the fingers appear to be missing.
The thumb is almost always intact.
It is the other four fingers that seem to have been shortened or completely hidden.
And this wasn’t something found only at Costco.
A similar pattern appears at Gargas cave in the Pyrenees where more than 100 hand stencils show missing or shortened fingers.
Hundreds of kilometers away, the same strange pattern appears again at Maltraiso Cave in Spain.
So what happened to these fingers?
Nobody knows for certain.
One possibility is that the fingers weren’t actually missing at all.
The artists may simply have folded some of their fingers down while blowing the pigment around their hands.
Some researchers have suggested frostbite, which could have damaged fingers in the extreme cold of the ice age.
Others believe the hand positions may have been a form of signaling with different finger positions carrying different meanings.
Another theory suggests that the fingers may have been deliberately removed or mutilated, but so far none of these explanations has been proven.
And because similar handprints appear across caves separated by hundreds of kilome, the mystery is difficult to simply dismiss.
But the hands weren’t the only traces of humans hidden inside Costco.
Deeper inside the cave, another image begins to stand out.
This one isn’t just an outline of a hand.
It is a humanlike figure.
Researchers call it the killed man.
The figure was engraved onto a piece of rock that had fallen from the cave wall long before humans ever entered the chamber.
It appears to be lying on its back with its legs raised into the air.
But then comes the strangest part.
Its head doesn’t look completely human.
It is rounded like the head of a seal with lines around it that resemble whiskers.
Even the mouth is unusual.
Four short lines were used to represent it.
The same kind of detail used elsewhere in the cave to depict actual seals.
So what does this image even mean?
We still don’t know.
But that uncertainty is exactly what makes it so fascinating.
Because among all the animals, handprints, and mysterious symbols covering Kascar’s walls, these rare human images offer something completely different.
But how did all these images end up 37 m beneath the sea?
To understand that, we need to go back thousands of years to a time when this cave looked completely different.
What sealed the cave?
During the last ice age, sea levels were roughly 100 to 120 meters lower than they are today.
Huge amounts of water were locked away in glaciers and massive ice sheets.
And that meant something important.
The cave wasn’t underwater.
In fact, it was roughly 6 km inland near the edge of a cliff overlooking a wide open landscape.
There was no seawater surrounding it.
No flooded tunnel, no underwater entrance, just dry land stretching out toward the distant coastline.
Then the climate began to change.
Around 10,000 years ago, the warming period known as the holosene began.
Temperatures rose, glaciers started melting, and enormous amounts of frozen water began flowing back into the oceans.
Slowly, the sea started moving inland.
Year after year, century after century, the Mediterranean pushed farther across the landscape.
The open land in front of the cave slowly disappeared beneath the rising water, but the sea kept coming.
Eventually, it reached the cave itself, and by roughly 9,000 years ago, the entrance had been submerged.
The dry route into the cave was gone.
And once the water filled the entrance, the chambers beyond it were effectively cut off from the outside world.
But nobody had sealed the cave.
There was no ancient door, no wall, no deliberate attempt to hide what was inside.
The planet itself had done it.
And strangely enough, that disaster became a form of protection.
For thousands of years, the flooded passage kept people, weather, sunlight, and the outside world away from the fragile paintings inside.
The cave became a kind of underwater time capsule until Hri Koskare found it.
But even after that discovery, there is still something nobody knows about.
The cave has a second entrance and for more than 30 years this route has remained completely hidden.
The second entrance.
In September 1991, a 32-year-old Marseilles diver, climber, and speliologist named Thierry Betton made a solo dive through the original flooded passage.
When he came back out, he had an idea.
There had to be another way into the cave.
Instead of entering through the dangerous underwater tunnel, perhaps there was a route from above.
Betton returned again and again.
After roughly 10 attempts, he finally found one in March 1992.
Working alongside his brother, Raises Betton, and diver archaeologist Luke Vanrell.
The passage was extremely narrow.
Cavers call it a chatier, which literally means cat flap.
To get through it, a person has to squeeze forward almost on their stomach.
But this wasn’t simply a second entrance.
It would become extremely important for the future of the cave.
The upper passage helps keep the cave slightly pressurized, which helps hold the water level down inside the chambers and protects the fragile artwork from being flooded again.
This also solves the problem divers have to face when they pass through the underwater passage.
During a mission debrief in 1995, Luke Vanro raised exactly this concern with French underwater archaeology authorities.
The importance of the upper route became even clearer as research at Costco slowed after attention and funding shifted toward Chauvet cave discovered in 1994.
But there was another reason to keep the entrance hidden.
Officials did not want people trying to enter the cave themselves.
So the exact location of the chatier was kept secret.
At any given time only around 10 people knew where it was and that secret lasted for more than 30 years.
It wasn’t publicly revealed until 2025 when prehistorian Thierry Felix described in a graphic novel.
Since September 2nd, 1992, Koscare Cave and three neighboring caves, Trieri, Figier, and Reinard, have been protected as French historic monuments.
A further maritime protection order was issued in December 2025.
Today, the real cave is restricted to authorized scientific missions under scientific director Cyro Mononttoya.
For everyone else, there is a replica cave.
On June 4, 2022, Koskeer Mediterranean opened at Bila Mediterranean in Marseilles.
Built by Clayber Rosillong, the company behind the chauvet 2 replica, the project cost roughly $23 million, including about 9 million from the re regional government.
Visitors descend in an elevator designed to look like a diving capsule.
Then they enter sixperson exploration pods and travel through a 2 and 20 meter reconstructed route that lasts about 35 minutes.
The replica attracted its 1 millionth visitor within just 15 months and is now approaching 2 million visitors with roughly one-third being students or young people.
But while millions of people can now experience a recreation of Costco, the real cave is facing a much more serious problem.
It is slowly disappearing.
In 2011, the water level inside the cave suddenly rose by 12 centimeters.
That single event caused irreversible damage to several horse engravings.
Researchers Michel and Luki Varrell publicly warned about what was happening.
Today, the water level is rising by roughly 3 millimeters every year.
Climate change is linked to this rise while microplastic pollution is also contributing to the weakening of the pigments and rock surfaces.
So researchers are racing to preserve what they can.
Nearly 2900 square meters of surviving wall and ceiling have now been laser scanned down to the centimeter.
A complete digital record is being created before more of the original cave is lost.
And that leaves Costco with a strange twist.
For thousands of years, the sea protected this hidden world by sealing it away.
Now, that same sea is slowly threatening to take it back.
So, after being hidden beneath the Mediterranean for more than 30,000 years, the story of Coster Cave is still not finished.
What began with a diver following a dark opening in the rock eventually revealed an entire prehistoric world.
But now the same sea that has protected those paintings is slowly rising and threatening to erase them.
So if this one cave can preserve so much information about the people, ancient humans, then what other hidden places are out there that we haven’t discovered yet?
Let me know your thoughts about this in the comments below.
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