[air hisses] I'm Steve Backshall, a naturalist and explorer.
I'm in Mexico on an expedition that will push me to extremes.
The terrain is just a nightmare.
This is the Yucatan Peninsula... First man down.
...home to the largest network of flooded caves... on Earth.
[exhales] I'm in.
In an environment under threat...
It's like a river flowing underneath the water!
...our mission is to find, explore... and map submerged passageways to help protect them.
Zero visibility and it's really tight.
We'll push ourselves harder and further...
Absolutely no idea what's up there in the darkness.
...to go where no one has been before.
We're several hundred metres in and we might as well be on the surface of the moon.
It's just essential that nothing goes wrong now.
Just blows your mind that somewhere like this can still exist in the 21st century.
This is what adventure is all about.
Beneath this jungle is the largest network of unexplored caves in the world.
The way into them is through openings called "cenotes".
What we want to try and find is a new cenote that's not been visited, that's never been explored.
Our mission is to find one that leads to a network of new connecting passageways.
The Yucatan has thousands of cenotes.
Near the coast, many are tourist attractions.
But head inland and it's estimated that as few as 1% have been discovered, let alone explored.
And the exciting thing is, this expedition could be the first to take light into cave systems that haven't been illuminated EVER.
The peninsula is composed of limestone, eroded by movement of rainwater and constantly changing sea levels over millions of years.
Now, it's a honeycomb of subterranean caverns.
Sometimes a void is so big, it can't support the weight of its own ceiling, which will collapse, leaving a hole in the jungle floor.
And it was the Maya, the ancient civilisation that dominated this part of the world 2,000 years ago, that named them cenotes.
They believed them to be a portal into an underworld called Xibalba, the Place of Fear.
Many who tried to enter this world... never came back.
But nowadays, it's the cenotes and the water they contain that are under threat from humans.
I mean... it's just an endless flat landscape.
There are no rivers or lakes here.
All the water is held underground in what's known as an aquifer.
All life here depends on this fresh water.
But scientists believe it's being contaminated by human pollution.
If we can find new cave connections running through the aquifer, we'll add to the scientific understanding needed to protect it.
I have to say as well, I have only just really got a handle on quite what it is we're trying to take on here.
Talk about needle-in-haystack time.
I mean, it goes on forever.
I'm in the coastal town of Tulum, my base for the next month, and with stakes as high as they are, I've brought in a crack team of cave divers to help me.
Robbie Schmittner is, arguably, the world's top underwater cave explorer and a personal hero.
Joining us is underwater camera operator Katy Fraser... That's very light, that's gonna fly.
...dive supervisor Scott Carnahan and, with over 20 years of cave-diving experience, Mexican instructor Bernadette Carrión.
With only a few days before we begin our exploration, we're heading seven kilometres west of Tulum for more training.
Around here are many cenotes that have already been explored.
They're the perfect place to fine-tune my cave-diving skills.
Let's go do it!
I'm hoping, with Bernadette's help, I can prove to myself and the team that I'm up to this potentially lethal challenge.
-Are we on nitrox?
-Yes, you are, at 30%.
Will you just check that I've got this the right way round?
No, you need to get it on the other tank.
So, diving is already one of the most logistically challenging things we do.
Cave diving is a big step up from that.
But that's not difficult enough for us, we're going to try and film it as well.
So that just makes it... Well, you can see all the kit lying around everywhere.
And, er, you know, What could possibly go wrong?
-Everything could go wrong.
-[laughs] I know all the basics now.
In this training dive, I need to bring everything together, to move at one with my equipment confidently.
-It doesn't feel right, Scott.
-Don't you think?
Is that supposed to be like that?
Pass it on to me.
[indistinct conversation] This feels like an awful long stretch.
I am really struggling with this today.
[Scott] There you go.
I'm much more familiar with gear that's set up for ocean diving.
This rig is a whole different story.
[sighs] We rely completely on our equipment in cave diving.
That's why we carry everything double and triple.
Three lights, two regulators each.
And that is because we cannot go up to the surface.
We're under the ceiling and sometimes we are 800 metres, a kilometre, in the cave.
So we need to fix problems there and make sure we can get back out.
Anything goes wrong and you're on your own.
[Bernadette] There's a saying in cave diving.
"Slow is smooth and smooth is fast."
So keep it slow, keep it nice.
Keep your trim, keep your buoyancy.
That should bring everything else from what you learn on your training into position.
[Steve] It's a really spooky sensation... heading into the darkness.
Kind of every part of your body is screaming out at you that it's a crazy thing to do.
The water in a cenote is filtered as it passes through the limestone.
The water's so clear, it almost feels like you're flying.
The big danger is that you lose that visibility and everything goes cloudy and you don't know where you are.
And that could happen from kicking up silt from the bottom, or even just from your bubbles.
So my bubbles go up to the ceiling.
Starts to bring down all this sediment.
You have to be very, very careful.
I'm throwing up a much bigger silt cloud than I was expecting.
Fortunately, this cave is already mapped and there are pre-laid lifelines to follow.
But when we explore new caves we'll lay our own.
If things go wrong, I'll lives will depend on them.
[sighs] Right now, I'm more worried about what Bernadette made of my performance.
You really seemed very calm and relaxed down there.
Your position, it looks good.
It don't look like a... as if you're going to die, at least, I think.
Bernadette's confidence in me is great to hear.
But I'm not so sure.
I still have a lot of work to do.
As the expedition phase gets closer, I'm spending as many hours as I can in the pre-mapped caves honing my skills.
Where we're headed, off the map, the passageways may barely be wider than I am.
My technique will need to be flawless.
But by far the biggest challenge is my own mind.
I need to supress panic, keep calm... and expect the unexpected.
[Steve] There's a crocodile!
Cenotes are very occasionally a sanctuary for young crocodiles.
I'm going to approach quite cautiously, cos I don't want to spook him.
He's swimming straight at Robbie's head!
Well done getting out the way of that, mate!
Taking a break from training, I use the opportunity to look for an animal that was once important to the ancient Maya.
[birds squawking] [twittering and squawking] [bird call] It's a great sound.
That is the voice of the cenote.
[bird call] These are motmots.
Once we get off in the forest and it's all featureless, we'll be listening for that sound, cos that'll tell us there's water nearby.
Motmots are only ever found near water and they only nest in limestone.
So if you hear them in the jungle, you must be near a cenote.
It's not surprising the motmots were very special birds to the ancient Maya.
It's like they're a guardian to the underworld.
I'm down to my last few hours of training with Bernadette before I head off the map with the rest of the team.
Today, she's testing me in a cave well-known for disorientating even the most experienced divers.
It's like a river flowing underneath the water!
My mind just can't take it in.
This is a halocline.
They can only occur here near the coast, where a dense layer of salty seawater meets a lighter layer of fresh water.
It's completely disorientating!
I can't tell where I am!
It's incredible, but I mustn't forget this is a potentially dangerous underwater cave.
As I complete my training, I'm constantly reminding myself to be wary.
But I think the risks are worth taking.
Our expedition is more than just exploration for the sake of it.
These cenotes are the only natural source of fresh water for human and animal life across the whole Yucatan.
It's a precious resource, but it's under threat.
Look at that!
After last night's rains, there's a slick running down the surface of the water.
Oil and other pollutants.
It's easy to see how the entire water system here could be contaminated from our waste on the land.
Exploring new cenotes and discovering how they're all interconnected will help assess the situation.
[he speaks Spanish] Robbie knows of one particular cave that's ripe for exploration.
It could be the way in to a new network of passageways.
Right there the vegetation is higher.
That's probably because it's more water under there in that area.
We're looking for a clearing to land our heavy diving kit.
[Robbie] We're right over it, but I can't see it.
-We're right over it?
But even with GPS coordinates, we're struggling to spot the cenote itself.
It's a lot of jungle.
The terrain is just a nightmare.
Robbie's spent years exploring the caves here.
And, as all life in the Yucatan depends on the fresh water they provide, a scientific project is now funding his work.
Every metre Robbie maps for them, adds to the scientists' understanding of this fragile life-support system.
I came here 20 years ago and I'm still here, it has endless possiblities to go exploring and find new places no one has been before.
He has single-handedly mapped 900 kilometres of the 1,400km that are known.
But thinks he has only scratched the surface.
But of the thousands of cenotes out here, this one, in particular, has eluded him.
Known as Satellité, he first heard rumours of it 20 years ago.
In 2017, he led a group in search of it and they found the entrance.
I wanted to get in there so badly to see what's down there but I couldn't get down there because it was too deep.
From up here, it's obvious that the jungle is too thick to land our gear by helicopter.
We're going to have to get it all out here on foot.
[thunder rumbles, rain splatters] So, logistically, Scott, we're talking about getting eight kilometres through the forest?
I think, realistically, we can't carry everything at one go.
I mean, there's not enough of us to carry all the tanks, so... We need to get in there, set up a base camp and see if it's worth carrying the rest of stuff out.
So how are you feeling about this, Katy?
Obviously, cave diving and filming in caves is your thing and is your job.
From a filming point of view, these caves are so aesthetically beautiful, it is really hard to get a bad shot and that's why I fell in love with them.
And to come across these places for the first time, and capture it and capture everybody's emotions and have it on film, will be really, really special for me.
Right, let's do this.
It's over 35 degrees Centigrade and approaching 90% humidity.
Ahead of us is a gruelling trek to Satellité.
I reckon we probably won't cover more than about ten kilometres, six or seven miles, on the map today.
But it's going to take us all day long.
This is hard and heavy going.
[cameraman] You all right?
If Satellité is the gateway into a larger network of passageways that we hope it is, we'll need to come back with even more heavy air cylinders.
Even so, Aldo's already loaded with 50kg.
Normally, every time we've been in the jungle, what do we have?
Jungle kit - hammock, food, water, Now we need jungle kit, climbing kit, -descending kit, diving kit.
Which explains why your load is so ridiculous.
[monkeys chatter] [bird call] -Hear that sound, Aldo?
Hear that sound?
What is it?
It's the birds that live in the cenote.
-[Aldo] Oh, the motmot.
-[Robbie] We must be close.
[Aldo] I hope so.
Check that out!
It is huge.
Robbie's been waiting a long time for this opportunity.
Could this ten-metre wide opening in the jungle be the doorway into the Maya underworld and countless new passageways?
[Robbie] Look how deep the water is back there.
When the light hits it.
[Aldo] You couldn't even see it from there, could you?
[Steve] No, we were circling over the top of this, Robbie and I, with the helicopter, and looking down.
Couldn't see it.
I mean, it sort of brings it home to you why... this has not been done, doesn't it?
It blows my mind that you can find places people haven't been.
I want to get down there!
[laughs] -[Steve] Do you think anyone's ever been in there before?
So what would your plan be?
How are we gonna tackle it?
[Aldo] We'll be able to rake ropes across there.
All the way down that root, basically.
-I think first things first.
We need to get some water out of there to drink.
I think we're all out of water.
Just a few metres away, this hole also opens into Satellité.
I haven't warranted water like this for quite a while.
This water drew the Maya here.
They might have even used the same well.
[straining] There we go!
-Is that nice?
The plan is for Robbie and I to recce the cave first by free-diving, with just masks.
We want to know there are passageways here before we commit to exploring it with air cylinders.
Really, really want to know if there's a cave passage here.
The place is just magical.
And I heard about it since 20 years and finally last year, somebody was able to show it to me.
It was always around like a ghost.
Now we're, first time, able to go down into it and see what's down there, it's...
I can't wait.
So you're going to be free-hanging about here.
-See the ropes there?
Now, it's Aldo's job to get us safely down the 20-metre drop, using rope-access skills and a bit of jungle improvisation.
-SB, are you happy with that?
[Steve] The, erm...
The logical part of my brain is telling me that this is a phenomenal privilege and something just extraordinary that very few people will ever get the chance to do.
The illogical part of my brain is... imagining monsters and crocodiles, and all kinds of other sinister things down the bottom of that.
Here goes nothing.
[Aldo] First man down.
[voice echoes] Oh, wow!
[voice echoes] Oh-oh!
This is so creepy!
Rather you than me.
[water splashing] [animal and bird noises] [exhales] I'm in.
This is seriously creepy.
Well... this is what it's all been about.
All right, Steve?
Robbie's coming down now.
[water splashes] That's them both in the hole at four thirty, okay?
At last, I'm in an unexplored cenote.
We need to find out whether this cavern has passageways leading from it.
It's an experience like none I've ever had before.
The first glimpse is tantalising, but we can't be sure what lies beyond in the darkness.
How far did you go in?
It just goes on forever.
It's huge and it's really deep.
[Aldo] How deep?
Can you guess how deep it is there?
Right here, it's about 15 metres deep.
Out to the sides, it could easily be 40.
-You can't tell.
You can't see.
Okay, shall we head out and make a plan?
-Let Robbie get up.
I don't think I've ever seen you look so excited.
I don't know what to say.
It's so much more than we expected.
So much more.
[Aldo] Okay, stop there, stop.
All stop, all stop, all stop.
-That was a nice lift.
Thank you very much.
[both laughing] -What do you think?
-Better than you expected?
I'm really surprised.
Honestly, I've just... My brain is popping.
I'm completely overwhelmed with it.
I don't have words.
That was, yeah, quite something.
This cenote is much, much bigger than anyone expected.
It could have connecting passageways, or it could just be one vast isolated cave.
The only way to truly find out is to use air tanks and lights to get a better look.
For that, we'll have to wait until morning.
So, the Maya, way back, hundreds of years ago, still had very advanced astronomy.
And they had names for all the constellations in the heavens.
And one of them they called Scorpion, named after that curled tail and the constellations that followed points along its length.
And that is exactly the same as the constellation Scorpio that the Romans and the Greeks recognised.
So these creatures found a position in the heavens because of that curled tail.
[cicadas chirping] [bird twitters] This morning, when we drop into Satellité, camera operator Katy will be critical, to light and record what we find.
That's going into your main harness point, so when you get to the bottom, spin that till it's red, pop that out and then just leave that at the bottom.
It's a priority for her to use one of our precious air cylinders, allowing her to explore deeper with her camera.
Robbie and I will free-dive around the dark edges.
My first deep exploratory scuba cave dive will have to wait.
If Katy's camera and lighting rig can help us find a passageway, we'll all dive it again with full scuba gear.
But now, we can finally see the giant cenote... in all its glory.
[Aldo] Is the bottom of the rope clear?
-[Steve] Rope is clear!
Heads, look up.
[water splashes] I Think we should go towards the wall back there and then circle all the way around and see if there's any cave entrances down there.
That sounds like a good plan.
With extra lighting, the magnitude of this cenote is becoming apparent.
These gigantic stalactites have taken hundreds of thousands of years to form, from the slow dripping of water when this cave was dry.
Though we're searching for signs of connecting passageways, it's the underwater life that draws our attention.
Everything here is evolved for life in a lightless world.
These blind cavefish don't react to torchlight.
Instead, these predators sense everything by vibration, including their prey.
They're extremely rare, but here, there are hundreds.
The fresh water of the cenote may be able to permeate the limestone, but it looks like there are no holes big enough for these fish to escape.
It's a sign that this is an isolated cenote.
All right, keep pulling.
I can see... [Aldo] And all stop.
It looks like it's just one magnificent subterranean chamber.
And our team were the first down there.
That's pretty special.
But I think if we're looking to find miles and miles of sunken cave passage, I think we're going to have to find somewhere else.
[indistinct chatter] It's been a huge achievement to explore a new cenote.
But as far as passageways are concerned, it's a dead-end.
Our search goes on.
The jungle is riddled with possibilities.
Part of Robbie's special talent is in detailed research, studying satellite imagery.
Looking for subtle differences in vegetation, or gaps in the canopy that could reveal the presence of water and a new cenote.
Using GPS coordinates, we're heading to a secret location.
So far, it has no name.
In the modern world, if you want to be an explorer, you want to be a cave diver.
But just prepare to do a lot of carrying.
[monkeys chattering] -Is this it?
-This is it.
Right in there, there's a beautiful pool of water.
I came here once before and I did a free-dive.
Just like... nothing.
Just a mask and light, to see if there is an opening, if there's a possible passage and it looks really promising.
[Steve] Well, let's get in and have a look, shall we?
[Robbie] Let's have a look.
Okay, everyone, watch your heads if you're coming in.
[water splashing] [Robbie] You see that?
The black hole down there.
That is the entrance.
That's where we wanna go.
So what's our plan here, Robbie?
How do we go about tackling this?
First going in, which should be me in this case, installing the guideline.
So if visibility goes bad, you only have to hold the line in your hand and come back the way we came in.
Are you sure we're going to get in through there?
We can fit through here, right there.
And from there on, I don't know what we'll find.
[air hissing] That tiny passageway looks scarily tight.
But I'm eager for my first exploratory cave scuba dive.
We're going to be the first ones laying our eyes on that new cave there.
It's just incredible.
It's a feeling you can't describe.
Robbie, how's your half mask?
Your line cutter...can live on the left side with the mask.
It's going to be fun to see Steve's, um, reaction to a virgin cave.
[Katy] It'll be interesting to see how he handles that.
[Steve] Right now, I've got a sense... of extraordinary excitement.
But that excitement is tinged with nervousness, you know.
This is what all the training's been for.
This is what it's been working up to - is getting in there and exploring.
I just have to get my head right.
That's got a spare light in it.
And I've got another spare light here.
Lanyards, cookies on your pipe?
I'm running through all my mental checklists, making sure I've got everything.
You know, I've done several thousand dives, but I've done fewer than a hundred in caves.
And that's not a lot.
It's not a lot to be going and exploring.
And so this is well out of my league.
And I guess we're about to find out if I'm properly out of my depth.
Joining me will be Katy, her support diver, Marty, and Robbie.
Every single extra person that you add adds an extra level of complication and added possibility of something going wrong.
If something goes wrong in cave diving, it goes wrong in a big way.
If you run out of light or air, and you're even a couple of hundred metres below the surface, you're not coming up again.
[Robbie] Okay, how the hell can we best do this?
We would use one-third of air to go into the cave.
One-third of the air in our tanks for the way back.
And one-third of the air for event or emergency.
Anything goes wrong during that dive, we will call the dive and we go straight out.
Back out to the exit, right?
Is that good, is that clear?
Lead the way.
Mind your head, Marty.
[Scott] Left tank.
-That's left tank.
-This is left tank.
That's left tank, that's right.
[Robbie] All right.
-Can we go?
Are you ready?
-Happy as I'll ever be.
Then let's do that.
Every metre of lifeline Robbie unfurls means we're one more metre away from the safety of the exit.
[Marty] Katy, we're calling the dive.
We're going up.
That was proper sketchy.
I didn't see anything.
No, just absolute zero visibility.
That wasn't good.
[sighs] [Robbie] So here's what I'm going to do.
I go back down there, find my reel.
Then I come back to get you guys.
Robbie is going alone to check the lifeline is still in place.
As dive leader, it's his call whether we make a second attempt to push through the silt cloud.
[Aldo] Is it disorientating quite quickly?
It's probably the thing more than anything else I've ever done that makes you want to panic quicker.
I'm not a panicky person, but you just feel your heartbeat just start to go up and your breathing start to go up.
And the mist descends.
-[Steve] All good?
Me, Steve, Katy, Marty... You guys, please keep your feet still.
Right, don't move too much, because that's what silted out the place in the first place.
Keep the line in your hand.
We're going in again, to try to push through that silt cloud... and see what's on the other side.
Steve, do you copy?
Scott, I can hear you.
I still have zero visibility here.
I'm keeping a really good hold on the line.
I hear you loud and clear.
[Steve indistinct] Steve, do you still copy?
Now they're beyond the abilities of this communication system.
Who knows how far inside they are right now?
We've no way to know until they come back out.
Oh, it's zero visibility.
Just squeezing myself in.
I can see Robbie's torch ahead of me.
I think the water may be starting to clear a little.
The visibility's starting to open out.
What an experience!
Absolutely no idea... what's up there in the darkness.
Oh, this is tight!
A size totally not built for this.
We're several hundred metres in.
And we might as well be on the surface of the moon.
It's just essential that nothing goes wrong now.
Robbie seems like he's found another lead.
This is absolutely magical!
The formations are just dazzling.
Absolutely endless amounts of stalagmites and stalactites.
They were formed here when this was dry cave.
And the thought that we're the first people ever to see them is almost beyond belief.
It's totally overwhelming.
In this first phase of the expedition, we've explored new cenotes... and we've documented new passageways.
Mapping this vast web of freshwater cenotes will help scientists better understand how far pollution is spreading.
This is somewhere that if we're not careful... could be destroyed before it's even close to being explored.
The scale and breadth... of the caves here in Yucatan is beyond my imagination, it goes on forever.
[laughs] And I think it's important for people to know that there are still old-fashioned explorations left to be done.
Robbie, thank you so much.
[Robbie laughs] -That was good!
-That was unbelievable.
♪ EXPEDITION WITH STEVE BACKSHALL IS AVAILABLE ON AMAZON PRIME VIDEO ♪ ♪