Original subtitles

There are still parts of our planet...

..yet to be explored.

Descending!

These are remote and unforgiving places...

I'm in.

..and the journey to reach them...

We're in very real danger of getting trapped here.

..will push me and a team of explorers...

Ugh. Ugh, ugh.

..to the limit.

STEVE SHOUTS

It looks like he's coming back.

The reward...

Amazing!

..breathtaking new discoveries.

When you're standing in a place that no-one has ever stood before,

it's very, very precious.

This time we're in the Yucatan Peninsula of Mexico.

Check that out.

We want to explore a vast, flooded cave system...

Are you ready? Happy as I'll ever be.

..deep beneath the jungle floor...

Absolutely no idea what's up there in the darkness.

..to find and map new passageways...

This is absolutely magical.

..but this is one of the most dangerous environments

on the planet.

It's just essential that nothing goes wrong now.

Oh, it's zero visibility.

I don't know where I'm going.

It 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, and 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.

Modern diving technology means it's now possible

to explore this underworld,

but it's definitely not without its dangers.

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.

You know, I've done several thousand dives,

but I've done fewer than 100 in caves.

That's not a lot.

So this is well out of my league,

and I guess we're about to find out if it's...

..if I'm properly out of my depth.

I did my first cave dive years ago,

but have spent the last few months in intensive training.

Unlike ocean scuba diving,

where I would normally carry a single rear-mounted cylinder,

in a cave, we'll carry two mounted on the side,

which will give us more breathing gas and a backup if one fails.

With cylinders in this position,

we'll be able to squeeze more easily through narrow spaces,

but the deeper we push,

the further we'll be from the safety of the surface and natural light.

We need to unfurl lifelines as we go,

which show us the way back out if our torches fail.

You ignore these rules at your peril.

There are rarely injuries in cave diving.

You either live...or you die.

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 going to fly.

..dive supervisor Scott Carnahan...

Who is it?

..and with over 20 years of cave diving experience,

Mexican instructor Bernadette Carrion.

With only a few days before we begin our exploration,

we're heading 7km 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.

With Bernadette's help,

I need to ready myself for the challenge ahead.

Scott? Yes, sir?

Are we on nitrox?

Yes, you are, 30%.

Bernadette, can you just check I've got this the right way round?

Now you need to get it on the other tank.

So, diving is already

one of 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, you know, what could possibly go wrong?

Everything could go wrong.

STEVE 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 just doesn't feel right, Scott.

Is that supposed to be like that?

Looks like the torch has just come on, Steve.

Just feels like an awful long stretch.

I'm really struggling with this today.

I'm much more familiar with gear that's set up for ocean diving.

This rig is a whole different story.

HE 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 why...

..because we can not go up to the surface.

We're under the ceiling and sometimes

we are 800 metres to 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.

Exactly.

There is a saying in cave diving -

slow is smooth and smooth is fast.

So keep it slow, keep it nice, keep your train,

keep your buoyancy and, erm...

..that should bring everything else

from what you learned on your training into position.

OK.

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 is 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 can happen from kicking up silt from the bottom...

..or even just from your bubbles.

So when my bubbles go up to the ceiling,

it 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, our lives will depend on them.

I'm keen to know what Bernadette made of my dive.

You really seem very calm and relaxed down there.

Your position, it looks good.

You don't look like as if you're going to die -

at least, I think.

Bernadette's confidence in me is good to hear...

..but there's still 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 suppress panic...

..keep calm...

..and expect the unexpected.

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.

Whoa! Amazing.

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.

WILDLIFE SCREECHING

It's a great sound.

That is the voice of the cenote.

BIRDSONG

These are motmots.

Once we get off in the forest and it's all featureless,

we'll be listening for that sound,

cos that will 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 sea water

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

can be contaminated from our waste on the land.

The coast of the Yucatan

is the fastest growing tourist hot spot in the world.

People come here in their millions to stay in luxury hotels

that are sited right on top of the underground cave systems.

Scientists are concerned that chemicals in waste water

from the town of Tulum are leaking underground

and finding their way into caves many kilometres away.

With tourist numbers set to increase tenfold by 2030,

the whole of Yucatan's underground water network

is at risk of being contaminated.

So how many of these cenotes have you tested, Arturo?

Well, we have more or less 50.

As more cenotes are monitored in and around the Tulum area,

more evidence of human contamination is being found.

How much do you need, Arturo?

Maybe full, please.

Water testing is just one step towards understanding the problem -

another is to appreciate how widespread

and interconnected the caves are.

For that, we need to explore and map.

Robbie knows of one particular cave that's ripe for exploration.

It could be the way into a new network of passageways.

We're looking for a clearing to land our heavy diving kit.

We're right over it. I can't see it. We're right over it, yeah.

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 is at the vanguard of a scientific project

which is charting the cave system.

He has single-handedly mapped 900km

of the 1,400km that are known,

but thinks he has only scratched the surface.

Only by mapping the caves

can we understand how water moves through them.

Then, together with water testing,

we'll know how far pollution is spreading

and hopefully where it's coming from.

Every metre Robbie maps for them

adds to the scientists' understanding

of this fragile life support system.

But of the thousands of cenotes out here,

this one in particular has eluded him.

Known as Satelite,

he first heard rumours of it 20 years ago.

In 2017, he led a group in search of it and they found the entrance.

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

So, logistically, Scott, what are we looking at here,

if we're talking about getting 8km inland through the forest?

Well, I think, realistically, we definitely can't

carry everything in 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 the stuff out.

So how are you feeling about this, Katy?

I mean, 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

that 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.

All we need now is just a symbol from the rain gods.

THUNDER RUMBLES

There we go. There you go - the gods say "yes".

Or "no".

Doesn't feel like "no" to me!

Satelite awaits us, far off in the dense jungle...

..but our point of departure

highlights the uglier side of urban life.

This is certainly the weirdest start to an expedition I've ever had.

It turns out that the best way for us to get into the forest

is through the town dump.

It's kind of a vision of hell,

but, you know, everywhere has to get rid of their rubbish somehow.

But here outside of Tulum in the Yucatan,

because of what we know about the geology here,

it's kind of got much more impact.

Loads of pollutants are going down into the cave systems,

and the fresh water that's flowing through them is the same fresh water

that the people here, and the animals, completely rely on.

Has everyone got mosquito repellent that needs it?

No, I haven't put it on yet.

I've called in an old expedition buddy

and former Royal Marine Aldo Kane.

He's the best in the business for getting teams into difficult places,

especially when ropes are involved.

Eh, this is vile. Can we go to the forest, please?

Yeah.

I think the smell is going to follow us for a while, isn't it?

Well, I'm used to being on expeditions with you, so...

THEY LAUGH

Right, let's do this.

It's over 35 degrees centigrade, and approaching 90% humidity.

Ahead of us is a gruelling trek to Satelite.

I reckon we probably won't cover more than about 10km,

six or seven miles on the map today,

but it's going to take us all day long.

This is hard and heavy going.

Oh!

Are you all right?

If Satelite 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 50 kilos.

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... Yeah. ..pre-diving kit. Yeah.

This explains why your load is so ridiculous.

BIRDS CHIRP

MOTMOTS CHIRP

Can you hear that sound, Aldo? Say again? Can you hear that sound?

What is it? Those are the birds that live in the cenote.

Ah, the motmot. We must be close.

I hope so.

Check that out.

No way.

It is huge.

Robbie's been waiting a long time for this opportunity.

Could this 10m-wide opening in the jungle

be the doorway into the Maya underworld

and countless new passageways?

Look how deep the water is back there.

It's all where the light hits it. STEVE GASPS

You couldn't even see it from there, could you? No, no.

We were circling over the top of this, Robbie and I,

with a helicopter and looking down. We couldn't see it.

I mean, it sort of brings it home to you

why this has not been done, doesn't it?

It still blows my mind that you can find places

that people haven't been.

I want to get down there.

You think anyone's ever been in there before?

No. Nobody.

So what would your plan be?

How are we going to tackle it?

We'll be able to rig ropes across there,

all the way down that route, basically. Yeah.

I think, first things first,

we need to get some water out of there to drink.

Yeah. Exactly.

Yeah, I think we're all out...all out of water.

I'll just be careful with that.

Just a few metres away,

this hole also opens into Satelite.

I haven't wanted water like this for quite a while.

This water drew the Maya here,

and they might have even used the same well.

There we go.

Is that nice? That's good.

We've just kept it really simple...

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.

I really, really want to know if there's a cave passage here.

The place is just so magical,

and I've 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, first time, are 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.

Do you see the ropes there?

Yeah.

Now it's Aldo's job to get us safely down the 20m drop,

using rope access skills and a bit of jungle improvisation.

SB? Yeah? Are you happy with that? Yeah. Right.

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.

OK. Here goes nothing.

First man down.

HIS VOICE ECHOES: Oh, wow!

Oh, it's so creepy.

Yeah.

Rather you than me.

I'm in.

Oh, wow.

This is seriously creepy...

..but this is what it's all been about.

Right, Steve, Robbie's coming down now.

That's them both in the hole at 4.30, OK?

At last, I'm in an unexplored cenote.

We need to find out whether this cavern has passageways

leading away 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.

How deep? Can you guess how deep it is there?

Right here it's about 15m deep.

Out to the sides it could easily be 40,

but you can't tell - you can't see.

Yeah.

OK, shall we head out and make a plan?

Yeah.

I don't think I've ever seen you look so excited.

I don't know what to say.

It is so much more than we expected. So much more.

OK, stop there. Stop. All stop. All stop. All stop.

That was a nice lift. Thank you very much.

What do you think? It's amazing.

Better than you expected?

Yeah, I'm really surprised. It's amazing.

Honestly, I've just...

My brain is popping. I'm just completely overwhelmed with it.

It's... Incredible. I don't have words.

Well done. That was... Great. 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.

Look at that.

There's around about 120,000 different kinds of moth

found in the world, and the ones around here are huge.

Can you see? It's not just the moths that get attracted by our lights -

it's beetles like this chafer as well.

This is amazing.

I've turned into a walking wildlife-attracting machine.

Ooh. Is that...?

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.

Morning.

How did you sleep? I slept like a baby.

Really nice bit of forest.

I was kind of lullabied to sleep at night by the tree frogs,

bats flapping through the trees all night long,

and then woke up this morning with the calls of the motmots.

Is that a stingy one?

Yeah.

Some of them in this part of the world,

if you get stung by them,

you can still feel it three months later. Ow.

Yeah. You have a shower or you get all sweaty or whatever

and you rub yourself and the spines that are embedded deep in your flesh

start stinging all over again.

That would hurt.

This morning, when we drop into Satelite,

our camera operator Katy will be critical

to light and record what we find.

That's you into your main harness point,

so when you get to the bottom, spin that until 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.

OK. Robbie and I will free-dive around the dark edges.

My first deep exploratory scuba cave dive will have to wait.

Going down.

If Katy's camera and lighting rig can help us find a passageway...

Gently. Mm-hmm.

..we'll all dive it again with full scuba gear...

OK.

..but now we can finally see the giant cenote...

..in all its glory.

Careful.

Is the bottom of the rope clear?

The rope is clear!

Thank you.

OK...

Heads, look up!

Yeah, 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 cave fish 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. Back in two.

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.

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.

Back in Tulum, Robbie's received some important news.

You remember that we did the water samples at the beautiful cenote,

you and Bernadette were diving, with the full facemasks,

with all the percolation coming out?

Nohoch Kiin. Yeah, Nohoch Kiin.

We did a sample there

and it's really, really polluted with chlorine.

Chlorine?! Chlorine.

But it's in the middle of the forest - how did that happen?

It must be connected to the town,

because that's where the chlorine comes in.

When they pump the potable water for Tulum from the wells,

they put chlorine in it, so any bacteria, everything would die,

and that's the only way the chlorine could come in there,

and we are about, as the crow flies, 7km away from town.

So, I mean, it must have flowed through tens of kilometres of cave

to get here from town. Yes.

It goes like 7km as a straight line,

and that would be probably 30km with caves,

and the only place the pollution could come from is from town. Right.

So what kind of levels above normal were the chlorine?

Four times as high.

Well, it shows that all of the waterways are connected...

Absolutely. ..and even when they're quite a long way away from town,

they're still receiving pollutants from people.

That's the most important thing of our explorations now,

that we can show this result,

and find out the things to get, in the long run,

protection for the aquifer and for the caves.

This result is shocking,

and it's all the more reason to step up our efforts

to find new cave connections.

Part of Robbie's special talent is 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.

A few months ago, he found just such a place

and hiked there to check it out.

It's a secret location.

It has no name and no-one has ever dived it.

This is the opportunity to get back on mission

in our search for new caves.

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.

Is this it? This is it.

What, seriously? Seriously.

Right in there, there's a beautiful pool of water,

crystal clear.

I came here once before and I did a free dive,

just, like, with nothing, just a mask and light,

to see if there is an opening, if there's a possible passage,

and it looks really promising.

Well, let's get in and have a look, shall we? Let's have a look.

OK, everyone, watch your heads if you're coming in.

Do you see that, the black hole back there, down there? Yeah.

That is the entrance. That's where we want to go.

So what's our plan here, Robbie?

How do we go about tackling this?

The first going in, which should be me in this case,

is installing the guideline, so if stability goes bad,

you only have to hold a line in your hand

and come back the way we came in.

Are you sure we're going to get in through there?

Oh, yeah, we can fit through here, right there.

From there on, I don't know what we'll find.

That tiny passageway looks scarily tight,

but I'm overwhelmingly drawn by what could be beyond.

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 and your line-cutter?

A little on the left side, by the look of that.

It's going to be fun to see Steve's reaction to a virgin cave.

It'll be interesting to see how he handles that.

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 has been for.

This is what it's been working up to,

is getting in there and exploring.

Yeah, now I just have to get my head right.

Pouch there, and that's got a spare light in it.

One spare light.

I've got another spare light in here. That's OK.

Line arrows and cookies are on your butt.

Yeah. All good. Awesome.

I'm running through all my mental checklists,

and making sure I've got everything.

What's that? Oh, do you want me to use it?

We've now got me,

Katy, the dive cameraperson,

and her support diver, so four of us in this cave,

and every single extra person that you add

adds an extra level of complication

and an added possibility of something going wrong.

If something goes wrong in cave diving, it goes wrong in a big way,

and everything happens quickly

and can be over fast.

OK. How can we best do this?

We would use one third of the 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 the event of an emergency.

If 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?

Yeah.

All right. Right. Let's do this.

Lead the way.

That's the left tank. That's the left tank?

That should be right? This is the left tank.

That's the left tank. That's the right.

Yep.

All right, everybody happy?

Can we go? Are you ready?

Happy as I'll ever be.

Marty? Let's go. Katy? Yeah.

All right, then, let's do this.

Katy, we are calling the dive and going up!

HEAVY BREATHING

That was proper sketchy.

What happened?

I didn't see anything.

No. Just absolute zero visibility.

That wasn't good.

So here's what I'm going to do.

I'll go back down there, I'll find my reel,

and then I'll 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.

Was it disorientating quite quickly?

Super disorientating.

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.

All good? All good.

Me, Steve, Katy, Marty...

You guys, please keep your feet still, all right?

Don't move too much,

because that's what silted out the whole place in the first place.

Right?

Keep the line in your hand.

OK.

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? Steve?

Yeah, I can hear you.

I still have zero visibility here.

I'm just keeping a really good hold on the line.

That's perfect. I hear you loud and clear.

Say again?

Steve, do you still copy? Steve?

Nah, now they're beyond the abilities

of this communication system.

Who knows how far inside they are right now?

We have no way to know until they come back out.

Ugh, 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.

Yes!

The visibility's starting to open out!

What an experience!

Absolutely no idea...

..what's off there in the darkness.

Oh, this is tight.

I'm 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.

Protecting this fragile environment and its precious resource...

STEVE SHOUTS JOYFULLY

..should now be a top priority.

Robbie, thank you so much.

ROBBIE LAUGHS

That was... That was good. ..unbelievable.

Next time, we push further into the Yucatan caves

on the trail of the Maya...

We've got a long, long way to go.

..uncovering ancient artefacts,

human remains and sinister secrets...

It is the stuff of nightmares.

..and I face my most terrifying underwater challenge yet.

HE SCREAMS