All language subtitles for BBC.Undiscovered.Worlds.Mexico.1of2.1080p.HDTV.x264.AAC.MVGroup.org.eng

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