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.

HE 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 Mexico.

Somewhere deep in there

is probably the thing I'm most frightened of in the whole world.

I'm entering the supernatural land of the ancient Maya.

Clear rope!

We'll journey underground into a secret underworld...

Look at that!

..the Maya called The Place Of Fear...

..to learn how they lived....

I just feel I've been connected back through time.

..and died.

It's a skull! There's a human skull!

This is more the kind of fear that I had when I was a kid,

and I was scared of monsters under the bed

and the stuff of nightmares.

I'm in the Yucatan Peninsula in Mexico.

All the way to the horizon, it's just completely flat,

and beyond the realms of the village, forest -

just thick, dense forest -

and underneath that is a peppered latticework of caves,

almost none of which have ever been explored.

Many of these caves are flooded.

I've spent the past month with Robbie Schmittner,

a world-leading cave explorer.

We've been mapping these caves to help scientists better understand

how to protect the precious fresh water they provide.

It completely blows my mind...

..to think that below...

..so much of the Yucatan are rooms like this that nobody has ever seen.

HE LAUGHS JOYFULLY

While Robbie continues mapping,

I'm heading further inland to explore a set of special caves.

The caves have played a unique part in the mythology of the ancient Maya

for at least 2,000 years.

The ancient Maya believed these caves were portals

to a supernatural underworld called Xibalba, "The Place Of Fear".

HORN HONKS

The human history here in Yucatan is absolutely fascinating.

I'm not an anthropologist or an archaeologist,

but you've got to say,

it's one of the most exciting places on the planet.

The Yucatan is rich in artefacts left by the ancient Maya,

a civilisation which stretched across Central America.

Scientifically advanced, they were master astronomers and builders...

..but they had a dark side.

Spiritual beliefs led them to carry out unusual

and often gruesome practices.

Many were performed in these caves.

To get a deeper understanding of the Maya,

I want to explore them and go further into them

than anyone has before.

To help me, I've chosen an elite team.

Underwater camera operator Katy Fraser...

That's very light. That's going to fly.

..dive supervisor Scott Carnahan...

Who is it?

..and former Royal Marine Aldo Kane,

who has the challenge of keeping us all safe.

Our journey begins at a secret location.

I'm meeting an archaeologist who'll take me to these caves.

Hey, Memo.

Guillermo De Anda, also known as Memo,

is a world authority on the ancient Maya.

This place is remarkable.

It's unbelievable, isn't it?

This is a sacred cenote for the ancient Maya.

He's spent his career examining the powerful relationship

the Maya had with these caves.

So this is where we're going, down here?

This is where we're going today.

Shall we get the lid off this thing? Yeah.

These ruins date back a few hundred years,

but beneath this tumbled down well is a submerged cave

that's much, much older -

one Memo thinks is a Maya portal to the underworld.

It contains a sinister secret that has helped Memo understand

how the Maya lived and died.

Whoa.

What's that, about 15 or 20 metres down?

Yeah, it's about 15 metres to the water.

Have you ever dived in a well before?

I have never dived in a well before, no.

It's working, so you're going to... Stop.

If we lift this back a bit...

We had to get down the well

and through a hole into the roof of the flooded cave beneath,

all in full dive gear.

Which, erm...?

Are these my cylinders here? Those are my cylinders? Yeah.

Before we dive, there's another cave just minutes away

that Memo also wants me to see -

more easily accessible,

he thinks it has a grisly connection to the well cave.

So, Memo, is it going to be any cooler inside the cave?

Yes. Much, much cooler.

Oh, that is good news.

I tell you, my English blood doesn't like this at all.

I know. Even we Mexicans...

..feel really, really hot this time of year.

Do many people come down here, Memo? No.

Fortunately this is, like, still very well preserved.

We're stepping right now, Steve, on a Maya stone stairway.

Only a select few know the location of these caves.

Secrecy is their only protection against vandals.

So we're looking at a piece of Maya history.

Seriously, I got a chill that went up the back of my neck

as soon as I saw that.

They look like they were left yesterday.

If you see, we call it Little Hands Cave or Manitas,

because there's a lot of children's hands.

And if you look towards your right...

Whoa, look at those over there! Positive handprints, yeah.

This is really pretty special... Yeah.

..that these are the handprints of a Mayan child

left over 1,000 years ago... Yes, yes. ..and they're still here,

as crystal clear as if a child just put their hand in paint

and whacked it on the wall.

It's interesting that I've seen prints like this in many caves

around the world, in New Guinea and Borneo and in Europe -

there's clearly something universal

about leaving your mark behind like this.

Do you think that this is like playing? Putting paint?

Or do you think that there could be something more to it than that?

You know, we cannot disregard that possibility,

but I don't think that that was the case,

because right where you are, if we look up...

..we should find some little feet, maybe a one- or two-year-old.

So there's no way that a child

could have deliberately put their hands and feet up there... Exactly.

..it had to be done...

..by someone else, by an adult.

I wish I could find exactly what the purpose

or what the symbolism or what the meaning of that is.

We believe it could be maybe a signature of something... Yeah.

..or a reminder of an event, or a rite of passage.

These might have been children that were sacrificed afterwards.

It was part of a ritual.

That's an incredibly macabre thought.

We're looking at the handprints of a Mayan child

from well over 1,000 years ago

that may soon after have been sacrificed.

Yes. Oh, yeah.

Yeah, maybe this was the last thing they'd do in their life.

There are over 100 handprints in this cave.

If Memo's theory is correct,

each one could belong to someone who was ritually sacrificed.

There are even prints from children as young as two.

If they were killed here,

their remains are missing from this cave.

Memo believes the flooded cave beneath the well

could hold the key to understanding why.

He's been diving in this well cave before,

and believes the site had a special

and grisly significance for the Maya.

He thinks they came to carry out human sacrifices.

That's pretty scary.

Effectively, we're going into a sunken mausoleum.

It is the kind of stuff that wakes you up in the middle of the night

in a cold sweat.

It is the stuff of nightmares.

It's not ideal to set up here because of the distance.

We'll work that out, or I can pull them in.

Aldo's rigged a frame that will lower us

and our equipment into the cave

and, just as importantly, get us back out again.

It is a complex operation.

We've got four divers in a confined space in an overhead environment,

in a cave, and you're on the end of ropes.

We can't get to you that quickly.

In fact, we probably won't get to you -

it'll be down to you to self-rescue to the ropes.

For once on an expedition,

I'm not thinking about the logistical problems.

OK, lean out a bit further. Try and get out past the edge.

Right, was there anything that needed changed on that?

Nope, negative.

Everyone happy with how that went?

These caverns are living graveyards, living mausoleums.

Two metres in the water.

This is more the kind of fear that I had when I was a kid,

and I was scared of monsters under the bed

and creatures in the cupboard.

It's that fear of the unknown,

and darkness and death,

and all the things that haunt us most.

Katy?

The Maya feared these caves

because they believed the gods responsible for human suffering

lived in them.

They offered these gods gruesome sacrifices to try and appease them.

HIS VOICE ECHOES: Descending!

How far out are we, Scott? More, Aldo.

OK, Steve, we're going to lower it on the working line. So get down.

Diver on the bottom? Diver's floating.

Stop.

All stop.

Guillermo? Guillermo, are you OK?

Can you hear me well? Yeah, I can hear you.

Take your time and get your systems right.

OK. Moving in closer.

We've been granted permission to dive here

on the understanding that we don't touch anything we find.

Guillermo, I see something.

OK.

It's a horn.

We're just descending down...

..underneath...

..the daylight...

..of the well above.

That is the skull of a bull.

What's this on the teeth, here?

Steve, I really don't know.

It looks like gold.

There is no natural goal here in the Yucatan, is there?

No. No, Steve.

There is no gold in Yucatan.

The unusual appearance of the teeth could be due to

natural discoloration caused by the diet of the animal,

and this skull can't be from the time of the ancient Maya,

because we know they didn't have cattle.

OK. What do you think? Shall we go down?

Let's go for it.

Look at that, Memo, it's a pot!

It's beautiful.

How old do you think this might be?

This might be 1,000 years old,

and look how well-preserved it is.

Wow.

They weren't just dropped in here by accident.

No. No, Steve, it would be too many accidents.

What would be in these pots?

Usually food or maybe a sacrificed animal.

Maya pottery was extremely fragile.

What we found suggests someone swam down and placed it here intact.

Guillermo, what's that?

Oh, where is it?

Just...just below you now, in your torchlight.

Just there.

It's a skull. There's a human skull.

There's something very unsettling about the sight of a human skull.

It might belong to somebody that was high-class, elite...

How do you know that?

If you look closely at the teeth,

you can see they have been modified for symbolic purposes

when this person was alive.

Memo thinks the person was decapitated,

and the head or skull then used as an offering.

There are bones everywhere.

Arms, legs, ribcages, feet - all given to appease the gods.

This is without any doubt a very sacred place.

Ancient Maya artwork shows that sacrifices were incredibly gruesome.

Flesh was stripped from dead bodies,

and they even inflicted terrible injuries on themselves

to offer up their own blood.

What's that? What have you got? It's a complete body.

How he got there, I wonder...

Memo has come across this sort of thing before.

I have seen evidence of cut marks, very deep,

either on the ribs...

..or either in the sternum.

That tells me that a knife got in that body

and very skilfully ripped apart the heart.

Sometimes this happened when the victim was still alive.

They ripped out their hearts and they were still alive?

Yes.

Those are... Those are under an overhang.

Yes. It looks like they were placed there, Memo.

There's no current in this chamber,

and all the bones are arranged close to each other.

They may have been put here deliberately -

no easy task.

It was hard enough for us to get in

with modern ropes and climbing equipment.

They would have had to hold their breath

and swim down to place this offering here,

to push and prod it into this position.

Everywhere you look, there's another body -

another skull...grinning at you from the darkness.

This place has so many stories to tell.

So many secrets, so many mysteries...

Everywhere you look, the memories of an uncomfortable ritual past.

This place has horrifying nightmare significance.

The cruelty the Maya engaged in to appease the gods of Xibalba

is evident everywhere.

These barbaric acts seem at odds with the sophistication

of their culture.

What we hear about the ancient Maya is all quite macabre.

It's all about human sacrifice and kind of quite grisly.

You know, the first time you encounter these remains,

it's very emotional.

I have kids.

And you try to understand why they do that.

There is...these two different...

..approaches to the Maya.

One of them is these beautiful buildings,

the great astronomy and science they develop,

and on the other side, we have this blood-letting.

But this was part of their cosmovision -

it was part of their religion.

It was part of their everyday life.

I wonder... If sacrifice was such a big part of their culture,

especially with children...

But how often were they actually doing that?

Cos I guess it would be counterproductive

to be killing that many children.

I'm not trying to justify anything but, you know,

I believe that sometimes those were acts of love...

..to sacrifice children.

Remember that the Maya believe in life after this life.

They were expecting this person to either go to another place

and carry a message, and they can bring it back...

So we don't know exactly,

but we do believe that most of these sacrifices

or offerings occur in trying to please the gods,

cos they were having problems with drought.

When there's no rain, there's no agriculture,

so there's no food production.

It's like, "Please send rain to my people because we're dying."

It's been an unsettling introduction to the world of the ancient Maya.

And how much more is still out there?

I believe...I believe we will find much more.

We're on our way to another Mayan cave, one that isn't flooded.

It's a labyrinth of dry passageways,

thought to stretch many kilometres beneath the jungle floor.

The cave is so vast, it's not been fully surveyed

for the Maya secrets it may contain,

but getting into it will be a challenge.

With our support,

Memo wants to explore a section of the cave comprehensively.

We know that we have at least one, possibly two descents on ropes,

so we're carrying as much rope as we can,

plus all of our climbing and ascending and descending gear.

I don't know exactly what we're going to find inside

but, from what I understand,

this is one of the most remarkable caves found anywhere in the planet,

and very few people have had the privilege of going inside

and seeing what we're about to see.

This cave was so special to the Maya,

they risked their lives to enter it.

We want to see just how far they managed to get.

It won't be easy.

In the last few weeks, someone fell to their death here.

We're taking every precaution.

Just so that everyone knows,

this is where the med kit is going to be.

If it's me that ends up incapacitated in there,

the med kit's here and the stretcher's here.

Unless both you and I take a catastrophic fall,

then we should be able to come out and... Let someone know.

..raise the alarm. OK.

If you're not back by 6am, we're coming in.

So is that 6am? Yeah.

Yeah, that's you.

Oh, it gets tight already.

It is.

Just go careful, mate.

Here, are we just passing bags through? Yeah.

You definitely said no more caves, Steve, a few years ago.

I know. I did...

Do you know what's amazing?

Just look at all this kit we've got, and head torches and lights...

Imagine doing it naked.

Why would you want to do it naked?

The way they did -

barefooted with burning torches.

Shout me when you're through.

So we're only just into the cave,

and already we've got our first drop down into the darkness.

It's uncertain how long it is.

How much are we sending down, Aldo?

Throw it all down that fall line.

See where... See right down through that cleft?

Yeah.

Perfect, mate. Good shot. Good shot.

So when you're coming down, don't run the rope over an edge

or somewhere that it could get jammed.

It's so humid in here.

You can see all the water droplets just floating around

in my headlight. Hey, Steve.

It's... Mainly, this is just for support.

Pretty much on your feet the whole way down.

Probably a 5m drop at the bottom.

Is the rope enough? Plenty.

I'm on the bottom level of the first abseil.

Order of march, Steve. Next, who do you want down?

I'm going to send Memo down next.

Yeah. Clear rope!

Are you OK, Memo?

I'm fine, thank you.

It's very slippery.

Yeah, yeah.

We're about an hour or so into the cave.

Memo said the earliest people were using this cave

was maybe 2,500 years ago,

so they would have been making these descents that I'm doing now

on completely natural fibre ropes.

It's just incredible.

It actually makes us with all our modern methods

feel a little bit pathetic.

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

It is hot.

It's taken us nearly an hour just to get the team down this 5m drop.

If they came down to this on a rope made from natural fibres,

how did they climb back up?

They may have had some kind of scaffold system,

or maybe wooden ladders,

but we haven't seen evidence of that.

After dropping down this challenging overhang,

we come across our first sign of the Maya.

Look at this, Steve.

What have you got?

This is a very important symbolic construction

which represents the mountain of creation. Yeah?

This is like a penis,

so this is a monument to life, to fertility.

I don't know, but maybe we're 60 or 100m below the ground?

The Maya managed to come all the way here,

build this fantastic altar,

made an offering,

and pray for rain or for harvest.

Wow. That's incredible. It is.

Although they referred to these caves as The Place Of Fear,

the Maya were still drawn to them.

As their civilisation crumbled and life became harder,

archaeologists think they ventured into these labyrinths

to honour the gods they held responsible.

We're four hours in.

We've reached a point the passageway spreads out

like the spokes of a wheel, pointing north, south, east and west.

So I think these caves run on all the cardinal points, bizarrely.

Yeah. The one that we should be going down is the west.

So that's... North is that way. And that's west. Yeah.

Smashed pots litter the cave.

They once held offerings,

and the Maya are thought to have broken them intentionally

to release the spiritual power of what was inside them.

What's that?

Ground stone.

What?! That must weigh 25 kilos.

How did they get that down a cave?

That's amazing.

It's kind of like a detective story, putting it all back together.

What have you got, Guillermo?

It's right here.

It's a beautiful, beautiful ceramic pot.

First time I've seen something like this. You've never seen this before?

No, and probably it's thousands of years that had been there.

It's perfect. It's even sculpted.

It's perfect. It's beautiful. It's unusual.

How on earth did they get in there?

They place it there - they hide it.

The ancient Maya sometimes do that. Whoa...

..and we can't even get it out.

Yeah. No. No way.

Yeah. It's stuck there. It's stuck, yeah.

There are discoveries around every corner here. Yes, of course.

Of course. We could spend months inside this cave

and keep discovering things.

It's so cool.

Isn't that amazing?

Whoa.

Oh! Oh... Oh, wow!

Look at that!

That is one of the most amazing things.

Look at it - it's completely three-dimensional.

Amazing.

Well, it's definitely a mammal, isn't it? 100%.

What were the important mammals to them thousands of years ago?

Tapir? Pig, perhaps? Could be a peccary.

It's so cool that they've taken this chunk of rock,

this part of the cave, and they've used its natural features

to form the head of the animal, and the body as well.

You can see it almost looks like it's leaping out the wall.

There's a parrot.

Look at that. That's amazing.

The face... That's so... It looks like it was done by a graffiti...

It looks like it was done by Banksy. Banksy.

THEY LAUGH

That's so good.

Imagine how far in we are at the minute.

How long have we been going for?

Erm, six hours at this point.

Six hours.

So they came six hours into a cave and created works of art.

The Maya believed some animals had the power to enter the underworld,

and they celebrated them in paintings like these.

Look at this, Steve.

It's a jaguar. It's a jaguar, yes.

I see it. Look at the face. It's looking back over its shoulder.

Look at the eyes. It's amazing.

It's really amazing, the motion. And I see the spots.

And look at the tail. That's definitely a jaguar tail...

That's amazing. ..which makes it different than the puma or cougar.

Yeah. It's really a jaguar.

It's beautiful.

Many of the oldest Maya deities took the form of an animal,

and this was the jaguar god of the underworld

known as the Night Sun.

The animal in front of it, what's that?

It really looks like a deer.

These two animals were able to go into the underworld

where we are right now... Yeah.

..and be able to survive and go back.

This painting represents one of the most revered

and powerful Maya gods.

The artists overcame extreme dangers to paint it here

in the deepest part of this section of the cave.

Nothing here is done by accident, is it?

Yeah, definitely nothing. Anything that's done this far into the cave,

that's taken this much sweat and effort. Yes, all this effort...

It's quite something to think of someone

standing barefoot where we are right now,

with a blazing torch to light them,

and painting, with charcoal, this drawing onto the wall

and leaving it here, and now we're sat here trying to decipher it,

trying to find its meaning.

It gives me goose bumps.

We are in the sanctum sanctorum of religion of the ancient Maya,

the centre of the Earth -

the underworld.

Memo has given us a glimpse into the incredible world of the Maya.

Their beliefs were so powerful that they risked it all

to reach these subterranean places of worship.

Everything about this place,

I just feel like I've been connected back through time

2,000 years to the people that used to walk through here. It's...

It's hard not to feel like that. Yeah.

It's time to say goodbye and move on,

but Memo's work in this cave will continue.

Thank you, Guillermo. No, thank you.

It was really a privilege to share with you and your friends.

And not many people have seen it, well, since the ancient Maya times.

Exactly, exactly, thousands of years ago.

The Maya didn't only worship the gods of the underworld -

they built pyramids to the glory of the gods

that existed in the heavens, too.

Yucatan is incredibly flat,

so any time you find a hill,

it's not going to be natural. It's artificial.

These blocks have been put here by the ancient Maya

and would once have been pyramids like that one there.

Over time they've crumbled, and become part of the dense jungle

and home for its wildlife.

These limestone blocks from the ruins are perfect

for animals to hide out in,

and this has obviously been the home of a pretty substantial snake.

That's its shed skin,

and the live animal must be at least 2m long.

No... Look at that.

HE GASPS

I'm watching one of the most macabre sights in nature right now.

This is the world's largest species of wasp, the tarantula hawk wasp,

which is as long as my thumb,

and it has a hold of a fully-grown Mexican red rump tarantula.

The spider, though, is not any longer putting up a fight

because it's been paralysed.

But this is not going to be her meal.

This is a meal for her young.

Here she goes, she's going to grab it again,

so she'll grab it in her jaws

and then start dragging it to her burrow.

Once she's got it there,

she'll lay one single egg onto the abdomen of the spider,

and that will hatch out and eat it from the inside out,

eating around the vital organs so the meat stays fresh

for as long as possible.

It's absolutely hideous. So macabre.

The Maya would have watched things like this

and had significance for it, because how could you not?

The wild world can be cruel,

and the Maya did gruesome things themselves

to stave off nature's cruelty.

It didn't help them.

Famine, drought and other natural disasters

could well have caused the collapse of the cities of the ancient Maya.

We're back exploring the caves under the Yucatan,

and my colleague Robbie Schmittner

now thinks he may have found the perfect place.

He's been mapping underwater caves in the region for years,

to help scientists protect the fresh water there

from the growing threat of pollution.

His search has identified a cave in the forest

that's thought to have Maya artefacts.

A lot of exploration is about following stories and legends.

So, we have a lead that there is a cave in the forest.

You never know -

I mean, it could turn into nothing at all -

but it's still super, super exciting.

The entrance has been explored, but not its furthest reaches.

We want to see what's in there,

and find out how far the ancient Maya journeyed into the cave.

Oh, cool. That's a proper cave.

It's a decent-sized entrance, isn't it? It's massive.

A few metres in and we come across our first signs of the Maya.

Check this out, Steve.

Whoa. That's an old altar.

That's amazing.

Look at that.

Wow. That's really cool.

And it has stucco, plaster all over it.

Lime plaster like this was used by the Maya from 600 BC.

When it was used on buildings outside,

it often didn't survive the weather,

but in here, it's been protected.

Look at that.

It's like a face, like a smiley face. Yeah.

That stalactite, that doesn't look like it's been placed there,

I think the altar's been built around it.

ALDO BLOWS RASPBERRIES

Mind blown.

Yeah. This is exactly what Memo was talking about. Look at this.

This is THE most sacred thing.

A stalagmite coming up -

that represents the phallus, that represents the male -

and this coming down,

the stalactite, which represents the female.

This is probably why that altar is right there,

because that is just screaming out

for everything from Mayan iconography and symbolism.

It looks like it could have been, perhaps,

something for carrying water in. Yeah.

What is it? Pottery? Yeah! Yeah, it's a bit of ceramic.

We didn't see anything like that in that other cave.

I've never seen anything like in the shape of that.

That's completely...unique.

I think you might have found something quite special here, Robbie.

It's a special place, definitely.

We're going to leave the daylight zone here right now.

We need a bit more light.

We're going to push on as far as we can into the cave,

going deeper than anyone has explored before.

It's not just stalactites coming down into the cave -

there are twisted tree roots dropping down, searching for water.

And right there...is a whip spider.

Very creepy-looking critters.

That's absolutely stunning.

It taps around in the darkness,

with these extended legs which function almost like antennae.

That's how it finds its prey.

It is genuinely quite worrying

when you think about how big this cavern is,

how much weight is above it,

and how little support there seems to be on the inside.

We're now hundreds of metres from the surface.

There are no human artefacts this far in,

but there are rock formations and they're staggeringly beautiful.

This is probably the most decorated cave system I've ever been in.

Everywhere you look,

there's something unique and new and fragile.

And in this corner,

there are thousands of what are called helictites.

It's where the stalactite seems to go completely crazy

and starts growing off in the wrong directions.

It's growing upwards, it's growing sideways,

sticking out all over the place,

and it's still a bit of a mystery why this happens.

The original idea was that it was due to wind movement,

that the air would start the calcite forming in different directions,

but I don't know, that seems a little bit far-fetched.

Another suggestion is it could be down to growth of bacteria

which just start going off in all kinds of crazy directions, whatever.

All of a sudden, it transforms these wonderful hanging structures

into something far more complex,

and every single one of them is different and unique,

and just a little sound...

PERCUSSIVE NOTES CHIME

..like a subterranean xylophone.

We're deep inside the cave,

and there's no evidence the Maya made it this far,

but something else catches our eye.

Whoa.

It's a lake.

Yeah.

A complete underground lake. Crystal clear.

It's a thrilling discovery.

This is what Robbie lives for.

I think we should get the scuba gear here and get in there and dive it.

Back there it looks all dark, even with the light.

That's where I would start searching first.

The dark shadow marks the start of a water-filled cavern

that will either lead to a dead end

or continue on for countless kilometres.

We decide to make camp at the cave entrance,

and prepare to dive in the morning.

Cave diving is often described

as one of the most dangerous types of exploration.

This will be one of the hardest dives I've ever done,

and our lives will depend on a single line.

I do need to prepare that guideline,

because when we measure the cave,

placing that guideline is our lifeline to come back out,

but it's also the line which we need to survey and make the cave map.

You can't get line knotted at the market...supermarket already.

We have to do that all the time before we go diving.

From Robbie's experience of the geology in this area,

we're expecting to encounter tight squeezes -

passageways barely big enough to get our bodies through.

This will push me to the limit of my diving abilities.

As far as my job is concerned of being in charge of safety,

generally speaking, on this expedition,

there is nothing, absolutely nothing I can do.

As soon as they leave the surface, they pretty much are on their own.

There is no rescue in there.

And the worst-case scenario is...it's body recovery.

There is no half measures, really.

Yeah, it's pretty...pretty scary.

We don't know if it's a very easy dive, what we're going to do.

We don't know what we'll find. It's exploration, right?

My main concern is that if I die tomorrow,

my last meal will have been a Pot Noodle.

Nobody's going to die tomorrow.

Cheers to that. Cheers to the dive. Cheers.

Cheers to that -

safe dive, safe journey and safe return.

Goodnight, everyone. Sleep good.

Night-night. Night, dude.

So, one side is the jungle and the other is the cave,

and somewhere deep in there...

..is probably the thing I'm most frightened of in the whole world.

Never been much good at...little narrow spaces and squeezes.

Get us coffee, Aldo. Hmm...

How did you sleep?

Yeah, very well.

I sleep better in a hammock than I do in a bed.

I love living like a caveman.

The simplicity of life...

Get up, start fire, have coffee.

I don't know if cavemen had coffee.

What?! OK, I don't want to be a caveman any more. Nah.

Robbie, do you want coffee? Sure.

How do you take it?

Milk, please.

Dive plan for this morning, Robbie?

Our main danger always, always in cave-diving is our own mind.

The most important thing is stay calm at all times.

We cannot panic. It's impossible. Not...

Not an option, right?

This is subtly different, what we're doing this time.

We're going for a slightly different system.

I'm not using the full facemask that allows me to talk underwater.

Instead, I'm going for a more conventional cave diving set-up,

which should be safer, more manoeuvrable

and help me to get to places

that I just wouldn't be able to with the big mask.

I'll go in front with the reel.

You know, stay close by the guideline -

always in reach distance.

If anything goes wrong, if anything is not ideal,

let's just call it and go back.

All right, sounds good.

All right. Let's do it.

1400 hours.

OK.

All right. Waiting on you, Robbie. All right, let's do this.

We're the first people ever to see these stalactites...

..and set our eyes on this.

This is an optical illusion caused when a shallow pocket of air

is trapped against the roof of the cave.

I don't know if the air in here is breathable.

It probably, actually,

from the fact my breathing is almost instantly going up quite a lot...

..probably means that there's not a lot of oxygen in here

and it's mostly carbon dioxide.

Oh, yeah. The air in here is not good. Definitely not.

HE BREATHES IN FROM OXYGEN CANISTER

There's always that slight sense of panic

as his heart rate starts going up, and your breathing starts going up,

because there's just not enough oxygen.

HE BREATHES IN FROM OXYGEN CANISTER

Robbie has just dropped down into a little crack and he's disappeared,

and he's gone completely,

which is pretty nerve-racking,

because it means that I'll have to follow.

But this little bell here,

nobody's ever been here before.

And where Robbie's gone now,

I'm not sure anyone should ever go, really.

Here, the cave drops down into a tight squeeze -

the smallest I've attempted.

HE SCREAMS

I can't move forward or backward.

I'm stuck.

HE SCREAMS

I force myself to slow down, calm down and think about my breathing.

I exhale, and the cave lets me go.

Robbie's waiting on the other side in a clear tunnel.

My only comfort is that I'm with one of the world's top cave divers.

He stays alive because he dives within his ability.

If he reaches his limit,

he always turns back and lives to dive another day.

We're approaching a second squeeze and I'm nervous.

This one is technically more difficult than the last,

but I'm determined to try.

Something on my harness is caught.

I'm through to the other side,

but now further away from safety than I've ever been.

Beyond here, I can see another squeeze and it's smaller still.

I don't think there's any way I can physically get through there.

This could be the decision of my life.

Robbie will continue on mapping, while I turn back to safety.

Wow. Wow.

Oh...

What an experience.

But we all want to know how much further Robbie managed to go.

You ran out of line?

What happened, man?

It keeps going and going. It does?

Yeah, you've got to see on here. It got really tight.

Watching Robbie's helmet cam footage

shows exactly what he had to go through to put all of his line out.

He reaches a squeeze so small,

he has to carry out the most complex manoeuvre in cave diving.

He removes both his air cylinders, passing them one by one

through the tiny restriction into the darkness below,

connected only by a length of hose and his mouthpiece.

His reward is a seemingly endless passageway on the other side.

I just pushed on and on and at one point it was,

"Oops, no more line."

And the cave keeps going.

To carry on without line

would be breaking the golden rule of underwater cave exploration.

How much line do you have on here?

It's about 500m. Half a kilometre?!

Half a kilometre. We've gone half a kilometre into the cave...

..and that's nothing, it hasn't even... Wow.

It keeps going. Wow.

That's amazing news.

Everything I'd done up to that point was the maximum that I'm capable of.

Being given the honour of being taken into that place,

it's Robbie's incredible experience and knowledge

that's allowed that to happen, and for that I owe him big time.

It's been an incredible journey.

The information being gathered by scientists

will help prevent further pollution

of these vast and interconnected waterways.

During our six-week expedition,

we've explored and mapped deep underground,

adding to the growing knowledge of both the Maya

and the area's precious water resource.

The thing that sets us apart as a species, as human beings,

is our desire to explore, to discover, to solve puzzles,

and to expand the limits of human knowledge.

Something led to the downfall of the ancient Maya.

One theory is that overdevelopment caused an environmental crisis

which saw them run out of fresh water.

In Yucatan today, there's another freshwater crisis

through pollution caused by over development.

Surely, we should be learning the lessons.