[ethereal music] [Steve] Somewhere deep in there... is probably the thing I'm most frightened of in the whole world.

[Scott] Clear rope.

[Steve] I'm Steve Backshall and I've brought a team of explorers to the Yucatan Peninsula in Mexico.

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

[Steve] Look at that!

[Steve] We'll journey into a secret underworld...

Descending!

...the Maya called "the place of fear".

The thing that sets us apart as a species, as human beings, is our desire to explore, to discover, to solve puzzles.

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

To learn how they lived...

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

...and died.

It's a skull!

It's a human skull!

This is more the kind of fear that I had when I was a kid and was scared of monsters under the bed.

The stuff of nightmares.

[muffled yelling] I'm in Mexico on an expedition that will push me to extremes.

This is the Yucatan Peninsula.

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.

But as far back as 2,500 years ago, the ancient Maya did penetrate deep into some of the caves.

We want to find these caves and explore them for ourselves.

I want to know why the Maya risked their lives exploring these dark, dangerous places.

And, if possible, push deeper than they or anyone else has explored before.

[Steve] 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 Maya are the indigenous people in this part of Central America.

Many still live here.

[car horn] At the height of their civilization, they built incredible pyramids to the glory of their many gods.

The Maya believed in 12 gods of Earth that resided in a terrifying underworld... called Xibalba, and caused every sort of human suffering.

They appeased them with bloody sacrifices.

They also believed some special caves were portals to this world.

We want to enter this hidden domain.

[Steve] Hey, Memo.

This place is remarkable.

You can feel the history here.

The man who can help us unlock the secrets of the underworld is archaeologist and Maya expert Guillermo de Anda... also known as Memo.

Yeah, it's all pre-Hispanic Maya temple.

This is the sacred entrance for the ancient Maya.

[Steve] So, this is where we're going?

Down here?

This is where we're going today.

Shall we get the lid off this thing?

Memo has brought us to a place full of Maya artifacts yet to be fully explained by the handful of people who've seen them.

We've come equipped to take Memo down and document what we find.

[Steve] Whoa!

These ruins are a few hundred years old.

But beneath them is a cave that's much, much older.

Because of what it contains, I'm not allowed to reveal its location.

[Aldo] What's that, about 15, 20 meters down?

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

Our safety expert, former Royal Marine Aldo Kane, has the job of getting us down the well shaft and into the cavern below.

[Aldo] Have you ever dived in a well before?

I have never dived in a well before, no.

First of all, we got to figure out how we get down the thing.

And also, more importantly, how we get someone out.

I reckon we use the van, use your vehicle, rig off the back of that, get a frame up.

It's easy enough to get you guys down inside there.

It's getting you out, A, if something happens and, B, just when you're tired at the end of a dive.

Also part of the team are dive supervisor Scott Carnahan... underwater camera operator Katy Fraser... and her safety diver, Gabriel Gasca.

Left, right.

Left, right.

The challenge is to get us all safely down into the well and back out again.

We'll be wearing full dive gear.

Are these my cylinders here?

-Yeah.

-Those are my cylinders?

And we can't damage any of it on the way down.

[Aldo] We're dealing with a lot of unknowns.

There's probably quite a lot that could go wrong.

Modern Mexicans built the well to bring up water, but that's not what Memo thinks motivated the ancient Maya.

To them, this cave seems to have been far more important.

Memo has been diving in this well cave before and believes the site had a special but grisly significance for the Maya.

He thinks they came here to carry out human sacrifices.

[Steve] That's pretty scary.

Effectively, we're going in to a sunken mausoleum.

It is the kind of stuff that wakes you up in the middle of the night in a cold sweat, what we're about to see.

It is the stuff of nightmares.

[Aldo] It's not ideal 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.

[Aldo] 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.

For once on an expedition, I'm not thinking about the logistical problems.

Lean out, a bit further, try and get out past the edge.

Everyone happy how that went?

[Steve] These caverns are living graveyards, living mausoleums.

[Scott] 2 meters to the water.

[Steve] This is more the kind of fear 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... and death and all the things that haunt us most.

But conquering those fears could lead to incredible discoveries.

Descending!

[Aldo] OK, Steve.

We're gonna lower on the working line!

[Steve] Send it down!

Oh, wow!

That is absolutely incredible.

Ah.

-[Scott] Past the hole.

-[Aldo] Roger.

- [Aldo] Diver on the bottom.

- [Scott] Diver's floating.

This sealed chamber is spherical and about 25 meters across.

A mound of rock in the center was once part of the ceiling, long since collapsed.

The well above now covers that hole.

[Steve] Guillermo, I see something.

[Memo] OK?

[Steve] It's a horn.

We're just descending down, underneath the daylight of the well above.

And in this area, it's possible that the animals have fallen in by accident.

That is the skull of a bull.

What's this on the teeth here?

[Memo] Steve, I really don't know.

It looks like gold.

[Steve] Putting it in the teeth of your cattle.

That seems crazy.

[Memo] I have never seen something like that.

The unusual appearance of the teeth could be caused by the diet of the animal.

OK, what do you think?

Shall we go down?

[Memo] Let's go for it.

Look at that, Memo.

It's a pot!

It's beautiful.

How old do you think this might be?

[Memo] This might be 1,000 years old.

Look how well preserved it is.

[Steve] Wow!

Maya pottery is extremely fragile.

It would have been smashed if someone had just thrown it down here from the ceiling.

[Steve] They weren't just dropped in here by accident.

[Memo] No, no, Steve.

It would be too many accidents.

[Steve] What would be in these pots?

[Memo] Usually, food, or maybe a sacrificed animal.

[Steve] Memo, what's that?

[Memo] Oh, where is it?

[Steve] Just below you now in your torchlight.

Just there.

It's a skull!

It's a human skull!

Whoa!

It looks like it's screaming.

It's like something out of a horror movie.

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

[Memo] This is... very important because it might belong to somebody that was high class, elite.

[Steve] How do you know that?

[Memo] If you look closely at the teeth, you can see they have been modified for symbolic purposes when this person was alive.

[Steve] I knew the ancient Maya were advanced with astronomy and mathematics.

I didn't know they had dentists.

[Memo] They mastered that technique.

[Steve] There are bones everywhere.

[Memo] Yes, this is, er, without any doubt a very sacred place.

[Steve] What's that?

What have you got?

[Memo] It's a complete body.

[Steve] Those are, those are under an overhang.

[Memo] Yes.

Yeah.

[Steve] It looks like they were placed there, Memo.

It was hard enough for us to get in with modern ropes and climbing equipment.

[Memo] Even though the ancient Maya couldn't dive, they came all the way into the cave.

Floating, maybe, in a little kayak, and made an offering.

The position of this body suggests that it was deliberately placed here, which could mean all the other artifacts were, too.

This is clearly not random.

It's some sort of shrine.

[Steve] Everywhere you look... there's another body... another skull, grinning at you from the darkness.

There are women, grown men and children.

This place has so many stories to tell, so many secrets, so many mysteries.

[Steve] Each body we found must have belonged to someone who earned their place here.

We'll never know their personal stories.

But Memo is convinced one thing bound them all in common.

They suffered a violent death.

[Steve] How do you know that these bodies were sacrificed?

[Memo] 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, very skillfully rip apart the heart.

Sometimes this happen with the victim was still alive.

[Steve] They ripped out their hearts and they were still alive?

[Memo] Yes.

Ancient Maya artwork shows gruesome offerings to the gods.

Skin and flesh were removed from victims and sometimes eaten.

Beheadings were common.

And some people even inflicted injuries on themselves to offer up their own blood.

[Memo] We learn a lot of the life of the ancient Maya through examining their death.

It's clear the Maya were capable of brutal killings to please the gods, but Memo thinks they were willing to sacrifice even those who were closest to them.

[Steve] Do many people come down here, Memo?

[Memo] None.

Fortunately.

This is, like, still very well-preserved.

There's another cave a short distance away from the well that Memo wants me to see.

[Steve] Seriously, got a chill went up the back of my neck as soon as I saw that.

[Memo] So, you're looking at a piece of Maya history.

We call it "little hands cave".

[Steve] This is really pretty special that these, these are the handprints of a Mayan child left - over 1,000 years ago.

- 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.

[echoes of child laughing] It's interesting that, I've seen prints like this in many caves around the world.

In New Guinea, in Borneo and in Europe.

There's clearly something universal about leaving your mark behind like this.

Right where you are, if we look up, we should find some, er, little feet, maybe a 1- or 2-year-old.

[Steve] So, there's no way that a child could have deliberately put their hands and feet up there.

It had to be done by someone else, by an adult.

[Memo] There was a lot of respect for caves.

And there was fear about caves.

We believe it could be, maybe, a signature of something or a reminder of an event, or a rite of passage.

These Maya had been children that were sacrificed afterwards.

It was part of a ritual.

[echoes of a child laughing] [Steve] 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.

[Memo] Yes, oh, yeah.

Yeah, maybe this was the last, the last thing they do on their life.

[Steve] They look like they were left yesterday.

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 2.

If they were killed here, their remains are missing from this cave.

[Bell tolling] All we hear about, about the ancient Maya is all quite macabre.

It's all about human sacrifice and kinda quite grisly.

Is that true, do you think?

[Memo] You know, the first time you encounter this, this remains, it's very emotional.

But this was part of their cultural vision.

It was part of their religion.

It was part of their everyday life.

I wonder, if such sacrifice was such a big part of their culture, especially with children, but how often were they actually doing that?

'Cause I guess it would be counter-productive to be killing that many children.

I'm not trying to justify anything but, er, 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.

[indistinct chatter] The cruelty the Maya engaged in to appease the gods of Xibalba is evident everywhere.

[Memo] We don't know exactly but we do believe that most of these sacrifices or offerings occur trying to please the gods, 'cause they were having problems with droughts, there's no rain, there's no agriculture, there's no food production.

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

The evidence we've seen so far suggests that to the Maya the caves were a direct line of communication with their underworld gods.

And how much more is still out here?

I believe...

I believe we will find much more.

Beneath the Yucatan Peninsula is a latticework of caves many hundreds of kilometers long.

And the Maya pushed further into them than I would ever have believed possible.

Memo's taking me further south to a gigantic labyrinth of dry passageways.

Its outline's been mapped but it hasn't been completely surveyed for the Maya secrets it contains.

This is where we can help Memo.

With our team and equipment, we can explore more comprehensively.

It won't be easy.

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

We're taking every precaution.

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.

[Memo] All right, let's go.

[Steve] OK, let's do it.

Shall we?

[Aldo] Yeah.

[Steve] Unless both you and I take a catastrophic fall, then we should be able to come out and raise the alarm.

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

- So is that 6am?

- Yeah.

Radios don't work through hundreds of meters of rock, so if we miss our deadline, the surface team will have to presume there's a problem and send help.

Yeah, that's you.

Oh, it gets tight already.

[Memo] It is.

[Aldo] Just go careful, mate.

- We're passing bags through.

- Yeah.

[Aldo] You definitely said no more caves, Steve.

-A few years ago.

-[Steve] I know.

I did.

[Aldo] Do you know what's amazing?

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?

-[Aldo] The way they did.

-[laughs] [Aldo] Bare-footed with burning torches.

Shout me when you're through.

[Steve] 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?

[Aldo] Er, throw it all down that fault line.

Perfect, mate.

- [Memo] Good shot.

- [man] Shot.

So, when you're coming down, don't run the rope over an edge or somewhere that it could get jammed.

[Steve] It's so humid in here.

[Aldo] Probably a 5-meter drop at the bottom.

Is the rope enough?

[Aldo] Plenty.

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

Order of march, Steve?

Next, who do you want down?

I'm gonna send Memo down next.

[Aldo] Yeah, clear rope.

You OK, Memo?

[Memo] I'm fine, thank you.

- It's very slippery.

- [Steve] Yeah, yeah.

It is hot.

If they came down this on a rope made from natural fibers, how did they climb back up?

They may have had some kind of scaffold system.

Or maybe wooden ladders.

But we have not seen evidence of that.

We're four hours in... and all around us are signs of the Maya.

[Aldo] It's weird, it's like stepping back in time.

[Steve] What's that?

[Memo] Ground stone.

[Steve] What?

Must weigh 25 kilos.

How'd they get that down a cave?

It's amazing.

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

What have you got, Guillermo?

[Memo] Ahh, it's right here.

It's a beautiful, beautiful ceramic pot.

First time I see something like this.

You've never seen this before?

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

[Steve] It's perfect.

It's even sculpted!

Ceramic pots were far more to the Maya than just containers.

They were hard to make, but were more than purely practical.

They had spiritual significance too.

[Memo] It's perfect, it's beautiful, it's unusual.

[Steve] How on earth did they get it in there?

[Memo] They place it there.

They hide it.

The ancient Maya sometimes do that.

[Steve] Whoa!

And we can't even get it out.

- Yeah, no, no way.

- [both laugh] - [Steve] It's stuck there.

- It's like stuck, yeah.

Though just a pot to us, these were treasured possessions to the Maya.

It's another deliberate offering to the gods.

There are discoveries round every corner here, aren't there?

Of course, we could spend months inside this cave and keep discovering things.

[Steve] So cool.

[Memo] Ain't that amazing?

[Steve] Whoa!

We're now several hundred meters underground and we're finding their signs at the bottom of every chasm and passageway.

[Steve] Ah!

Oh, wow!

Look at that!

[Aldo] That is one of the most amazing things.

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

Amazing.

What do you reckon it is?

It's definitely a mammal, isn't it?

100%.

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

[Steve] 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.

That's so good.

[Aldo] Imagine how far in we are at the minute.

How long have we been going for?

Er, six hours at this point.

Six hours.

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

I'm amazed at how deep underground we're finding these animal paintings.

Another two hours further on and we're as far away from the surface as we've been on this whole expedition.

It's seems this cave was a test to anyone courageous enough to enter.

[Memo] Look at this, Steve.

It's a jaguar!

-[Memo] It's a jaguar.

-It's a jaguar.

I see it!

[Steve] It's looking back over its shoulder.

[Memo] It's amazing.

It's really amazing.

The motion.

And look at the tail.

- It's definitely a jaguar tail.

- [Steve] That's amazing!

[Memo] Which makes it different than the puma or cougar.

It's really a jaguar.

[Steve] It's beautiful.

The animal in front of it.

What's that?

[Memo] 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.

[Steve] Nothing here is done by accident, is it?

Yeah, definitely.

Anything that's done this far into the cave that's taken this much sweat - and effort.

- This effort.

[Steve] 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 goosebumps.

The Maya would have seen jaguar coming in and out of caves.

This is a shrine to supernatural beings in animal form.

The Maya thought the animals could pass to and from the underworld.

We are in the sanctum sanctorum of religion of the ancient Maya, the center of the Earth, the underworld.

Memo has given us a glimpse into the world of the Maya and how their powerful beliefs drove them deep into these dry caves.

[Steve] 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... [Memo] It's hard not to feel like that.

Yeah.

They celebrated their gods in dry caves like these.

But they believed the portals to the underworld were often in special flooded caves.

While Memo's work will continue here, we will need to look further afield.

-Thank you, Guillermo.

-No, thank you.

It's... it's really a privilege to share with you, my friends.

-Not many people have seen this.

-It's great.

Well, since the ancient Maya times.

- Exactly, exactly.

- [Steve laughs] Thousands of years ago.

All around the Yucatan is evidence of how successful the Maya cities were.

But around 1,200 years ago, they were mysteriously abandoned.

Yucatan is incredibly flat.

So, anytime you find a hill, it's not gonna 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.

These pyramids were built as temples to the Maya gods.

Many think they were designed to reach up to the sky in a way that mirrored a cave reaching down to the underworld.

And where they crumble and become overgrown with jungle, it's created a habitat that's just crying out to be explored.

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 2 meters long.

No!

Look at that!

Huh!

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 paralyzed.

But this is not gonna be her meal.

This is a meal for her young.

Here she goes.

She's gonna grab it again.

So, she'll grab it in her jaws and then start dragging it to a 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?

[owl hooting] I'm back on the hunt for a Maya cave that hasn't yet been fully explored.

And further to the south, near the coastal town of Tulum, one of our team might have found what we're looking for.

This is Robbie Schmittner.

He's spent a career exploring and mapping new caves.

He's heard about a cave which has evidence of the Maya in it and a deep passageway leading to water that no one has explored.

To the Maya, a cave flooded with water marked an entrance to the underworld.

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, it could turn into nothing at all.

But it's still super, super exciting.

[bird chirping] Oh, cool.

Oh, it's a proper cave.

-[Robbie] Check this out, Steve.

- Whoa!

[Robbie] That's an old altar.

[Steve] That's amazing.

Look at that.

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

- Yeah.

- [Steve] That stalactite, that doesn't look like it's been placed there.

I think the altar's been built around it.

[Aldo sighs] Mind blown.

[Steve] Yeah.

The entrance of the cave is littered with signs of the Maya.

[Steve] Looks like it could have been, perhaps, something for carrying water in?

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

It's a special place, definitely.

And the further in we go, the more we find.

-[Aldo] What is it?

Pottery?

-[Steve] Yeah.

Yeah, it's a bit of ceramic.

That's completely unique.

[Robbie] We're gonna leave the daylight zone here right now.

We need a bit more light.

As far as we know, we're the only modern explorers who have been this far into the cave.

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.

But harmless to me.

Taps around in the darkness with these extended legs which function almost like antenna.

That's how it finds its prey.

We're now several hundred meters from the surface and the Maya trail has gone cold.

[Steve] Whoa!

- It's a lake.

- [Robbie] Yeah.

[Steve] A complete underground lake.

[Robbie] Crystal clear.

We've found what we'd been looking for.

This was the point where the mortal world of the ancient Maya ended and their supernatural world began.

We brought technology that can take us beyond this point to the place that the Maya feared the most - Xibalba.

[Robbie] 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 we'll start searching first.

The dark shadow marks the start of a water-filled cavern that might lead to a dead end or continue submerged for thousands of meters.

This is an opportunity for Robbie to map any new passageways.

And our findings will help conserve these caves.

We decide to camp at the cave's entrance and prepare to dive.

[birds squawking] Cave diving is often described as one of the most dangerous kinds of exploration.

I'm trained to master my control underwater... to avoid kicking up silt and to carry backups of every piece of vital equipment.

In mapped caves, we follow a pre-laid line to avoid getting lost.

But when we explore new caves, we have to take our own with us, and it is literally our lifeline.

[Robbie] I do need to prepare that guideline, 'cause when we measure the cave, 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 have to get through squeezes, passageways barely big enough for a person to pass through.

Having to do this myself fills me with utter dread.

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.

In the worst case scenario, it's body recovery.

There is no half measures, really.

Yeah, it's pretty scary.

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

We don't know what we find.

It's exploration, right?

[Steve] My main concern is that if I die tomorrow, my last meal will have been a pot noodle.

[Robbie] Nobody's gonna die tomorrow.

-Cheers to that.

-Cheers to the dive.

-Cheers.

-Cheers to that.

Cheers to that safe dive, safe journey, and safe return.

Goodnight, everyone.

Sleep good.

-Night-night.

-Night, dude.

Night, John.

[Steve] To one side is the jungle.

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.

[Steve] Get us a coffee, Aldo.

How did you sleep?

[Aldo] 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 anymore.

[Steve] Robbie, do you want coffee?

[Robbie] Sure!

How'd you take it?

[Robbie] Milk, please.

[Steve] Dive plan for this morning, Robbie?

[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.

That's impossible.

Not... Not... Not an option.

Right?

Thanks to our modern dive equipment, we have the ability to go through the portal to a place that, for the Maya, was the preserve of gods.

Their fear was at least partly supernatural, whereas mine is the very real fear of never coming out.

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

We're going for a slightly different system.

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

Instead, I'm going for a more conventional cave diving set-up, which should be safer, more maneuverable and help me get to places that I just wouldn't be able to with a big mask.

I go in front with the reel.

You know, stay close by the guideline always in reach distance.

Anything goes wrong, anything is not ideal -let's just call it and go back.

- All right.

Sounds good.

All right.

That's it.

In the dive team we have, Robbie, myself, camera operator Katy and Marty, her support diver.

[Aldo] 1400 hours.

[Steve] OK. - All right.

- I'm with you, Robbie.

[Robbie] All right, let's do it.

The Maya have led me and the team to places no human has ever set eyes on.

This optical illusion is caused by a shallow pocket of air trapped against the roof of the cave.

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

It probably, actually, from the fact my breathing's almost instantly going up quite a lot, probably means there's not a lot of oxygen in it, that it's mostly carbon dioxide.

So, I'll have to put my regulator back in just a second.

Oh, yeah.

The air in here is not good.

Definitely not.

Here, the cave drops down into a tight squeeze... the tightest I've ever attempted.

I can't go forward or backward.

I'm stuck.

[Steve's muffled yelling] I force myself to slow down... calm down... and think about my breathing.

I exhale... and the cave lets me go.

Robbie is waiting on the other side in a clear water tunnel.

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

Robbie 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 another squeeze and it's smaller still.

This one is more technically difficult than the last... but I'm determined to keep calm.

Something on my harness is caught.

Through to the other side.

I'm now further away from safety than I have ever been in my life.

Beyond here, I can see a third squeeze and it's smaller again.

This could be the decision of my life.

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

Robbie will continue on, mapping, while Katy and I follow the lifeline back out.

Wow!

Wow!

What an experience!

The question is... how much further did he go?

You ran out of line?

- I ran out of line.

- [Steve laughs] What happened, man?

Keeps going and going.

- It does?

- Yeah.

You gotta 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 encounters a squeeze so small he has to carry out the most complex maneuver in cave diving.

He removes both 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.

[Robbie] I just pushed on and on and at one point I'm like, "Whoops!

No more line."

And the cave keeps going.

[Steve] To carry on without line would be breaking the golden rule of underwater cave exploration.

[Steve] How much line do you have on here?

There's about 500 meters.

- Half a kilometer?

- Half a kilometer.

You've gone half a kilometer in the cave?

And that's nothing, that hasn't even... - Wow.

- Keeps going.

Wow.

That's amazing news.

We've been able to push deeper into these caves than the Maya ever did.

Yes!

Come on!

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

Being given the honor of being taken into that place, it's Robbie's incredible... experience and knowledge that's allowed that to happen, and, you know, for that I owe him big time.

The thing that sets us apart as a species, as human beings, is our desire to explore, to discover, to solve puzzles.

When I was a kid, I was so disappointed in thinking that I'd been born in the wrong generation and that all of the exploration had been done.

And I was wrong.

There are still fabulous, dark parts of the world that are left to be explored.

It's so exciting, just never knowing what you're gonna find next.

That's what exploration's all about.

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