NARRATION

We've made it to Norseman, and tee off at the world's longest golf course.

Dr Derek Muller

I'm gonna go the driver. Nailed it.

Anja Taylor

Nailed a tree.

Simon Pampena

Can we go now?

NARRATION

Excitement levels couldn't be higher as we start our journey across the Nullarbor.We've left Lake Lefroy and our mining friends in Kalgoorlie to cross the Eyre Highway and the dry, hot, seemingly endless plain known to many weary travellers as the 'Nullarboring'.

John Eyre, the first European to stumble across this region, described it as 'a hideous anomaly, a blot on the face of nature, the sort of place one gets into bad dreams'.

But for those who know where to look, the Nullarbor is anything but dull.

Anja Taylor

I think it's here. We're close. According to this, straight up ahead, and it should be just at the end of this little section.

Dr Derek Muller

I've heard that before.

Anja Taylor

We're about to embark on one of the most visually spectacular, exciting and challenging science expeditions to be had. Hang on, what's that?

Dr Derek Muller

I just see some rocks.

Anja Taylor

That thing. Look!

Dr Derek Muller

I can't even see it.

Simon Pampena

Derek, Derek, be careful.

Anja Taylor

There they are. There they are.

NARRATION

This is the entrance to Weebubbie Cave. What we're seeing here is a fraction of the beast inside.

Anja Taylor

Look at this thing!

Dr Derek Muller

That is something. Whoa!

Anja Taylor

That is ginormous!

Ian Lewis

It does go further.

Anja Taylor

It's not exactly 'wee', is it?

Ian Lewis

And when you get to there you're only halfway to the bottom. The rest of it is down a giant slope into a room.

NARRATION

In fact, the Nullarbor is littered with gigantic underground caves and passages. That's because it's made up of the largest continuous block of limestone in the world. Formed when the whole plain was underwater, it built up over millions of years from the skeletons of tiny sea creatures. When the sea retreated, the limestone was exposed to the elements and cave formation began.

Ian Lewis

Here it comes!

NARRATION

Weebubbie is the deepest cave in the Nullarbor...

Man

Lowering!

NARRATION

..plunging 100 metres to intersect the major aquifer of the Eucla Basin. It's a mecca for cave divers. And today we're on a rare mission of discovery with sports scientist Dr Peter Buzzacott, subterranean ecologist Dr Stefan Eberhard, and Ian Lewis, a karst geomorphologist.

Anja Taylor

So, you drew this in 1972?

Ian Lewis

1972 was the first cave-diving expedition here. And then I came back and did this vast map. This is what we did in those days - draw up maps like this.

NARRATION

40 years ago, Ian led the first scuba diving expedition under the Nullarbor here at Weebubbie.

Ian Lewis

We discovered everything from here, all through to the hidden lake in here.

NARRATION

Prior to this, only a handful of foolhardy adventurers had explored the dry caves of this region, locating telltale holes - or dolines - from the air.

Ian Lewis

This is a carbide lamp that burns acetylene, and cavers and the old explorers of the Nullarbor used to use these. And I brought Fred's hat. He died recently. He was ninety five. But he had a great life and he was involved with exploring the dry cave here in the 1950s. So I brought it out here to celebrate Fred's life and fifty years caving here.

Anja Taylor

Ohh.

Simon Pampena

Beautiful.

Ian Lewis

So, this is what Fred would've looked like. Probably I look about his age, I think.

Anja Taylor

Down we go.

Dr Derek Muller

Doesn't look so bad.

NARRATION

The walls of this doline are absolutely massive. And getting us and the gear down is a monumental task, not for the faint-hearted or inexperienced. Even in a wetter climate, it's hard to fathom how these structures grew so large. If the water table's down there, I mean how does it end up carving out this whole hole?

Ian Lewis

OK, this hole was not carved. This is broken by falling in. It's all jagged. And it fell into the tunnel, which is lower down, that was dissolved by the water.

Dr Derek Muller

So the chamber formed before the hole formed?

Ian Lewis

Exactly. You can't have an entrance without a chamber to start with.

Anja Taylor

It's very Indiana Jones. There's rungs broken.

NARRATION

It drops from a sweltering 30-plus outside to a cool 18 degrees in the cave. Still, there's 200 metres of rubble to scramble over with scuba tanks in the pitch black. So there's nothing easy about this adventure.

Simon Pampena

Can't go to the loo in the rocks. I'm gonna go down to the water.

NARRATION

With tiny headlamps, it's impossible to see more than a few metres in front. But throw a little light on the subject, you could literally fly a plane through here.

Anja Taylor

It's huge!

Ian Lewis

It's enormous.

Anja Taylor

It's so enormous!

Ian Lewis

One of the mysteries is WHY is it so enormous? These massive, big tunnels. How can we explain their size?

Anja Taylor

I mean it's supposedly rainwater, isn't it? It must've been torrential.

Ian Lewis

People think huge rivers must have flowed through here, but there is no evidence of that.

NARRATION

Millions of years ago, when the Nullarbor uplifted, these layers of porous limestone fractured and weakened. As rainwater seeped through the limestone and gathered in the water table below, it opened up the fractures, dissolving them into chambers, which collapsed into giant caves like these. But Ian doesn't think that's the whole story.

Ian Lewis

If it was just sitting there dissolving away very slowly, it couldn't account for the size and the space. So we are beginning to wonder whether there are some other mechanisms that helped the rock dissolve much faster.

NARRATION

To get the bottom of the mystery, Peter Buzzacott is searching for clues in the lake. 50 metres ahead, and another hour of schlepping our heavy gear. But it's worth it. Whoa! Wow!

Simon Pampena

Oh, wow!

Dr Derek Muller

I love the colour. That's incredible!

Simon Pampena

That is beautiful!

Anja Taylor

It's so blue!

Ian Lewis

It's just beautiful, and it curves out of sight way down that big tunnel about twice the distance that you can see.

Dr Peter Buzzacott

Here, we get down to, say, 50 metres deep. So we get a snapshot much further back in time as cave divers.

Ian Lewis

We're able to start to see from underwater what the water's doing to the limestone.

NARRATION

This is my first cave dive ever. so I'm pretty spoilt, if not a little nervous.

Dr Peter Buzzacott

This is where you join a very exclusive club.

Anja Taylor

Yes. I'm so very privileged to be doing this as my first cave dive. Ready?

Dr Peter Buzzacott

Well, you'll be seeing something now that so very few people will ever see.

NARRATION

The water is so breathtakingly clear at Weebubbie, it's like flying through another planet.

Dr Peter Buzzacott

We all say that. 'Cause you can't see the water. It's invisible.

Ian Lewis

We measured a visibility at five hundred feet. I've never been anywhere in the world that does that.

Dr Peter Buzzacott

It's the best visibility in the world.

NARRATION

It's ironic that visibility is the one thing real cave dwellers can't appreciate.

Simon Pampena

What's that?

Dr Stefan Eberhard

It's a slater.

Simon Pampena

That's just like the ones I find in the garden.

Dr Stefan Eberhard

See how they're completely white?

Simon Pampena

Yep.

Dr Stefan Eberhard

They're cave slaters.

Simon Pampena

Right.

Dr Stefan Eberhard

They're called troglobytes.

Simon Pampena

Troglobytes.

Dr Stefan Eberhard

Not troglodytes, which are people that live in caves. But these are troglobytes, which are animals that live in caves.

Simon Pampena

So, you're a troglodyte.

Dr Stefan Eberhard

And so are you.

Simon Pampena

Right, yes. I hope not forever. And these are troglobytes.

Dr Stefan Eberhard

Yes.

Dr Stefan Eberhard

If you look closely, they've got long antennae. And they don't really seem to be affected by our light. They've probably got no eyes.

Simon Pampena

Right.

Dr Stefan Eberhard

You don't need eyes when you live in a cave when it's completely dark. So that's an evolutionary adaptation to this environment. Every time we look, we find new species. So, this is just the tip of the iceberg.

Simon Pampena

Ohh! Iceberg is not a bad word for it. Ohhh! Ohhh!

Dr Derek Muller

I don't know if those are good noises or bad noises.

Simon Pampena

Whoo! Oh, Man!

Dr Derek Muller

That's quite cold.

Simon Pampena

It's very cold!

Dr Derek Muller

It's fresh. It's very fresh. What temperature do you reckon it is in here?

Simon Pampena

At a guess, I'd probably say about, maybe, 10 to 15.

Dr Derek Muller

No way! They measured the temperature in here. It's, like, 17 or 18 degrees.

Simon Pampena

Oh, right.

Dr Derek Muller

You know that the temperature actually works out to be the same as the air temperature and the same as the rock temperature and the same as the yearly average temperature on the surface.

Simon Pampena

Wow!

Dr Derek Muller

So it all kind of averages out down here.

Simon Pampena

It's very consistent.

Dr Derek Muller

It is.

NARRATION

Temperature is exactly what we're interested in down here.

Dr Peter Buzzacott

The water in these caves, in a big cave like this - you have a big lake, you have a big chamber - it should be the same temperature as the average air temperature outside because there's plenty of opportunity for heat exchange.

NARRATION

Pete's temperature loggers show the lake is just over 18 degrees, as you'd expect. But in other caves on the Nullarbor, something strange is happening. The water is warmer. In a cave called Murra-el-Elevyn, Pete made a startling discovery.

Dr Peter Buzzacott

It was 19 degrees. And everyone said, 'Oh, great, the water's warm.' But I laid there at night thinking, 'Why?' So I started measuring the temperatures to see where the warm water might come from. And I put my little temperature loggers through the cave and I found out that it was 19.5 degrees over there.

NARRATION

The further in he swam, the warmer it got.

Dr Peter Buzzacott

And I followed the temperature through the cave till finally, three hundred metres in, after a maze of passages left and right, I came across an area you could feel warm water coming out through this broken rubble on the floor. And that was the most exciting thing.

NARRATION

Exciting, because if warmer water is pouring in at Murra, it could be happening in caves all over the Nullarbor. Other caves nearby reach temperatures of up to 24 degrees Celsius.

Dr Peter Buzzacott

Suddenly it becomes interesting that there's this whole region of warm water which is unexplainable.

NARRATION

Could Pete's discovery help solve the mystery of why these caves grew so large?

Dr Peter Buzzacott

If it's a 24-hour, 7-days-a-week, 365,000-days-per-millenium flow of water coming into a cave, that enormous volume could not, I don't think, move through a cave without having some sort of formative effect.

Anja Taylor

That was so amazing!

Dr Peter Buzzacott

You know when we were at the end looking in, we were probably looking back 5 million years.

Anja Taylor

Really?

Dr Peter Buzzacott

Something like that.

Anja Taylor

Thank you so much. That was such a good... Oops, sorry. Coming to give you a hug. That was such a good experience.

Dr Ian Lewis

Ah, the diver emerges.

Anja Taylor

If I wasn't so cold, I'd go straight back in again. Pete's temperature data shows Weebubbie Cave also gets warmer further in. But what does it all mean?

Anja Taylor

You're a cave formation expert. I was just wondering what you thought about Pete's hot spring discovery. Do you think that's an exciting thing?

Ian Lewis

Well, I was a bit reluctant about the hot spring theory. Until I had a good chat with him and so on. But I'm very interested in the area in caves where water temperatures are elevated.

Anja Taylor

Why did you have reservations?

Ian Lewis

Because they aren't hot springs. They're not gushing out of the ground and we all jump in and have a lovely swim or whatever else. You've got warmer water coming slowly through the limestone as he's been able to measure it. And that's of great interest, in a geological sense, for caves developing.

Anja Taylor

Why? Why is that interesting?

Ian Lewis

That might mean that that water is coming from somewhere else, or it's charged up in minerals and so on. And that has big implications for how big caves might form here.

NARRATION

Ian's team has already shown how geothermal activity has shaped other caves in Australia. Giant sinkholes and caves in Mount Gambier, South Australia, line up with volcanoes in the area.

Ian Lewis

Gases from the volcanoes around Mount Gambier has come up into the limestone and made the water much more acidic. And that explains why it's dissolved such huge spaces.

Anja Taylor

But there's no volcanoes here.

Ian Lewis

No, there's no volcanoes here. But you don't have to have volcanoes to generate heat, or to generate changes in groundwater chemistry. The whole continent of Australia is floating on a magma. It doesn't form volcanoes, but it's able to warm and generate gases and so on that can work their way up through weaknesses in the rock, up through into the limestone and the groundwater and then acidify it.

NARRATION

Peter and Ian believe the elevated temperatures and structures of the Nullarbor caves may indicate similar geothermal processes.

Ian Lewis

It's a pretty new idea, but on a scale that we've found it in Australia, in the Mount Gambier area and the scale of the caves here, it starts to really open our eyes and say, this is a seriously new way to look at how large caves may form.