Underland

A mile to the west of a hill

marked by nine burial mounds,

there stands an old ash tree.

Her crown reaches skyward.

Her roots dive deep into the land.

Many have been this way before,

passing through the old ash and into the world beneath.

What is this strange song that calls us below?

Why do we seek the void?

...southern areas of the state and High Sierra.

Strong gusts and heavy rain moving overhead.

Travel conditions poor right through to morning.

Meanwhile, northern areas continue to enjoy

milder conditions than usual.

Our journey begins.

Somewhere beneath the skin of the Earth.

Where darkness thickens.

And sounds... stir.

Yet, even here, just inches below the surface,

this is a place so alien to ours above.

Where nothing is familiar.

And all is strange.

As I descend, the first thing that hits me

is the smell.

It's kind of a smell of a cave in the making.

Wet concrete and trash, maybe sewage.

You know, after years,

decades exploring the underground,

those smells now I completely associate

with the feeling of freedom.

When you're in the main system,

all of the smaller systems dump into it.

You wouldn't necessarily know

if it was raining on the surface

until all of those veins start pumping.

It turns into a subterranean flash flood in an instant.

Maybe the past 20 years of my life

I've spent exploring underground spaces.

I actually began my career in archaeology.

Eventually, I ran into these urban explorers.

These incredible groups of people

who were sneaking into abandoned places

and photographing them.

And it seemed to me

like a kind of archaeological practise.

I feel like I was really pulled into the underground.

This is just now what I do.

I guess I've made a whole career

out of writing about things that I find underground.

As a species, our presence on the Earth

extends further into the ground

than our buildings do into the sky.

One day, after we've gone,

I think this will be all that's left.

So what I wanna know is:

what do these traces say about us, about humanity?

What does it say about who we actually are as a species?

It's so dark in the cage.

You can't see much at all.

It just adds to the sense of fear.

Even for people who go underground a lot, I guess,

most of them never go two kilometres down.

Even when I was a child,

I always was trying to make sense

of the world around me.

My grandmother would call me 'Miss Why'.

I'd ask her things like what are the stars in the sky?

Why are they there? How was our universe born?

Why do we exist?

The answers just got thinner and thinner until,

at the end, you realise you've asked a question

that nobody in the world knows the answer to.

As scientists, when we know we need to

try to look for hard things,

we are often pushed to the extremes.

Extreme energies or temperatures or distances.

In this particular case, we're being pushed

to extreme depths.

That's because sometimes the experiments

you want to conduct are so sensitive

that you just can't do them on the surface.

There's too much interference

from all the radiation that bombards the planet.

But when you're underground,

at the bottom of a deep mine,

you can pretty much filter all that out.

That means that you can look for things

that you just can't see up there.

In my case, the thing we're looking for,

if we can actually find it,

would go some way toward answering the question

of how our universe developed and why we're even here.

The only problem is, this thing we're looking for

has never been directly detected.

Ever.

So, the question for me is:

How do you catch a ghost?

Deeper we dream.

Far beneath the roots of the old ash tree.

Down through an ancient rift of stone.

Where living walls show we have long been making

these journeys below.

Though those early human eyes encountered

simply so much darkness.

Since they could not fill that dark with light,

they filled it with story.

In some of these stories,

this world was a perfect inversion

of the human realm.

Where the feet of the dead walked soul to soul

with those of the living.

In others, it was a place from which

the roots of a giant world tree drew its power.

One which linked the heavens and the Earth

to this land of eternal night.

To do what I do, descending is not enough.

I need to find ways to augment my vision.

That's when you really start to see.

Sometimes in the drains,

you encounter what looks like trash.

But then you look closer and you see what it really is.

People live down here.

People who don't have homes at the surface

and who need to escape the heat

or the constant surveillance by police.

In cities, they often say that wealth rises up

into towers, into skyscrapers.

But it's also true that poverty sinks.

And it sinks underground.

Unfortunately, that means that when it rains,

people die.

Part of what I do is to document stories like these

to make them visible.

The underground is full of secrets like this.

Documenting those secrets is the preoccupation

of a global community of people.

CMHQ.

There are no windows.

They'd be separated completely from the outside world.

Canteen.

Looks like there's some bits left.

1978-dated newspaper.

But wherever we are, the drive,

the obsession of this community is about

a feeling that this stuff is just too precious

to rot in the dark.

Unseen.

So we have to do everything we can to figure out ways

to light it up,

and to bring that back to the surface.

These labs have to be kept incredibly clean.

Only the equivalent of a teaspoon of dust is allowed

to pass through them over the course of an entire year.

That's to protect all the equipment

that we have down here.

Equipment that's helping in the search for

what we call dark matter.

Dark matter is...

Well, we don't know what dark matter is.

That's the issue.

Black as the midnight sky on a moonless night.

We think it's a substance or a material,

just one that we haven't been able to detect directly.

It's never been seen, heard or touched.

That's why some call it 'The Ghost'.

The only reason we know The Ghost is there is because

we can see the gravitational effect it has

on things like the motion of galaxies.

Even the path of light.

That's how we've actually been able to map

how dark matter appears to be distributed

across the universe.

And from these maps,

we can see it's pretty prevalent.

In fact, it makes up around 85%

of the total mass of the universe itself.

It's kind of the big flagship sub-GV experiment...

That makes it all the more frustrating

that we don't actually know what it is.

The leading theory is that

it's actually made up of particles.

But just ones that mostly pass through regular matter.

But if, even if only rarely,

one of those dark matter particles

were to collide with say the nucleus of an atom,

that could tell us a lot about what dark matter actually is.

The challenge is, how do you build something

that could catch one of those events as it actually happens?

All around the world, in deep underground labs,

we've built these massive detectors.

Some of them are more than 15 metres across

and they sit in flooded caverns of ultra-pure water,

which filters out any cosmic radiation

that might have made it this far down.

Down here, only a ghost would get through.

The hope is that if we run the right experiment,

one of these detectors, these giant eyes,

will see a collision between a dark matter particle

and something in our world.

All the cabling down the sides is connecting...