found their way to ancient Egyptian tombs,

there were warnings they ignored.

"Do not open this tomb.

"Do not steal the riches inside.

"You will be cursed and you will die."

It's not true. Not just the curses aren't real,

but those warnings were just an urban legend.

The robbers wouldn't have been able to read them.

And even if they could, the messages weren't warning of curses.

They were just the ancient equivalent of a 'no trespassing' sign.

This is Olkiluoto, an island on the far west of Finland.

And here, they're building a tomb for something that really is dangerous.

In a few years, the deep bedrock here

will contain the world's first long-term storage site

for high-level nuclear waste,

right next to some of the reactors that produce it.

This isn't a video about whether nuclear power is good or bad.

I think it's good, and so, it seems, does Finland.

The third reactor here is being tested now and it's due to go online next year.

But this video is about a different question.

We do have nuclear power in the world,

so where do we put the waste?

I didn't know how it would feel until I stood here.

It is very strange to have 400 metres of rock above my head

and knowing that inside that solid plug of clay there

there is high-level radioactive waste.

This new tunnel complex is called Onkalo, which translates as 'cavity'.

And long-term storage really does mean long-term:

100,000 years.

To put that in perspective,

100,000 years ago, humans hadn't made it to Europe yet;

the continent was still frozen in the middle of an ice age.

Our species were just hunter-gatherers.

This facility is being designed to last for geological time.

And yes, despite the fact

that there are hundreds of nuclear reactors in the world,

right now there is no permanent geologic storage facility anywhere

for high-level radioactive waste, the spent nuclear fuel, the really nasty stuff

that could cause disasters if it leaks.

Now, the problem isn't urgent.

The world doesn't actually produce that much of it, relatively speaking.

A century's worth of high-level waste from all the current reactors in Finland

will fit in the final version of this one new facility.

But right now, around the world, high-level radioactive waste

is generally stored near reactors,

first in water ponds that require active cooling,

and then in concrete casks that'll last,

well, about as long as a concrete building would;

not long enough.

The US has been trying to work out a storage site for years,

but their one serious attempt, Yucca Mountain,

has been mired in politics for decades.

Finland, here, is closest to building that solution.

- The geology in Finland is very old.

We are talking about two thousand million years

is the age of what we have below our feet here.

And in that way, it's really stable

and it represents the most ancient parts of this Earth.

High-level waste will be disposed of with cast-iron copper canisters,

and the canisters are put into the hole in the deposition tunnels

and then these are surrounded by bentonite clay.

Clay has a low hydraulic conductivity,

which means that the water doesn't transport through the clay.

The spent fuel, when it's taken from the reactors into the disposal,

it's highly radioactive.

But with time, the radioactivity will decrease.

After 500 years, people can stand beside the canister

and it's not dangerous.

But there are some other long-lived radionuclides

which might be harmful

in case they were to come up to the surface

and into the drinking water, into the wells or biosphere,

and therefore the geological disposal

will isolate the waste for 100,000 years.

- This area isn't prone to earthquakes or other natural disasters.

Geologically, it's about as peaceful as you can get.

So, the plan is: dig the tunnels

and then, for 100 years, put the casks of waste in,

and then backfill each tunnel with clay so they're encased in solid rock.

And once the site is full, fill in every bit of it,

every last tunnel, and then leave it forever.

There are multiple levels of containment.

The idea is that even if one or two of them fail,

everything will still be safe for 100,000 years,

unless someone digs it up.

When the US government was thinking about using Yucca Mountain as its storage site,

they commissioned a now-famous report on how to mark a site like this.

How do you warn people away,

even if our civilization has collapsed,

our language has fallen apart,

and no one knows what radiation is?

There were all sorts of ideas, from a spike field to 'forbidding blocks'.

But all those would ultimately be a big sign saying,

"Hey, there's something interesting under here; dig!"

- The facilities will be closed in about 100 years,

6,500 tonnes of uranium.

We have not decided what is the best way to mark that.

- There was one other idea in that research paper:

Don't mark the site at all.

One the repository is full,

fill it in, seal it shut, and then hide it

so there's nothing remarkable at the surface.

There are very few reasons to dig this deep, particularly here.

The odds are slim that some future archaeologist

would deside to dig 400 metres down in this remote location.

And if they did, if someone ignored all the warnings

or arrived after everything had been eroded away up top,

well, if they did break through all the containment

in search of treasure,

what they'd get is radiation poisoning and a particularly unpleasant death.

If the grave-robbers that dug up Egyptian tombs

had started rapidly and obviously dying from a real curse,

maybe they would've taken the hint.

Thank you so much to all the team at Olkiluoto

who showed me and all the camera teams who are down here today around.

You can find out more about them in the links in the video description.

All right, thank you. Your turn!