Not only do Roman and his team of engineers need to relieve up left pressure, they also have to check that the structures connection to the mountain is rock solid.

That's what I'm going to look at next.

I gotta go meet Man called Thomas, who is well as well as working at the dam Bi's of a keen mountain climbers who skips about this place like a mountain goat.

Beating them there in some sort of cave and he's arranged for me is a special treat.

The opportunity to ab sail down.

It's kind of thing he loves doing.

So set up ropes people to help me everything I need.

But I mean, I think he's misunderstood the meaning of the word treat.

Really, that's not treat.

This is like, you know, missile.

Definitely helmets.

What difference is that gonna make?

We got to talk on our fate.

Oh, I'm going.

Okay.

That you ready to go?

Oh, God, this is going against my Yep.

Yep.

Looking down looking.

Oh, good.

Oh, I can't keep this up.

That's not at sleep.

Me?

I well bottled.

I really, really don't like heights.

It actually went something like this.

Well done.

Good.

Oh, that's good.

Okay.

Started that, uh, come to me.

Time up.

Well, you told us.

Do what?

Climb up there.

Yeah.

Fear of heights or not.

It appeared there was no way of ducking out of this one, because you know me.

Okay.

Okay.

No, this is just stupid.

Uh huh.

Oh, it's homos.

Hello.

Very unhappy, Thomet.

Thank you.

Right.

And you go, Mark.

Tell me.

Didn't work the day you give Thomas.

In the 19 fifties, engineers first dug out a cave here to test the strength of the rock and check.

It was strong enough to hold a dam so his caves go.

This is an important one.

That's make sure the rock was the strength to hold down.

That's how much down worked.

The strength of the men's set.

It really is holding mass machine society inside the rookie strong we can build that was received with No.

I think I have a case because they will tend to be as good back then stands for Terry.

Don't worry.

We'll see in the next to go after satisfactorily testing the rock on one side of the valley.

It then took 800 workers, six years and nearly 4.5 1,000,000 tons of concrete to build this enormous structure on.

After a latte work and £85 million they discovered they'd made a colossal mistake about 40 years ago.

When they finally finished the epic task of building all of this and started to fill the reservoir behind it, they noticed small leaks, period.

Not a good moment.

I'm probably not a good feeling here.

They investigate it.

And it turned out that the rocks in the mountain on this side was slightly stronger, slightly stiffer than the rocks in the mountain on this side, which meant an imbalance.

And if there's one thing you want in a damn especially one of this shape on this record, breakingly thin its symmetry.

They thought about it and decided to build a supporting structure.