Tide goes in, tide goes out.

It's just some water moving

because the moon and sun are pulling on it.

But those words: "just some water" hide an incredible amount of matter,

and "pulling on it" hides an unimaginable amount of force.

And nowhere is that more obvious than right here.

I'm just north of the Arctic Circle in Norway,

half an hour's drive from a town called Bodø.

The land here is mostly what Douglas Adams called 'lovely crinkly edges':

and this particular lovely crinkly edge holds the Saltstraumen Maelstrom:

the most powerful tidal current on the planet.

And the reason it's so powerful:

that way is the sea.

That way is a fjord, a tidal inlet.

Twice a day here, the sea rises and falls by about two metres,

so the water level in the fjord tries to equalise.

The catch is that pretty much all the water it needs to do that,

400 million cubic metres give or take,

has to go through this channel.

There are other routes that it could take,

but the land masses make those slow and shallow.

They're winding country roads

whereas this is like a twelve-lane freeway.

It's... well, it's a perfect storm.

There's a huge mass of water on both sides providing pressure,

and the channel starts wide and steadily narrows to the centre both ways.

It's the right width, and the right shape,

to create this maelstrom of whirlpools and currents and vortices.

What surprised me the most, coming here and standing next to it,

is how much it changes, moment to moment, second to second,

every little twist in the tide and the water

creates a new vortex and a new boil.

I had to get close enough to stick my camera in on a long pole,

and honestly?

It terrified me.

Tour groups can take an inflatable boat out into the channel,

and you'll see occasional fishing vessels taking advantage

of all the marine life that gets shunted through here.

But unlike the whirpools of myth and legend, this maelstrom

isn't going to pull a modern, bouyant, powered boat down to the depths,

although a swimmer on their own would be in a lot of trouble,

and a large ship might lose control and be dashed against the rocks.

But all this force, all this water,

this is just a fraction of what's being pulled around

out on the ocean.

It's just here it's visible, it's on a human scale, it's threatening,

so here we pay attention to it.