North Antrim Coast Waterworld

Rathlin Island stands proud of the Atlantic Ocean.

An impressive craggy wilderness just six miles from Ballycastle.

The island's old name is Reachra -

the High Rocky Place.

But our mission will uncover the life

that's hidden below these spectacular cliffs.

Only a few explorers have ever been here.

I'm in Bruce's Cave where, legend has it,

Robert the Bruce watched a spider try and try again to spin a web.

And this is where our ambitious quest begins,

our 21st-century odyssey into Waterworld.

I've always looked out over water

and imagined the fabulous creatures that live in it,

and now I'm finally getting the chance to find out.

I learned to scubadive especially for Waterworld, so I'm a novice,

but the rest of our team is first class.

Have we got the right one?

You've lived and worked in Egypt, on the Red Sea.

I know this is a special day for you as well.

Absolutely. Every time I come back to this dive site it's always a joy.

The soft corals and sponges down there are next to none,

except possibly the drop-off in Strangford.

But the two of them, top of the range all the way in Europe

and most of the places in the Red Sea.

This gear is extraordinarily heavy.

I'm very calm... I'm very...

I hope everybody keeps their fingers crossed

that I can do this after all the training I've put in.

Here we go!

-Ready for this?

-Yeah.

'This mask allows me to speak to Bernard

'and the surface from the deep.'

Bathed in summer sunshine, the landscape is glorious.

But my mind is fixed on submarine delights.

With Jim at my side, we slip into Waterworld.

This is unbelievably exciting.

We're heading down, below the kelp forest.

This tangled canopy devours sunlight, smothers the rock

and prevents other creatures getting a foothold.

The wildlife that's waiting for us below 20 metres

looks more like plantlife.

Colourful dead men's fingers, slender hydroids

and delicate anemones cling to the edge of a darkening abyss.

Bernard has long been fascinated

by the mysteries of this secret kingdom.

I was always interested in little creatures of all sorts.

From when I was about eight I used to collect butterflies and things.

When I went underwater I suddenly could see

there's a lot of stuff here that not many people know much about.

This is where the sponges really start.

This grey sponge is elephant ear sponge.

They're very common round here, but you wouldn't take them to the bath.

They're full of silica, which is glass. They'd cut you to pieces.

We did a big survey all around the Northern Ireland coast

about 20 years ago

and that threw up that Rathlin was particularly unusual

and had a lot of different sorts of sponges,

more than anywhere else, I think.

You're used to seeing photographs and film of the tropics underwater,

but we're not used to seeing much film of underwater around the UK.

And on Rathlin in particular,

you get these beautiful gardens of soft coral, sea anemones, sponges,

all brightly coloured and all just mixed in there

but in a very attractive way.

I think there's a perception, especially in the UK,

that all of the animals and plants were named by the Victorians,

and we've been able to show here on Rathlin

is that even the North Atlantic,

there are still large number of sponges that don't even have a name.

Bernard's a scientist with the Ulster Museum,

where his samples are examined under microscope.

He's identified up to 30 potentially new sponge species here.

Some may exist nowhere else in the world.

Hunting for more, he leads us deeper still,

past encrusting anemones.

They may look like flowers, but they're mini-predators,

armed with deadly stinging tentacles -

and there's more.

Sea slugs are one of my favourite creatures.

Probably the first creatures that I got interested in

when I was diving and going underwater.

I think they are charismatic invertebrates, I would call them,

because divers all round the world now are really tuned into sea slugs.

And one of the other big discoveries that people made in recent years

was that some of them were taking the noxious chemicals from their food.

So the sponge or the bryozoan has something in it

that prevents fish from eating it.

The nudibranch can then reprocess some of those chemicals

and put them into their own body,

and then, in order to warn the fish that they mustn't be eaten,

would then go into quite bright colours.

And in an archway frosted with soft corals,

we're touched by the void.

When I go underwater I just feel so privileged

to just be able to go down there and know that I'm seeing things

that perhaps nobody's seen before.

And you can discover something completely new.

It seems to me it's the last frontier

where you can find a whole load of different sorts of animals,

different sorts of plants,

and be probably seeing some of those for the first time.

It looks like a tropical sea.

The archway is festooned, bejewelled, encrusted with life!

'All too soon we're low on air.'

OK, guys, I'm out of here!

A reminder that we can only be brief visitors

in a beautifully alien world.

Subtitles by Red Bee Media Ltd