0:00:32 > 0:00:35Rathlin Island stands proud of the Atlantic Ocean.
0:00:35 > 0:00:40An impressive craggy wilderness just six miles from Ballycastle.
0:00:41 > 0:00:44The island's old name is Reachra -
0:00:44 > 0:00:46the High Rocky Place.
0:00:46 > 0:00:48But our mission will uncover the life
0:00:48 > 0:00:51that's hidden below these spectacular cliffs.
0:00:54 > 0:00:57Only a few explorers have ever been here.
0:00:58 > 0:01:02I'm in Bruce's Cave where, legend has it,
0:01:02 > 0:01:08Robert the Bruce watched a spider try and try again to spin a web.
0:01:08 > 0:01:12And this is where our ambitious quest begins,
0:01:12 > 0:01:17our 21st-century odyssey into Waterworld.
0:01:18 > 0:01:21I've always looked out over water
0:01:21 > 0:01:25and imagined the fabulous creatures that live in it,
0:01:25 > 0:01:28and now I'm finally getting the chance to find out.
0:01:28 > 0:01:33I learned to scubadive especially for Waterworld, so I'm a novice,
0:01:33 > 0:01:36but the rest of our team is first class.
0:01:40 > 0:01:42Have we got the right one?
0:01:42 > 0:01:45You've lived and worked in Egypt, on the Red Sea.
0:01:45 > 0:01:47I know this is a special day for you as well.
0:01:47 > 0:01:51Absolutely. Every time I come back to this dive site it's always a joy.
0:01:51 > 0:01:55The soft corals and sponges down there are next to none,
0:01:55 > 0:01:59except possibly the drop-off in Strangford.
0:01:59 > 0:02:01But the two of them, top of the range all the way in Europe
0:02:01 > 0:02:03and most of the places in the Red Sea.
0:02:07 > 0:02:10This gear is extraordinarily heavy.
0:02:12 > 0:02:14I'm very calm... I'm very...
0:02:14 > 0:02:16I hope everybody keeps their fingers crossed
0:02:16 > 0:02:19that I can do this after all the training I've put in.
0:02:21 > 0:02:24Here we go!
0:02:24 > 0:02:25- Ready for this?- Yeah.
0:02:25 > 0:02:28'This mask allows me to speak to Bernard
0:02:28 > 0:02:30'and the surface from the deep.'
0:02:39 > 0:02:44Bathed in summer sunshine, the landscape is glorious.
0:02:44 > 0:02:47But my mind is fixed on submarine delights.
0:02:49 > 0:02:54With Jim at my side, we slip into Waterworld.
0:02:58 > 0:03:02This is unbelievably exciting.
0:03:27 > 0:03:31We're heading down, below the kelp forest.
0:03:31 > 0:03:35This tangled canopy devours sunlight, smothers the rock
0:03:35 > 0:03:38and prevents other creatures getting a foothold.
0:03:44 > 0:03:47The wildlife that's waiting for us below 20 metres
0:03:47 > 0:03:49looks more like plantlife.
0:03:59 > 0:04:03Colourful dead men's fingers, slender hydroids
0:04:03 > 0:04:08and delicate anemones cling to the edge of a darkening abyss.
0:04:11 > 0:04:14Bernard has long been fascinated
0:04:14 > 0:04:17by the mysteries of this secret kingdom.
0:04:17 > 0:04:20I was always interested in little creatures of all sorts.
0:04:20 > 0:04:23From when I was about eight I used to collect butterflies and things.
0:04:23 > 0:04:27When I went underwater I suddenly could see
0:04:27 > 0:04:31there's a lot of stuff here that not many people know much about.
0:04:36 > 0:04:38This is where the sponges really start.
0:04:40 > 0:04:44This grey sponge is elephant ear sponge.
0:04:45 > 0:04:51They're very common round here, but you wouldn't take them to the bath.
0:04:51 > 0:04:56They're full of silica, which is glass. They'd cut you to pieces.
0:05:00 > 0:05:03We did a big survey all around the Northern Ireland coast
0:05:03 > 0:05:04about 20 years ago
0:05:04 > 0:05:07and that threw up that Rathlin was particularly unusual
0:05:07 > 0:05:10and had a lot of different sorts of sponges,
0:05:10 > 0:05:11more than anywhere else, I think.
0:05:21 > 0:05:25You're used to seeing photographs and film of the tropics underwater,
0:05:25 > 0:05:29but we're not used to seeing much film of underwater around the UK.
0:05:29 > 0:05:31And on Rathlin in particular,
0:05:31 > 0:05:36you get these beautiful gardens of soft coral, sea anemones, sponges,
0:05:36 > 0:05:39all brightly coloured and all just mixed in there
0:05:39 > 0:05:41but in a very attractive way.
0:05:48 > 0:05:52I think there's a perception, especially in the UK,
0:05:52 > 0:05:55that all of the animals and plants were named by the Victorians,
0:05:55 > 0:05:57and we've been able to show here on Rathlin
0:05:57 > 0:06:00is that even the North Atlantic,
0:06:00 > 0:06:04there are still large number of sponges that don't even have a name.
0:06:09 > 0:06:12Bernard's a scientist with the Ulster Museum,
0:06:12 > 0:06:15where his samples are examined under microscope.
0:06:15 > 0:06:20He's identified up to 30 potentially new sponge species here.
0:06:20 > 0:06:23Some may exist nowhere else in the world.
0:06:26 > 0:06:29Hunting for more, he leads us deeper still,
0:06:29 > 0:06:31past encrusting anemones.
0:06:31 > 0:06:36They may look like flowers, but they're mini-predators,
0:06:36 > 0:06:39armed with deadly stinging tentacles -
0:06:39 > 0:06:40and there's more.
0:06:40 > 0:06:42Sea slugs are one of my favourite creatures.
0:06:42 > 0:06:45Probably the first creatures that I got interested in
0:06:45 > 0:06:47when I was diving and going underwater.
0:06:47 > 0:06:51I think they are charismatic invertebrates, I would call them,
0:06:51 > 0:06:56because divers all round the world now are really tuned into sea slugs.
0:06:56 > 0:07:00And one of the other big discoveries that people made in recent years
0:07:00 > 0:07:04was that some of them were taking the noxious chemicals from their food.
0:07:04 > 0:07:07So the sponge or the bryozoan has something in it
0:07:07 > 0:07:10that prevents fish from eating it.
0:07:10 > 0:07:13The nudibranch can then reprocess some of those chemicals
0:07:13 > 0:07:16and put them into their own body,
0:07:16 > 0:07:20and then, in order to warn the fish that they mustn't be eaten,
0:07:20 > 0:07:22would then go into quite bright colours.
0:07:24 > 0:07:29And in an archway frosted with soft corals,
0:07:29 > 0:07:30we're touched by the void.
0:07:30 > 0:07:33When I go underwater I just feel so privileged
0:07:33 > 0:07:37to just be able to go down there and know that I'm seeing things
0:07:37 > 0:07:40that perhaps nobody's seen before.
0:07:40 > 0:07:43And you can discover something completely new.
0:07:43 > 0:07:45It seems to me it's the last frontier
0:07:45 > 0:07:49where you can find a whole load of different sorts of animals,
0:07:49 > 0:07:51different sorts of plants,
0:07:51 > 0:07:54and be probably seeing some of those for the first time.
0:07:56 > 0:07:58It looks like a tropical sea.
0:07:58 > 0:08:05The archway is festooned, bejewelled, encrusted with life!
0:08:08 > 0:08:11'All too soon we're low on air.'
0:08:11 > 0:08:14OK, guys, I'm out of here!
0:08:16 > 0:08:19A reminder that we can only be brief visitors
0:08:19 > 0:08:22in a beautifully alien world.
0:08:25 > 0:08:28Subtitles by Red Bee Media Ltd