0:00:30 > 0:00:34The world's greatest wilderness, the open ocean.
0:00:36 > 0:00:39It covers over half the surface of our planet.
0:00:45 > 0:00:49Here, there is nowhere to hide and little to eat.
0:00:54 > 0:00:57It's the marine equivalent of a desert.
0:01:00 > 0:01:03CLICKING AND SQUEAKING
0:01:09 > 0:01:13And patrolling this desert, spinner dolphins.
0:01:25 > 0:01:26They stick together...
0:01:34 > 0:01:38..in a super-pod, 5,000 strong.
0:01:43 > 0:01:47That maximises their chances of finding something to eat.
0:01:55 > 0:01:59Like all who live here, they must go to extraordinary lengths
0:01:59 > 0:02:02to make their home in the big blue.
0:02:25 > 0:02:30There are rare moments when these empty seas can explode with life.
0:02:37 > 0:02:42Lanternfish, off the Pacific coast of Costa Rica.
0:02:42 > 0:02:44They're scarcely bigger than minnows,
0:02:44 > 0:02:49but what they lack in size they make up for in numbers.
0:02:52 > 0:02:55They are one of the most numerous fish anywhere.
0:03:02 > 0:03:04Normally, they only come to the surface at night,
0:03:04 > 0:03:06to feed on plankton,
0:03:06 > 0:03:10but this immense shoal has risen during the day,
0:03:10 > 0:03:13almost certainly in order to spawn.
0:03:20 > 0:03:24For the dolphins, this would be a bonanza.
0:03:28 > 0:03:30CLICKING AND SQUEAKING
0:03:35 > 0:03:40They have located the shoal using their echo-sounding calls.
0:03:50 > 0:03:53But they have to get to it quickly.
0:03:58 > 0:04:00They are not the only hunters here.
0:04:00 > 0:04:04Yellowfin tuna have also detected the shoal.
0:04:13 > 0:04:18And behind them, with their two-metre wingspans, mobula rays.
0:04:27 > 0:04:31Now sailfish, one of the fastest fish in the sea,
0:04:31 > 0:04:33have joined the chase.
0:04:39 > 0:04:43The lanternfish may return to the deep at any moment.
0:04:49 > 0:04:52But now the dolphins have got here.
0:05:20 > 0:05:24They swim beneath the shoal, pinning it to the surface
0:05:24 > 0:05:28and forcing the lanternfish to pack more closely together.
0:05:28 > 0:05:31CLICKING AND SQUEAKING
0:05:34 > 0:05:38And now the sea begins to boil.
0:05:46 > 0:05:51The tuna charge into the shoal at over 40mph.
0:06:20 > 0:06:24The slower-swimming rays arrive at last.
0:06:31 > 0:06:33With their immense mouths agape,
0:06:33 > 0:06:36they scoop up the lanternfish by the hundred.
0:06:39 > 0:06:43The shoal has now been largely dispersed,
0:06:43 > 0:06:46and the sailfish pick off the survivors.
0:06:48 > 0:06:52In just 15 minutes,
0:06:52 > 0:06:56all that's left is a silvery confetti of scales.
0:07:07 > 0:07:11But here, such feasts are only too infrequent.
0:07:20 > 0:07:24Whilst the dolphins perform great feats of endurance,
0:07:24 > 0:07:27others are driven to even greater extremes
0:07:27 > 0:07:30to find food in this ocean desert.
0:07:48 > 0:07:50A sleeping giant.
0:07:54 > 0:07:56A sperm whale.
0:08:03 > 0:08:07This family is resting between bouts of feeding.
0:08:15 > 0:08:16Who knows what the owners
0:08:16 > 0:08:19of the biggest brain in the planet dream about.
0:08:22 > 0:08:24CLICKING
0:08:27 > 0:08:29One has a calf.
0:08:30 > 0:08:35It's about two weeks old but still dependent on its mother's milk.
0:08:35 > 0:08:36It's hungry.
0:08:38 > 0:08:40CLICKING
0:08:40 > 0:08:44It communicates with its mother using a pattern of clicks.
0:08:46 > 0:08:48CLICKING CONTINUES
0:08:51 > 0:08:53But its mother slumbers on.
0:08:59 > 0:09:03The calf, covered in sucker fish, of which it can't yet rid itself,
0:09:03 > 0:09:06has to be patient.
0:09:32 > 0:09:37Sleep over and refreshed, the whales move on.
0:09:39 > 0:09:43Sperm whales don't wait for their prey to rise to the surface.
0:09:43 > 0:09:47They swim down into the depths to find it.
0:09:52 > 0:09:56They take a series of heavy breaths...
0:09:58 > 0:10:00..to saturate their blood with oxygen.
0:10:02 > 0:10:06Then down they go.
0:10:14 > 0:10:18This entire family dives together in search of squid.
0:10:25 > 0:10:29A mother will push her body to the limits of her endurance,
0:10:29 > 0:10:33and already it's hard for her calf to keep up with her.
0:10:34 > 0:10:36CLICKING
0:10:43 > 0:10:47The calf sticks to its mother as closely as it can...
0:10:49 > 0:10:55..touching her frequently, as if for reassurance.
0:10:55 > 0:10:58FASTER CLICKING
0:11:08 > 0:11:11But 300 metres down,
0:11:11 > 0:11:15it seems the calf can't hold its breath any longer.
0:11:15 > 0:11:18CLICKING CONTINUES
0:11:39 > 0:11:43In their early years, calves are forced to sit out the hunt.
0:11:48 > 0:11:51The adults continue their dive.
0:11:56 > 0:11:58CLICKING
0:12:04 > 0:12:06The mother changes her calls
0:12:06 > 0:12:10into a series of louder and more rapid clicks.
0:12:16 > 0:12:20She's now using sonar to hunt down shoals of squid.
0:12:28 > 0:12:32At 800 metres, a burst of clicks.
0:12:32 > 0:12:34CLICKING BECOMES VERY FAST
0:12:38 > 0:12:40CLICKING STOPS
0:12:40 > 0:12:42Then silence.
0:12:45 > 0:12:47She's made a catch.
0:12:59 > 0:13:02A calf can have a long wait at the surface.
0:13:09 > 0:13:13A mother returns from the deep after as much as an hour.
0:13:15 > 0:13:18She has a stomach full of squid.
0:13:37 > 0:13:41Finally, this hungry calf can take some milk.
0:13:46 > 0:13:50It's one of the richest produced by any mammal,
0:13:50 > 0:13:54and the calf guzzles a bathful of it a day.
0:14:10 > 0:14:16It may be six years before a calf masters the art of deep diving
0:14:16 > 0:14:20and is able to find food for itself.
0:14:36 > 0:14:41The emptiness of the big blue is what makes life so hard for hunters.
0:14:47 > 0:14:52But it's this emptiness that makes it comparatively safe for prey.
0:15:01 > 0:15:06A baby turtle, hatched just days ago, is leaving the crowded,
0:15:06 > 0:15:11dangerous waters of the coast and heading for the open ocean.
0:15:18 > 0:15:22To start with, they fill their little stomachs with plankton.
0:15:24 > 0:15:27But soon they need something more substantial.
0:15:38 > 0:15:40Only recently have we begun to solve
0:15:40 > 0:15:45the mystery of where baby turtles disappear to in their early years.
0:16:01 > 0:16:04Hundreds of miles offshore, in every ocean,
0:16:04 > 0:16:08there are communities of young castaways.
0:16:16 > 0:16:20So anything that floats attracts them.
0:16:27 > 0:16:29A log.
0:16:29 > 0:16:32It may have been at sea for several years,
0:16:32 > 0:16:35and it has already become the centre of a small community.
0:16:37 > 0:16:40Young puffer fish are here for the same reason.
0:16:45 > 0:16:48A floating log is just the kind of refuge
0:16:48 > 0:16:51this young turtle has been looking for.
0:16:52 > 0:16:57Here, there's not only seaweed on which to graze, but barnacles.
0:17:10 > 0:17:13But it's important to stay under cover.
0:17:19 > 0:17:23A young ocean-going silky shark is here, too.
0:17:41 > 0:17:44It's learning what tastes good.
0:17:48 > 0:17:50And what doesn't.
0:18:09 > 0:18:13We now know that many young turtles stay in such places
0:18:13 > 0:18:16for several years, until adulthood.
0:18:29 > 0:18:34Even if it means facing the full force of the high seas.
0:18:41 > 0:18:45The sun beating down on the deep blue
0:18:45 > 0:18:48warms the surface waters so that they evaporate.
0:18:55 > 0:18:59As the vapour rises, it condenses into clouds.
0:19:01 > 0:19:06They rapidly build into gigantic, burgeoning towers,
0:19:06 > 0:19:11which eventually generate violent storms, some 1,000 miles across.
0:19:27 > 0:19:31Hurricane-force winds sweep across the open ocean,
0:19:31 > 0:19:35building waves that can rise to 30 metres tall.
0:19:49 > 0:19:52Out here, ships have been known to sink without trace.
0:20:06 > 0:20:13130 million containers are shipped across the oceans every year.
0:20:14 > 0:20:19And on average, four of them fall into the sea every day.
0:20:33 > 0:20:37In 1992, a few were lost
0:20:37 > 0:20:40that contained a consignment of bath toys...
0:20:45 > 0:20:50..including 7,000 plastic ducks like these.
0:21:01 > 0:21:05They started their travels 1,000 miles off Alaska.
0:21:09 > 0:21:14Some drifted right across the Pacific Ocean and reached Australia.
0:21:20 > 0:21:22Others were carried north
0:21:22 > 0:21:26and landed on shores between Russia and Alaska.
0:21:28 > 0:21:31They even found their way into the High Arctic.
0:21:45 > 0:21:48One duck, having been at sea for 15 years
0:21:48 > 0:21:51and crossing three oceans,
0:21:51 > 0:21:55eventually landed on the west coast of Scotland.
0:22:02 > 0:22:06Their travels vividly illustrate how a network of currents
0:22:06 > 0:22:12connects all our oceans into one gigantic circulatory system.
0:22:26 > 0:22:31Many of the inhabitants of the big blue rely on these currents
0:22:31 > 0:22:33to carry them to feeding grounds.
0:22:48 > 0:22:50The blue shark.
0:22:55 > 0:22:58It travels over 5,000 miles a year,
0:22:58 > 0:23:03riding on the currents, supported by its broad wing-shaped fins.
0:23:08 > 0:23:12This one may not have eaten for two months.
0:23:18 > 0:23:22But the currents can carry promising traces of fatty oils
0:23:22 > 0:23:26from many miles away and will lead it to its next meal.
0:23:34 > 0:23:38After days of travel, the smell of food gets stronger.
0:24:00 > 0:24:03A dead whale, recently struck by a ship.
0:24:07 > 0:24:10This could be a real feast,
0:24:10 > 0:24:13but the blue shark must be cautious.
0:24:22 > 0:24:24Great white sharks...
0:24:27 > 0:24:29..ten times heavier than a blue...
0:24:32 > 0:24:35..are highly possessive around a whale carcass.
0:25:02 > 0:25:06Great whites are eager to feed on energy-rich whale blubber,
0:25:06 > 0:25:09which we now know forms a major part of their diet.
0:25:22 > 0:25:24Once the great white has had its fill,
0:25:24 > 0:25:29smaller sharks, like the blue shark, tackle what's left of the carcass.
0:25:39 > 0:25:42As the oils from this dead whale spread more widely,
0:25:42 > 0:25:45more and more blue sharks appear.
0:26:06 > 0:26:10Within days, the carcass will be stripped of its blubber.
0:26:19 > 0:26:22Then, no longer kept buoyant by its oil,
0:26:22 > 0:26:26it will sink into the depths below.
0:26:29 > 0:26:32The blue, with its reserves of fat replenished,
0:26:32 > 0:26:36can now survive for another two months without eating.
0:26:59 > 0:27:05Over half of all animals in the open ocean drift in currents.
0:27:14 > 0:27:17Jellyfish cross entire oceans
0:27:17 > 0:27:21feeding on whatever happens to tangle with their tentacles.
0:27:26 > 0:27:29Some can grow to a metre, even two metres, across.
0:27:31 > 0:27:33And when, by lucky chance,
0:27:33 > 0:27:37they encounter a patch of sea rich in plankton,
0:27:37 > 0:27:39their numbers explode.
0:28:22 > 0:28:25It's such a successful strategy
0:28:25 > 0:28:29that jellies are one of the most common life forms on the planet.
0:28:39 > 0:28:43But among the jellies, and looking somewhat like them,
0:28:43 > 0:28:46is a rather more complex and sinister creature.
0:28:51 > 0:28:54The Portuguese man-o'-war.
0:28:59 > 0:29:03It floats with the help of a gas-filled bladder,
0:29:03 > 0:29:05topped by a vertical membrane.
0:29:07 > 0:29:09With that serving as a sail,
0:29:09 > 0:29:12it maintains a steady course through the waves.
0:29:36 > 0:29:42Long threads trail behind it, some as much as 30 metres long.
0:29:46 > 0:29:50Each is armed with many thousands of stinging cells.
0:29:52 > 0:29:56A single tentacle could kill a fish or, in rare cases, a human.
0:30:02 > 0:30:05But among its lethal tentacles lurks
0:30:05 > 0:30:10a man-o'-war fish that feeds by nibbling them.
0:30:15 > 0:30:19Whilst this fish has some resistance to the stings,
0:30:19 > 0:30:21it must still be extremely careful.
0:30:25 > 0:30:28Most other fish are not so lucky.
0:30:49 > 0:30:52A tentacle has caught this one and reels it in.
0:31:03 > 0:31:06It's already paralysed.
0:31:10 > 0:31:15Specialised muscular tentacles transfer the victim to others
0:31:15 > 0:31:20that digest the catch, liquefying it with powerful chemicals.
0:31:26 > 0:31:29Eventually, all that is left...
0:31:32 > 0:31:34..is a scaly husk.
0:31:42 > 0:31:44This voracious man-o'-war
0:31:44 > 0:31:47may collect over 100 small fish in a day.
0:32:10 > 0:32:15For the most part, the big blue seems featureless...
0:32:17 > 0:32:21..a place where the winds blow, uninterrupted by land.
0:32:26 > 0:32:30But beneath the surface there are long mountain ranges,
0:32:30 > 0:32:34deep trenches and isolated volcanic peaks
0:32:34 > 0:32:38that make it far more varied than the human eye can see.
0:32:44 > 0:32:48We are only just discovering in any detail
0:32:48 > 0:32:51how the inhabitants of the big blue exploit that.
0:33:01 > 0:33:05A lonely whale shark on a special journey.
0:33:11 > 0:33:17She is as long as a small aircraft and she weighs over 20 tonnes.
0:33:21 > 0:33:26Like many sharks, she does not lay eggs but gives birth to live young.
0:33:42 > 0:33:46She carries up to 300 of them in her swollen belly.
0:33:53 > 0:33:55She may be the biggest fish in the sea,
0:33:55 > 0:34:00but the place where whale sharks give birth has not yet been found.
0:34:06 > 0:34:11Today, however, we may be a step closer to solving this mystery.
0:34:13 > 0:34:16We have known that great numbers of whale sharks,
0:34:16 > 0:34:20at certain times of the year, appear around the Galapagos Islands.
0:34:22 > 0:34:25Here they assemble around a tiny islet
0:34:25 > 0:34:29that rises abruptly from particularly deep water.
0:34:33 > 0:34:35It's known as Darwin Island.
0:34:44 > 0:34:48Here, swirling currents bring up nutrients from the deep,
0:34:48 > 0:34:51so enriching these waters that they attract
0:34:51 > 0:34:55great concentrations of fish from far and wide.
0:35:03 > 0:35:07Thousands of hammerhead sharks also assemble here.
0:35:10 > 0:35:13They are nearly all female.
0:35:20 > 0:35:23They, too, it seems, have come here to breed.
0:35:31 > 0:35:35The whale shark receives an extraordinary welcome.
0:35:54 > 0:35:59Silky sharks, themselves three metres long,
0:35:59 > 0:36:01bounce against her rough skin...
0:36:03 > 0:36:07..perhaps to scrape off any parasites they might have.
0:36:12 > 0:36:16These sharks could be a danger to any newly born young.
0:36:19 > 0:36:23So, perhaps to avoid them,
0:36:23 > 0:36:25the whale shark dives...
0:36:28 > 0:36:31..down to around 600 metres.
0:36:57 > 0:37:00And there she may release her young.
0:37:03 > 0:37:04In these great depths,
0:37:04 > 0:37:08away from the predators that hunt in the waters above,
0:37:08 > 0:37:10and with abundant food,
0:37:10 > 0:37:14her babies could grow and eventually disperse.
0:37:24 > 0:37:26No-one, it is true,
0:37:26 > 0:37:29has ever seen young ones in these little-visited depths.
0:37:35 > 0:37:39But the fact that hundreds of expectant whale sharks
0:37:39 > 0:37:41come here every year
0:37:41 > 0:37:44is strong evidence that somewhere here
0:37:44 > 0:37:49lies the nursery of the biggest fish in the sea.
0:38:15 > 0:38:19There are almost 30,000 sizeable islands
0:38:19 > 0:38:21scattered across the world's oceans.
0:38:25 > 0:38:27One of them is South Georgia...
0:38:31 > 0:38:34..an ideal place for those ocean dwellers
0:38:34 > 0:38:38who are compelled to land in order to breed.
0:38:41 > 0:38:43The wandering albatross.
0:38:53 > 0:38:57It may spend as much as a year continuously at sea.
0:39:00 > 0:39:02Searching for food,
0:39:02 > 0:39:07gliding on wings that are 3.5 metres across -
0:39:07 > 0:39:09the biggest of any living bird.
0:39:22 > 0:39:28The entire world population of 16,000 wanderers
0:39:28 > 0:39:29nest on South Georgia
0:39:29 > 0:39:33and half a dozen or so of the other smaller islands
0:39:33 > 0:39:35that lie in the Southern Ocean.
0:39:40 > 0:39:43It's spring and this bird is returning
0:39:43 > 0:39:46to the nest site it's always used.
0:39:48 > 0:39:51THEY CALL TO EACH OTHER
0:39:55 > 0:39:59Its lifelong partner is already here.
0:39:59 > 0:40:02THEY CALL AND CLUCK
0:40:07 > 0:40:09In South Georgia,
0:40:09 > 0:40:13individual birds have been studied for their entire lives,
0:40:13 > 0:40:17revealing that older pairs, in their late 30s,
0:40:17 > 0:40:20will go to extraordinary lengths to give their young
0:40:20 > 0:40:22the best possible start in life.
0:40:42 > 0:40:45This chick is now several weeks old,
0:40:45 > 0:40:48but still has its warm, downy coat.
0:41:02 > 0:41:07The chick will need a regular supply of regurgitated fish and squid.
0:41:29 > 0:41:32With food so scarce in the open ocean,
0:41:32 > 0:41:36both parents may have to scour thousands of square miles
0:41:36 > 0:41:39just to provide enough for one meal.
0:42:02 > 0:42:07Ageing parents struggle on all through the Antarctic winter
0:42:07 > 0:42:10to raise a chick that is big, strong and healthy.
0:42:30 > 0:42:32After some 130 days,
0:42:32 > 0:42:36the youngster begins to replace its down with flight feathers.
0:42:54 > 0:42:58Finally, nine months after their egg was laid,
0:42:58 > 0:43:01this chick is ready to leave.
0:43:23 > 0:43:27Of all the chicks they've reared in recent years,
0:43:27 > 0:43:31such a favoured chick will have the best chance of survival.
0:43:34 > 0:43:36But it will also be their last.
0:43:40 > 0:43:43Elderly parents never recover from their exertions.
0:43:45 > 0:43:50They will soon leave this island, never to be seen again.
0:44:07 > 0:44:09Surviving in the open ocean
0:44:09 > 0:44:12has always tested animals to the limit...
0:44:14 > 0:44:17..but today they face a new additional threat.
0:44:23 > 0:44:25Plastic.
0:44:28 > 0:44:30Just over 100 years ago,
0:44:30 > 0:44:33we invented a wonderful new material
0:44:33 > 0:44:36that could be moulded into all kinds of shapes
0:44:36 > 0:44:39and we took great trouble to ensure that it was hard-wearing,
0:44:39 > 0:44:43rot-proof and virtually indestructible.
0:44:45 > 0:44:47Now, every year,
0:44:47 > 0:44:51we dump around eight million tonnes of it into the sea.
0:44:54 > 0:44:59Here, it entangles and drowns vast numbers of marine creatures.
0:45:05 > 0:45:10But it may have even more widespread and far-reaching consequences.
0:45:18 > 0:45:22A pod of short-finned pilot whales.
0:45:22 > 0:45:24THEY CLICK AND BUZZ
0:45:30 > 0:45:33They live together in what are, perhaps,
0:45:33 > 0:45:36the most closely knit of families in the whole ocean.
0:45:44 > 0:45:49Today, in the Atlantic waters off Europe, as elsewhere,
0:45:49 > 0:45:52they have to share the ocean with plastic.
0:46:03 > 0:46:06A mother is holding her newborn young.
0:46:09 > 0:46:11It's dead.
0:46:22 > 0:46:25She is reluctant to let it go
0:46:25 > 0:46:29and has been carrying it around for many days.
0:46:37 > 0:46:40In top predators like these,
0:46:40 > 0:46:43industrial chemicals can build up to lethal levels...
0:46:50 > 0:46:53..and plastic could be part of the problem.
0:46:57 > 0:46:59As plastic breaks down,
0:46:59 > 0:47:01it combines with these other pollutants
0:47:01 > 0:47:05that are consumed by vast numbers of marine creatures.
0:47:18 > 0:47:21It's possible her calf may have been poisoned
0:47:21 > 0:47:24by her own contaminated milk.
0:47:36 > 0:47:39Pilot whales have big brains.
0:47:39 > 0:47:43They can certainly experience emotions.
0:47:45 > 0:47:47Judging from the behaviour of the adults,
0:47:47 > 0:47:51the loss of the infant has affected the entire family.
0:47:59 > 0:48:03Unless the flow of plastics and industrial pollution
0:48:03 > 0:48:06into the world's oceans is reduced,
0:48:06 > 0:48:11marine life will be poisoned by them for many centuries to come.
0:48:17 > 0:48:21The creatures that live in the big blue are perhaps more remote
0:48:21 > 0:48:23than any animals on the planet.
0:48:27 > 0:48:29But not remote enough, it seems,
0:48:29 > 0:48:34to escape the effects of what we are doing to their world.
0:49:02 > 0:49:05The biggest challenge of filming in the vastness of the open ocean
0:49:05 > 0:49:07is to find your subject...
0:49:12 > 0:49:14..and the Blue Planet team wanted to film
0:49:14 > 0:49:17one of the most elusive of them all,
0:49:17 > 0:49:21the rarely witnessed "boiling sea".
0:49:26 > 0:49:30Until now, this feeding frenzy has been the stuff of legends.
0:49:35 > 0:49:39After some promising sightings off the north-east coast of Australia,
0:49:39 > 0:49:41the team heads out to investigate.
0:49:45 > 0:49:48We know it's a phenomenon, we know it's out there,
0:49:48 > 0:49:50the scientists have documented it,
0:49:50 > 0:49:52the fishermen have told us about it,
0:49:52 > 0:49:56so we know it's happening, but no-one has been crazy enough
0:49:56 > 0:50:00to attempt to go out there and actually film it - except for us.
0:50:02 > 0:50:06The team start their search 100 miles out in the Pacific Ocean.
0:50:09 > 0:50:12The reason it's called a "boiling sea" is that the tuna
0:50:12 > 0:50:15are actually coming out of the water
0:50:15 > 0:50:19and attacking lanternfish and it creates a lot of white water.
0:50:22 > 0:50:24To film the boiling seas,
0:50:24 > 0:50:28the team must first find a large shoal of bait fish,
0:50:28 > 0:50:30most likely to be lanternfish
0:50:30 > 0:50:32rising to the surface at night to spawn.
0:50:34 > 0:50:35A few days out,
0:50:35 > 0:50:41Adrian thinks he may have spotted a giant shoal on the echo-sounder.
0:50:41 > 0:50:44What we're seeing is a very, very dense layer
0:50:44 > 0:50:46at about 200 metres' water depth.
0:50:46 > 0:50:49And so, the fact that we've got this would suggest
0:50:49 > 0:50:54that we have a very deep and dense layer of fish.
0:50:54 > 0:50:58One of the best ways to film at such depths in the open ocean
0:50:58 > 0:51:02is to use an ROV - a remotely operated vehicle -
0:51:02 > 0:51:04carrying a light-sensitive camera.
0:51:07 > 0:51:10But working with such heavy equipment in the high seas
0:51:10 > 0:51:12is a risky operation.
0:51:13 > 0:51:16CLANK, LOUD THUD
0:51:26 > 0:51:30Fortunately, a team of technicians is on hand,
0:51:30 > 0:51:33and 24 hours later, they're ready to relaunch.
0:51:37 > 0:51:39Adrian drops them on top
0:51:39 > 0:51:42of what he hopes is a large shoal of lanternfish.
0:51:44 > 0:51:46Good news is, we've just put the ROV down,
0:51:46 > 0:51:48we're down at 250 metres,
0:51:48 > 0:51:51which means we've gone to almost the end of the cable
0:51:51 > 0:51:54and nothing's blown up so we're back in business.
0:51:56 > 0:51:58But there's almost nothing there,
0:51:58 > 0:52:00just a thick layer of plankton.
0:52:05 > 0:52:07Over the next three weeks,
0:52:07 > 0:52:10they don't find a single lanternfish.
0:52:17 > 0:52:19This trip is the perfect illustration
0:52:19 > 0:52:21of why we know so little about the ocean -
0:52:21 > 0:52:23we came out looking for something,
0:52:23 > 0:52:25we've searched and searched and searched,
0:52:25 > 0:52:26and we still haven't found it
0:52:26 > 0:52:28even with every single tool you could wish for.
0:52:28 > 0:52:31As it turned out, the team had been filming
0:52:31 > 0:52:34at the very start of El Nino -
0:52:34 > 0:52:36an unpredictable climatic event
0:52:36 > 0:52:39when sea temperatures can suddenly rise
0:52:39 > 0:52:41and disrupt the spawning behaviour of fish.
0:52:45 > 0:52:48It would be 18 months before conditions would improve
0:52:48 > 0:52:51and the team could continue their quest.
0:52:56 > 0:53:00The other side of the Pacific Ocean, off Costa Rica.
0:53:05 > 0:53:08This time, rather than searching for their prey,
0:53:08 > 0:53:11the team are looking for their predators.
0:53:11 > 0:53:15BUZZING AND CLICKING
0:53:24 > 0:53:25But in the endless blue,
0:53:25 > 0:53:29even finding a massive pod of dolphins isn't easy.
0:53:33 > 0:53:3720 miles offshore, series producer Mark Brownlow
0:53:37 > 0:53:39leads an aerial filming team
0:53:39 > 0:53:42scanning thousands of square miles of ocean...
0:53:45 > 0:53:47..but there's not a dolphin in sight.
0:53:59 > 0:54:03Day three, no spinner dolphins.
0:54:04 > 0:54:06Getting worried now...
0:54:11 > 0:54:14Finally, after ten days on the open ocean,
0:54:14 > 0:54:17they get their reward.
0:54:19 > 0:54:21OK, dolphins!
0:54:22 > 0:54:24Woohoo!
0:54:24 > 0:54:25Yes!
0:54:25 > 0:54:26Spinners!
0:54:30 > 0:54:35With dolphins in sight, the dive team race to intercept them.
0:54:37 > 0:54:38Spinners, look!
0:54:40 > 0:54:44The plan is to film the dolphins' feeding frenzy from underwater.
0:54:47 > 0:54:49Several hundred dolphins jumping all over the place,
0:54:49 > 0:54:51it looks like this could be it.
0:54:56 > 0:54:57They catch up with the dolphins.
0:55:02 > 0:55:04But they're too late.
0:55:08 > 0:55:09Ah, dear.
0:55:09 > 0:55:11- Nothing, Rog?- Just...nothing.
0:55:11 > 0:55:13Just fish scales and bones,
0:55:13 > 0:55:15it's like turning up at a battle
0:55:15 > 0:55:17just to see all the dead bodies left over.
0:55:19 > 0:55:22To stand a chance of filming the dolphins' feeding,
0:55:22 > 0:55:25the dive team need to be more proactive.
0:55:33 > 0:55:36Rachel hitches a ride, following them underwater,
0:55:36 > 0:55:39searching for any clue to where they might go next.
0:55:51 > 0:55:54Finally, after three weeks of searching,
0:55:54 > 0:55:59the dive team catch a huge feeding event,
0:55:59 > 0:56:02a massive shoal of lanternfish
0:56:02 > 0:56:05being rounded up by hundreds of spinner dolphins.
0:56:10 > 0:56:12That was incredibly intense.
0:56:12 > 0:56:13A very large bait ball
0:56:13 > 0:56:16spread over probably the size of a football field.
0:56:19 > 0:56:22Things coming in and over your shoulder, over your head,
0:56:22 > 0:56:23it was incredible.
0:56:27 > 0:56:28For the first time,
0:56:28 > 0:56:32the aerial team can record the epic scale of this spectacle.
0:56:32 > 0:56:37You know, we heard these stories of boiling seas, but it's real!
0:56:37 > 0:56:38They're huge.
0:56:40 > 0:56:44The vastness of the ocean wilderness
0:56:44 > 0:56:46made capturing this extraordinary event a great challenge.
0:56:51 > 0:56:53But this is the reward,
0:56:53 > 0:56:55a moment of unparalleled drama
0:56:55 > 0:56:59in the immense expanse of the big blue.
0:57:04 > 0:57:09Next time, we journey into the bountiful green sea.
0:57:12 > 0:57:16These are enchanted worlds, home to strange creatures...
0:57:20 > 0:57:24..where only the most ingenious will triumph.
0:57:29 > 0:57:36To find out more about our oceans with this free poster, call...
0:57:36 > 0:57:41or go to...
0:57:41 > 0:57:45and follow the links to the Open University.