0:00:02 > 0:00:05Our planet is the greatest living puzzle in the universe.
0:00:05 > 0:00:08A collection of worlds within worlds.
0:00:09 > 0:00:13Each one a self-contained ecosystem bursting with life.
0:00:17 > 0:00:19But how do they work?
0:00:21 > 0:00:24The intricate web of relationships
0:00:24 > 0:00:27and the influence of natural forces
0:00:27 > 0:00:31makes each microworld complex and unique.
0:00:33 > 0:00:35So, to discover their secrets,
0:00:35 > 0:00:38we need to explore them one by one,
0:00:40 > 0:00:43untangle their interlocking pieces
0:00:43 > 0:00:47and, ultimately, reveal the vital piece,
0:00:47 > 0:00:49the key to life itself
0:00:49 > 0:00:54hidden deep within each of nature's microworlds.
0:01:04 > 0:01:08The ocean covers 71% of the Earth's surface.
0:01:08 > 0:01:12And we may think it's brimming with life from shore to shore,
0:01:12 > 0:01:16but, actually, that's not quite true.
0:01:16 > 0:01:19In fact, in most of this ocean, life can be hard to come by.
0:01:20 > 0:01:25Instead, most marine species are condensed into intense pockets,
0:01:25 > 0:01:27crammed along the coasts.
0:01:32 > 0:01:36And, in tropical seas, these colourful coastlines
0:01:36 > 0:01:39are the most diverse ecosystems in the ocean -
0:01:39 > 0:01:41the coral reef.
0:01:44 > 0:01:48These coral reefs are found across the world,
0:01:48 > 0:01:51but one of them is the master of them all.
0:01:51 > 0:01:53It is simply the largest
0:01:53 > 0:01:54living structure on Earth.
0:01:56 > 0:01:58The Great Barrier Reef.
0:02:01 > 0:02:02Of all the reefs on the planet,
0:02:02 > 0:02:05how has this one managed to achieve this incredible size?
0:02:08 > 0:02:12But scale isn't the only success of this microworld.
0:02:14 > 0:02:17The water that this reef basks in
0:02:17 > 0:02:19lacks many of the nutrients vital for life.
0:02:22 > 0:02:25So such a vibrant ecosystem should have struggled
0:02:25 > 0:02:26to get a foothold here.
0:02:29 > 0:02:32So how has this enormous place managed to survive?
0:02:35 > 0:02:36To understand this,
0:02:36 > 0:02:38we first need to find out
0:02:38 > 0:02:41what it was that allowed the Great Barrier Reef to get so big.
0:02:46 > 0:02:49The reef stretches along the north-eastern coast of Australia
0:02:49 > 0:02:51for over 2,000 kilometres -
0:02:54 > 0:02:58a distance equivalent to that between Britain and North Africa.
0:03:02 > 0:03:05Underwater, it's a marine metropolis
0:03:05 > 0:03:08and home to a hustling, bustling whirlwind of life.
0:03:11 > 0:03:15But this microworld encompasses more than just a reef.
0:03:15 > 0:03:17All along its lengths are lagoons,
0:03:17 > 0:03:20islands and seagrass meadows.
0:03:22 > 0:03:26On the western boundary is the Australian mainland.
0:03:27 > 0:03:28And to the east,
0:03:28 > 0:03:30the open Pacific Ocean,
0:03:30 > 0:03:33where the outer wall of the Great Barrier Reef
0:03:33 > 0:03:35meets the deep blue sea.
0:03:36 > 0:03:39In our search for the key factors that make this place so big,
0:03:39 > 0:03:43we need to look at some of these habitats in more detail.
0:03:46 > 0:03:49And, to begin, the one that dominates all others...
0:03:50 > 0:03:53..the underwater world of coral,
0:03:53 > 0:03:57home to the majority of species on the Great Barrier Reef.
0:03:59 > 0:04:03The fish are perhaps the most conspicuous of all the animals here
0:04:03 > 0:04:05and there are some 1,500 species,
0:04:07 > 0:04:09each with their own way of making a living.
0:04:14 > 0:04:16Clownfish live amongst anemones.
0:04:17 > 0:04:19There are more than seven species of these fish
0:04:19 > 0:04:21that live along this reef
0:04:21 > 0:04:24and each species depends on the presence
0:04:24 > 0:04:27of their host anemone to survive.
0:04:27 > 0:04:30Spiny damselfish are another reef resident.
0:04:30 > 0:04:35And they face the challenges of parenthood in a watery world.
0:04:35 > 0:04:39They keep their school of miniature offspring in sight at all times,
0:04:39 > 0:04:44only venturing away from the safety of the reef to grab a quick snack
0:04:44 > 0:04:47before returning back to the cover of their coral cave.
0:04:50 > 0:04:53This diversity of fish is only the beginning
0:04:53 > 0:04:55of the Great Barrier Reef story.
0:04:56 > 0:05:00This microworld is jam-packed with thousands of other species,
0:05:00 > 0:05:01like mantis shrimp
0:05:01 > 0:05:02and feather stars.
0:05:04 > 0:05:07All these lives play out against a backdrop
0:05:07 > 0:05:09of hundreds of coral species,
0:05:09 > 0:05:12which can stretch as far as the eye can see.
0:05:13 > 0:05:17This huge array of life may be impressive,
0:05:17 > 0:05:20but it doesn't set the Great Barrier Reef apart from other reefs.
0:05:22 > 0:05:25Amongst all of these players, it isn't the fish,
0:05:25 > 0:05:27the invertebrates or even the corals
0:05:27 > 0:05:30that explain why this reef is so big.
0:05:32 > 0:05:35We need to look further.
0:05:35 > 0:05:37We need to visit the other environments
0:05:37 > 0:05:40that make up the Great Barrier Reef.
0:05:40 > 0:05:43And one of these is seagrass.
0:05:45 > 0:05:49Remarkably, seagrass is actually a flowering plant -
0:05:49 > 0:05:51the only one adapted to live in the ocean.
0:05:53 > 0:05:56And like huge grasslands, such as the Serengeti,
0:05:56 > 0:06:00seagrass provide important feeding grounds for herbivores.
0:06:06 > 0:06:11More related to the elephant than to any dolphins or whales,
0:06:11 > 0:06:13these are herbivores that can hold their breath.
0:06:15 > 0:06:17This is a dugong.
0:06:20 > 0:06:24Meadows of seagrass stretch right along the Great Barrier Reef
0:06:24 > 0:06:29and cover up to 40,000 square kilometres of the sandy bottom.
0:06:29 > 0:06:33This huge area allows impressive numbers of dugongs
0:06:33 > 0:06:36to congregate in these southern stretches of the coast.
0:06:39 > 0:06:42But what part do these vast areas of seagrass play
0:06:42 > 0:06:44in the Great Barrier Reef's size?
0:06:47 > 0:06:49Well, the seagrass supports massive creatures
0:06:49 > 0:06:51and provides additional habitat...
0:06:53 > 0:06:57..but this seagrass is just a product of the reef, not its cause.
0:06:59 > 0:07:02In fact, the seagrass actually depends on the reef
0:07:02 > 0:07:05for the material in which it grows.
0:07:08 > 0:07:09Sand.
0:07:10 > 0:07:13And some of that sand comes from animal life.
0:07:22 > 0:07:25The aptly-named parrotfish is a conspicuous sight
0:07:25 > 0:07:27to come across on any reef.
0:07:29 > 0:07:30And, by following these fish,
0:07:30 > 0:07:33we soon see where some of this sand comes from.
0:07:46 > 0:07:50The fish's teeth are fused into a parrot-like beak
0:07:50 > 0:07:53that crunches its way through the calcium structure
0:07:53 > 0:07:56of dead and living coral alike.
0:07:58 > 0:08:03But what seems to be pure destruction has a surprising twist.
0:08:03 > 0:08:05The digested bits of coral skeleton
0:08:05 > 0:08:08are ground down inside the humphead parrotfish
0:08:08 > 0:08:11into a fine calcium powder
0:08:11 > 0:08:13and excreted as sand...
0:08:16 > 0:08:19..sand that builds up with other bits of calcium rubble
0:08:19 > 0:08:24and, with wind and waves, becomes mounds that push up above the water.
0:08:24 > 0:08:26The beginnings of islands
0:08:26 > 0:08:29that allow other creatures to prosper on the reef.
0:08:31 > 0:08:36There are some 600 islands dotted all along this coast.
0:08:36 > 0:08:39Oases of life that have been colonised by plants
0:08:39 > 0:08:42and become vibrant ecosystems of their own.
0:08:44 > 0:08:47Some of the islands in the southern reaches of the reef
0:08:47 > 0:08:52have become central to generation after generation of green turtles.
0:08:54 > 0:08:56Every year, 38,000 female turtles
0:08:56 > 0:08:59come to the sandy beaches of the Great Barrier Reef
0:08:59 > 0:09:01to lay their eggs.
0:09:06 > 0:09:10This female was born on this same beach over 45 years ago
0:09:10 > 0:09:16and she's now returned to lay her own clutch of around 100 eggs.
0:09:16 > 0:09:19It might sound like a lot but of each 1,000 eggs laid,
0:09:19 > 0:09:23only one turtle might survive to adulthood.
0:09:28 > 0:09:30So the reef's protected sandy beaches
0:09:30 > 0:09:34provide vital nesting habitat for an ancient and iconic reptile.
0:09:39 > 0:09:43But the islands have brought other life to the reef too.
0:09:43 > 0:09:45They have become important breeding grounds
0:09:45 > 0:09:48for the 20 or so species of seabirds.
0:09:50 > 0:09:54And white-capped noddies are here in their thousands.
0:09:55 > 0:09:57They produce a single egg
0:09:57 > 0:10:00and build their nests among the trees and shrubs
0:10:00 > 0:10:02of coral sand islands.
0:10:06 > 0:10:08Without the sand that built these islands,
0:10:08 > 0:10:12these birds would never be able to nest here in such great numbers.
0:10:15 > 0:10:17So sand plays a vital role
0:10:17 > 0:10:20in creating the islands and seagrass
0:10:20 > 0:10:24that allow a rich diversity of animals to make the reef their home.
0:10:28 > 0:10:32But sand is just one of the reasons it's so vast.
0:10:41 > 0:10:46The Great Barrier Reef has 2,900 reef systems,
0:10:46 > 0:10:49600 tropical islands
0:10:49 > 0:10:53and more than 270,000 square kilometres of coral.
0:10:54 > 0:10:57To find the ultimate reason for this immense size,
0:10:57 > 0:10:59we need to look wider still.
0:10:59 > 0:11:03And the answer is right under its feet.
0:11:05 > 0:11:06Seen from space,
0:11:06 > 0:11:08the reef sticks out from the coast.
0:11:08 > 0:11:10It's sitting up on a huge platform.
0:11:11 > 0:11:15This is an extension of the continental shelf -
0:11:15 > 0:11:17over 2,000 kilometres long
0:11:17 > 0:11:19and up to 200 kilometres wide -
0:11:19 > 0:11:21that provides the reef
0:11:21 > 0:11:24with the perfect shallow-water stage on which to grow.
0:11:28 > 0:11:31But there's another reason why it's so massive.
0:11:31 > 0:11:33In the northern section of the reef,
0:11:33 > 0:11:36a gap in the coral allows a huge current
0:11:36 > 0:11:38to pour in from the Coral Sea.
0:11:39 > 0:11:41It's the East Australian Current
0:11:41 > 0:11:43and it's far from just a trickle.
0:11:51 > 0:11:54This movement of water can be up to 100 kilometres wide
0:11:54 > 0:11:56and 500 metres deep.
0:11:58 > 0:12:02It carries over 30 million cubic metres of water per second.
0:12:05 > 0:12:09And the current is a vehicle that carries life.
0:12:11 > 0:12:13Combined with wind-driven currents,
0:12:13 > 0:12:16planktonic animals are transported for hundreds of kilometres
0:12:16 > 0:12:19up and down the reef.
0:12:19 > 0:12:21It's a great highway that has spread life
0:12:21 > 0:12:24to all its individual parts
0:12:24 > 0:12:27and it's joined up this massive ecosystem
0:12:27 > 0:12:30to make the longest reef on the planet.
0:12:37 > 0:12:40So the Great Barrier Reef's immense size
0:12:40 > 0:12:43is a product of the huge shallow-water platform
0:12:43 > 0:12:46created by Australia's continental shelf,
0:12:46 > 0:12:49combined with the effects of a great current
0:12:49 > 0:12:52that spreads colonising life right along the coast.
0:12:53 > 0:12:56And it's this unique set of circumstances
0:12:56 > 0:12:58that has led to such success.
0:13:02 > 0:13:06But survival on this coast is far from guaranteed.
0:13:07 > 0:13:12The Great Barrier Reef is an ecosystem in an impoverished ocean.
0:13:14 > 0:13:16So how does it actually work?
0:13:22 > 0:13:24It's almost unbelievable
0:13:24 > 0:13:27but all of this is created by a tiny creature.
0:13:30 > 0:13:33An animal just one millimetre across,
0:13:33 > 0:13:35which barely looks alive at all.
0:13:38 > 0:13:39The coral polyp.
0:13:41 > 0:13:43This is a reef-building coral
0:13:43 > 0:13:46and it literally manufactured the Great Barrier Reef
0:13:46 > 0:13:47from the ground up.
0:13:48 > 0:13:51It's made by a creature you can barely see with the naked eye,
0:13:51 > 0:13:54and it's created enough coral reef
0:13:54 > 0:13:56to cover the entire land area of the United Kingdom.
0:13:58 > 0:14:01270 billion square metres.
0:14:03 > 0:14:04How does it do it?
0:14:06 > 0:14:11Reef-building corals are made up of tiny polyps
0:14:11 > 0:14:15and they each live in a little box of calcium carbonate.
0:14:15 > 0:14:20This stony structure is called their skeleton
0:14:20 > 0:14:22and they deposit it beneath them in order to grow.
0:14:26 > 0:14:30Each colourful structure here is a colony
0:14:30 > 0:14:35and each has slowly built up their fortified-calcium castle
0:14:35 > 0:14:37of all shapes and sizes.
0:14:53 > 0:14:55They look like sturdy rock giants
0:14:55 > 0:15:00but, remarkably, the living tissue forms a thin veneer on the outside
0:15:00 > 0:15:04of sometimes metres of inanimate coral rock.
0:15:08 > 0:15:13This tiny layer of life makes corals incredibly fragile
0:15:13 > 0:15:15and vulnerable to attack.
0:15:20 > 0:15:24The dainty mouths of angelfish might appear harmless
0:15:24 > 0:15:26but they actually eat coral.
0:15:28 > 0:15:30And, for the coral under attack,
0:15:30 > 0:15:34the daytime is best spent hiding indoors.
0:15:36 > 0:15:39But, at night, when the reef goes quiet,
0:15:39 > 0:15:41the coral polyps emerge,
0:15:41 > 0:15:43and, finally, we get to see the animals within.
0:15:45 > 0:15:49All across the tropics, stone coral statues come to life.
0:15:52 > 0:15:55The coral animals are relatives of the anemone
0:15:55 > 0:15:57and are equipped with stinging tentacles
0:15:57 > 0:16:00that they use to gather the plankton floating by.
0:16:17 > 0:16:21These corals are clearly far from lifeless rocks.
0:16:24 > 0:16:26And, on some of these crammed reefs,
0:16:26 > 0:16:30these tiny animals will even wage war on their neighbours.
0:16:34 > 0:16:38The growing corals send out digesting tentacles
0:16:38 > 0:16:41that, over hours, can eat away at their neighbours
0:16:41 > 0:16:43in a quest for dominance.
0:16:43 > 0:16:47And their thin layer of living tissue is soon stripped back
0:16:47 > 0:16:49to their limestone skeleton.
0:16:51 > 0:16:55But, for the most part, these corals can grow peacefully.
0:16:55 > 0:16:56And, in good conditions,
0:16:56 > 0:16:59will deposit new layers of their calcium skeleton
0:16:59 > 0:17:01at up to five centimetres per year.
0:17:05 > 0:17:09This remarkable process is what lays down
0:17:09 > 0:17:11the limestone building blocks of the reef.
0:17:13 > 0:17:16Between them, the polyps of the Great Barrier Reef
0:17:16 > 0:17:20lay down one billion tonnes of calcium carbonate every year,
0:17:20 > 0:17:22creating the foundations
0:17:22 > 0:17:26for this awe-inspiring shallow-water ecosystem.
0:17:32 > 0:17:34But building their skeletons is demanding
0:17:34 > 0:17:37of both energy and nutrients
0:17:37 > 0:17:41and the coral animals can't manage such a feat alone.
0:17:45 > 0:17:48The crystal-clear water of the Great Barrier Reef
0:17:48 > 0:17:50might seem like the stuff of postcards and dreams...
0:17:53 > 0:17:58..but this bright blue sea is missing something vital for life.
0:18:01 > 0:18:03It lacks the nutrients
0:18:03 > 0:18:06that microscopic plants and animals need to grow -
0:18:07 > 0:18:09the nitrates and phosphates
0:18:09 > 0:18:11that fuel plankton blooms,
0:18:11 > 0:18:13which are the base of the food chain.
0:18:17 > 0:18:20So, despite feeding every night,
0:18:20 > 0:18:23the coral simply can't get enough energy to survive here
0:18:23 > 0:18:25from plankton alone.
0:18:27 > 0:18:29But deep in its evolutionary history,
0:18:29 > 0:18:32coral developed a clever trick to get around this.
0:18:38 > 0:18:40The key to this is a remarkable association
0:18:40 > 0:18:43between the coral and a single-celled algae,
0:18:43 > 0:18:44called zooxanthellae.
0:18:48 > 0:18:54These tiny single-celled algae are incorporated into the coral tissues,
0:18:54 > 0:18:56at up to five million per square centimetre.
0:19:00 > 0:19:04The coral's waste products feed the algae with the nutrients they need.
0:19:07 > 0:19:12And, in return, the algae develops inside the coral,
0:19:12 > 0:19:16photosynthesising, using sunlight to create oxygen and sugars.
0:19:19 > 0:19:22These much-needed sugars are the vital energy
0:19:22 > 0:19:24that the coral needs to grow.
0:19:27 > 0:19:30So this unique partnership allows coral to thrive
0:19:30 > 0:19:33in the nutrient-poor waters along this coast.
0:19:35 > 0:19:40But this relationship also sets the rules for this entire ecosystem.
0:19:43 > 0:19:45By having an algae within its tissue,
0:19:45 > 0:19:50the coral effectively becomes half plant and half animal.
0:19:50 > 0:19:55And so, like plants, coral has become a slave to the sun.
0:19:57 > 0:20:00It needs the sunlit waters of the tropics to grow.
0:20:07 > 0:20:10But these shallow tropical waters
0:20:10 > 0:20:13leave the coral exposed
0:20:13 > 0:20:15because each Australian summer,
0:20:15 > 0:20:18destructive forces move towards the coast.
0:20:54 > 0:20:57150kmph winds.
0:20:57 > 0:20:59Destructive waves.
0:20:59 > 0:21:01Tropical cyclones develop over the warm seas
0:21:01 > 0:21:03of the South-Western Pacific
0:21:03 > 0:21:06and hit parts of the Great Barrier Reef every year.
0:21:14 > 0:21:17Huge waves smash the fragile coral skeletons,
0:21:17 > 0:21:19breaking them up into rubble
0:21:19 > 0:21:21which tumbles down the reef edge...
0:21:52 > 0:21:55..leaving behind a wasteland of coral skeletons.
0:21:58 > 0:22:00This coral takes the brunt of the storm
0:22:00 > 0:22:04and, in doing so, protects other parts of the reef.
0:22:08 > 0:22:12The seagrass is sheltered in protected lagoons
0:22:12 > 0:22:15and the rich island ecosystems are kept intact.
0:22:17 > 0:22:21The sturdy outer wall of coral protects this coastline
0:22:21 > 0:22:25from the wild forces of the Pacific Ocean that roll in every year.
0:22:29 > 0:22:31But if life in these shallow waters
0:22:31 > 0:22:34is threatened on such a regular basis,
0:22:34 > 0:22:37how has this coral reef managed to survive?
0:22:41 > 0:22:43The secret to this reef's success
0:22:43 > 0:22:46is that it's bigger than any storm.
0:22:48 > 0:22:51And to see how this saves the reef from ruin,
0:22:51 > 0:22:55we need to take a look at what happens when the dust settles.
0:22:59 > 0:23:05This coral graveyard is not an untimely death but an opportunity.
0:23:05 > 0:23:07An opportunity for new life.
0:23:11 > 0:23:14Not all of the reef was damaged in the storm.
0:23:14 > 0:23:16Even in the worst cyclones,
0:23:16 > 0:23:2085% of it can escape damage
0:23:20 > 0:23:24and new life comes from these undamaged parts each November
0:23:24 > 0:23:28when the reef experiences an incredible reproductive event.
0:23:32 > 0:23:34In the moonlit night,
0:23:34 > 0:23:36the corals of the Great Barrier Reef
0:23:36 > 0:23:39start to release eggs and sperm into the water
0:23:39 > 0:23:42and it becomes alive.
0:23:55 > 0:23:58Tiny coral nomads take shape.
0:24:01 > 0:24:05But they don't just settle on the reef where they were released.
0:24:08 > 0:24:12This is where the ocean current comes into play.
0:24:14 > 0:24:18It carries the larvae of coral, fish and invertebrates
0:24:18 > 0:24:19all along the reef.
0:24:23 > 0:24:27This allows bits of damaged reef to be re-colonised with life.
0:24:35 > 0:24:38A single coral polyp.
0:24:39 > 0:24:42The beginning of a new colony.
0:24:48 > 0:24:50And a renewed stretch of reef.
0:24:53 > 0:24:56The new life created by the annual coral spawn
0:24:56 > 0:25:00helps rebuild damaged parts of the reef...
0:25:02 > 0:25:04..allowing them to flourish once more.
0:25:10 > 0:25:12This process repairs the ramparts
0:25:12 > 0:25:15and can help replenish the reef edge
0:25:15 > 0:25:18before the next cyclone arrives.
0:25:21 > 0:25:23It might seem like a pointless reconstruction
0:25:23 > 0:25:27but the re-growth of coral on a bedrock of dead calcium skeleton
0:25:27 > 0:25:32is vitally important to the continuation of life here.
0:25:32 > 0:25:34And, to see just why this is,
0:25:34 > 0:25:36we need to look back in time.
0:25:41 > 0:25:43Just some 10,000 years ago,
0:25:43 > 0:25:47this whole stretch of coastline was completely different.
0:25:47 > 0:25:50Since the end of the last ice age,
0:25:50 > 0:25:52sea levels have been rising
0:25:52 > 0:25:54and as they rise,
0:25:54 > 0:25:57flooded land gives way to shallow seas.
0:26:00 > 0:26:05The sea surface was about 100 metres lower than it is today
0:26:05 > 0:26:09and this shallow-water ecosystem used to be on dry land.
0:26:14 > 0:26:16And the sea hasn't stopped there.
0:26:16 > 0:26:18It continues to rise
0:26:18 > 0:26:21and threatens to flood the reef too,
0:26:21 > 0:26:25leaving it in the dark depths of the seabed.
0:26:25 > 0:26:29This could have spelled the end for the Great Barrier Reef
0:26:29 > 0:26:31but not yet.
0:26:31 > 0:26:34In the same way that it recovers from cyclones,
0:26:34 > 0:26:36the reef builds up on itself,
0:26:36 > 0:26:38layer by layer.
0:26:38 > 0:26:42The dead coral skeletons of the past are cemented together
0:26:42 > 0:26:44by colourful coralline algae.
0:26:46 > 0:26:49A living mortar that binds the calcium reef together.
0:26:53 > 0:26:57From here, new life is brought on the currents to colonise
0:26:57 > 0:27:00and the reef spreads ever upwards.
0:27:00 > 0:27:03Keeping up pace with the rising seas
0:27:03 > 0:27:06and creating the shallow-water ecosystem that we see today.
0:27:09 > 0:27:11This exceptional process not only ensures
0:27:11 > 0:27:14that the coral stays near the surface
0:27:14 > 0:27:17so that its algae can photosynthesise
0:27:17 > 0:27:18but, in doing so,
0:27:18 > 0:27:22it allows this whole shallow-water ecosystem to exist.
0:27:25 > 0:27:30The Great Barrier Reef is more than just coral and fish.
0:27:30 > 0:27:35It's a network of thousands of islands, reefs and other habitats
0:27:35 > 0:27:38that allow animals both large and small to prosper.
0:27:40 > 0:27:42The reef is remarkably resilient.
0:27:45 > 0:27:49Not only can it withstand some of the most ferocious forces of nature
0:27:49 > 0:27:53but it has also managed to exist in nutrient-poor waters
0:27:53 > 0:27:55that could otherwise be lifeless.
0:27:59 > 0:28:03But the reason this reef has got bigger than any other on Earth
0:28:03 > 0:28:05is the massive shallow-water platform
0:28:05 > 0:28:07of the Australian continental shelf,
0:28:09 > 0:28:12the great current that runs right along the coast
0:28:12 > 0:28:15and the simple partnership between a one-millimetre coral
0:28:15 > 0:28:17and a single-celled algae.
0:28:19 > 0:28:23These are the secrets of the Great Barrier Reef.
0:28:51 > 0:28:55Subtitles by Red Bee Media Ltd