"The Great Barrier Reef is huge." "It stretches over 2,000km along Australia's north-east coast." "It's so big, it's clearly visible from space." "The planet's largest living structure." "And it's not simply a collection of coral gardens, but a network of very different habitats, where animals are dependent on each other." "There's a complexity of life here on a scale found nowhere else in the world," "but it doesn't exist in isolation." "Violent storms are unwelcome visitors." "Ocean voyagers converge here from many parts of the globe." "Some, here to breed." "Others, come to feed." "It means that the Great Barrier Reef's an international hub, home to some of the greatest wildlife spectacles on Earth." "Great Barrier Reef" "This green sea turtle's a summer visitor." "She's travelled hundreds of kilometres across the ocean and she's heading to the very beach where she was born." "She's come to lay her eggs." "and she's not alone." "Out here, on the edge of the reef, she's joined by thousands more female turtles." "They're all driven by the same instinct, to return home to nest." "It's the largest breeding population of green sea turtles in the world." "After her long journey, she takes a few days to rest and recover." "Butterfly fish provide a cleaning service, clearing away dead skin and the parasites acquired from many months at sea." "The turtles all converge on small islands made of coral rock and sand, known as coral cays," "but not all are suitable for nesting, some are sandbanks exposed only at low tide," "and many others have beaches that are swamped at high tide." "Islands with deep sand and a covering of vegetation are more stable" "and one island in particular seems just right." "In the far north of the Great Barrier Reef is Raine Island." "It's so wild and so special, few people are permitted to land." "It's one of the most protected islands on Earth." "This speck in the ocean is barely a kilometre long, yet it attracts thousands of turtles and enormous flocks of seabirds." "The birds have flown in from New Guinea and Japan to the north," "Fiji to the east and even from the Asian mainland thousands of kilometres away." "In summer, Raine Island's the most crowded destination on the Great Barrier Reef." "It may look chaotic, but there's some order here." "Brown boobies are everywhere but other species prefer specific nesting sites." "Red-footed boobies hang out on branches, a scarce commodity on the outer reef." "Caspian terns from Japan nest on the sand." "Frigate birds find low-growing shrubs" "and red-tail tropic birds hide amongst the limestone rocks." "The birds, like the turtles, are here to breed." "But the turtles, unlike the birds, are about to face the biggest challenge of their visit to Raine Island." "By late afternoon, they move towards the prime-nesting beach that surrounds the entire island." "when she leaves the weightlessness of the sea, her heavy body becomes a real burden." "her entire weight presses down on all her vital organs." "She's beautifully adapted to a life at sea but ill-equipped to move about on land." "Her progress is slow and probably painful." "It's quite common for 5,000 turtles to emerge in one summer evening but tonight is anything but ordinary." "26,000 turtles are coming at the island from all sides." "A world record." "It's going to be a long night." "These green turtles are only one of the many visitors to the reef." "Another ocean voyager is heard before it's seen." "Whale calls" "It's a dwarf minke whale, one of the smallest of the great whales." "While the turtles have swum from as far away as islands in the South Pacific, the whales have travelled considerably further, all the way from the Antarctic." "They come to the Ribbon Reefs, south of Raine Island, to calve in the warm, tropical waters or to mate." "Whale watching's become a local tourist attraction but some whales turn the tables, they go people watching." "These are adolescent whales, yet to make a hit with the opposite sex, and they are extremely inquisitive." "By hanging onto the rope, the position of the people is predictable, so these 7-metre long whales are quite unafraid." "The mechanical twang of their call is so powerful it vibrates through the body." "Being on nodding terms with a minke whale is a new experience for most visitors." "And to be with the whales not on humans terms, but on theirs is something people find quite extraordinary," "to be in the audience of the ultimate underwater ballet." "More musical sounds announce the arrival of even bigger whales." "Humpbacks." "They're one of ten species of whale that visit the reef each year." "They were once hunted, almost to extinction, but numbers here have bounced back to 15,000, about half the pre-whaling population." "Like minkes, humpbacks come up from the Antarctic to mate and calve." "This mother gave birth a couple of weeks ago, and already her calf weighs over two tonnes and is more than six metres long." "For the turtle mother, the final and what may turn out to be the most arduous part of her journey, has only just begun." "With thousands of turtles arriving at the same time, their trails criss-cross the sand like tank tracks on a battlefield." "There are just too many turtles for the space available." "She spends much of the night heaving her bulk back and forth across the sand, searching for a vacant nest site." "Such is their enthusiasm for digging, neighbours are in real danger of being buried alive." "Some even dig up eggs that have already been laid." "After several hours searching, the female finds a suitable place, where the sand is still moist." "Her flippers may be a liability on land, but now they come into their own." "Using her front flippers, she first digs a protective hollow for herself." "Then, with her back flippers, she delicately scoops out a deep pit." "Each one of her clutch of 100 eggs is the size of a ping-pong ball." "A soft shell prevents them from breaking as they drop into the hole." "When she's finished, she'll cover the nest and over the coming weeks her eggs will incubate in the warm sand." "By morning, she joins the mass evacuation of the island." "Most of the exhausted turtles head back to the sea at the same time, creating an even bigger pile-up at the water's edge than when they arrived." "For a few stragglers, it's a race against time." "A couple of hours after dawn, the heat of the sun kicks in, and the temperature on the sand will soar." "And, on one part of the island, there's a major obstacle that wasn't there when they arrived." "Rocks exposed at low tide make the return to water anything but easy." "Being reptiles, sea turtles have little control over their body temperature." "As there's no shade anywhere, those left on the beach risk being cooked alive." "For the unlucky few, this will be the last journey they'll ever make." "When turtles leave the sanctuary of the ocean, thay are taking a gamble." "Some lose, but a few of those left behind, have a chance of returning to the sea." "If turtles trapped by the rock wall can make it to a pool, they might survive." "The seawater cools their bodies." "All they have to do is wait for the incoming tide and whatever that will bring." "Raine Island is part of the outer barrier reef, so it's right next to the open ocean." "Here the mottled hues of the shallow reef meet the dark blue of deep sea." "The reef wall plunges down vertically to the ocean floor 1,000 metres below." "It's here that reef life and creatures of the deep coexist." "For migration to the reef is not only from across the ocean, but also up from the depths." "A diver can only go down to the 100 metres, so the only way to see what's living much deep down is by using a remotely operated vehicle, an ROV." "It enters an alien world, pitch black, with crushing water pressures," "and extreme cold." "At four degrees, it's the same temperature as the sea in the Antarctic." "The ROV reaches a depth of 800 metres, not quite at the bottom, but not far off." "A pile of coral sand at the base of the reef wall slopes gently into the abyss, and here we find signs of real deep-sea creatures." "Some, like this sea anemone, are familiar." "Others are less well known, like this chambered nautilus." "It's an ancient relative of octopus and squid, a living fossil, the last survivor of a group of animals that dominated the world's oceans 500,000,000 years ago." "It moves around by jet propulsion, squirting water backwards, in order to go forwards." "At night, it's the nautilus's turn to migrate." "It swims up towards the surface to feed on shrimps beside the reef wall, returning back down during the day." "This is a baby nautilus, the first time one's been filmed in the wild." "It's no bigger than three centimetres across, yet it makes the same daily up and down journey as its plate-sized parents." "But that pales into insignificance when compared to the daily vertical migration of these microscopic animals called zooplankton." "At sunset, all these tiny creatures swim upwards, and, under cover of darkness, they graze on floating algae close to the surface." "Many of them are fish larvae." "In fact, almost every fish species on the Great Barrier Reef starts life in the plankton." "There are billions upon billions of them making the roundtrip, the greatest daily migration on Earth." "They all travel an extraordinary distance, size for size, it would be like a person running a marathon twice a day." "That's if they're not caught on the way up." "On the reef wall, at about 150 metres deep, are huge sea fans." "They look like plants, but they're colonies of animals, whose branching arms capture the rising plankton." "Hidden amongst the branches is a pygmy seahorse." "It's a tiny fish that also feeds on plankton." "At little more than a centimetre long, fully grown, it's one of the world's smallest fish." "Contrast that with this monster, a tiger shark." "Like the tiniest marine life, it too rises up from the ocean's depths, but unlike the plankton, it's planning to stay a while." "Tiger sharks travel over 800km to reach Raine Island, and each year they show up at exactly the same time," "the time when turtles are nesting." "Turtles trapped in rock pools begin to refloat on the incoming tide." "For some, it's a second chance." "Now all this female must do is cross the lagoon to reach the reef edge and the safety of the open ocean." "But swimming in with the incoming tide is her number-one predator." "It's crossed the reef and is heading towards the beach." "A tiger shark could dismember this turtle and can even saw through her shell." "She's on high alert." "She turns rapidly and tilts, presenting her widest profile." "but the tiger shark doesn't give chase." "It can't be bothered." "There's a much easier way to get a meal." "It's been waiting for fresh turtle carcasses to float out on the rising tide." "And that's an amazing thing for a shark to do." "No mindless killer, this shark is smart." "It pitches up at the peak of the turtle nesting season and simply waits for the tide to deliver its food." "And it's well-equipped to tackle a carcass." "It clamps onto a flipper or the head, and uses the weight of its body to rip from side to side, slicing through flesh and bone, with razor sharp, saw-like teeth." "Vibrations from the commotion and the odour of mashed turtle flesh are carried on the currents, attracting more sharks to the feast." "They have impeccable manners." "Sharks of this size could do each other harm, so they take turns." "Smaller sharks deferring to the larger ones." "Tiger sharks are generally solitary, so this gathering of 25 is unusual." "The largest number of tiger sharks seen in one place at any one time," "and there is no feeding frenzy." "When you hunt the dead, there's no need to hurry." "It means the living can slip away, for now." "She'll not go far, for she'll be back again, up to eight times during a single breeding season to deposit up to 100 eggs on each visit." "Only then can she head back to her feeding grounds, and who knows what will happen to her there." "Away from the Great Barrier Reef, sea turtles are caught to eat." "And sharks are not immune." "These tiger sharks bear the scars of hooks from long-line fishing." "It's not surprising then that the older sharks are conspicuous by their absence." "While at Raine Island, the sharks and turtles are in a sanctuary, but even sanctuaries can come under threat." "And not necessarily from us." "Brief thunderstorms are a welcome break from the heat and humidity, but they can also build into the mother of all storms." "They're known as hurricanes in America, but here they're called cyclones." "A cyclone can be over 500km across, with winds swirling around the eye at 300km per hour." "It's the most destructive force the Great Barrier Reef must face." "And in February 2011, this part of the reef was hammered by the biggest storm in living history," "Cyclone Yazi." "'This is a special broadcast of 9 News with Peter Overton." "'Live in the cyclone...'" "'Good evening and welcome to a special edition of 9 News, 'live from Airlie Beach." "'The Cyclone Yazi bears down on North Queensland." "'These are the latest satellite images of the biggest cyclone" "'Australia has experienced in more than 100 years." "'Estimated to have the same intensity as Hurricane Katrina, 'it's a category 5, you can't get anything more powerful.'" "'We are expecting to wake up tomorrow morning 'to scenes of devastation and heartbreak, that's unprecedented, 'not only in Queensland, but Australia's history.'" "Daybreak exposed the ferocity of Cyclone Yazi." "Island resorts were destroyed." "And a massive storm surge smashed into marinas, destroying everything it touched." "In the direct path of the cyclone, waves pulverised the reef" "and huge swells seriously damaged corals 500km from the eye of the storm." "Cyclones form when humidity and air temperature build, and that's not the only time high temperatures directly affect the Great Barrier Reef." "In summer, the air temperature can soar." "Close to the ground, sunlight and heat reflected by the sand compound the problem." "On Raine Island, the turtle eggs are incubating safely below the ground, but for the seabird chicks, the cay becomes a searing furnace." "Birds must find shelter wherever they can." "Bizarrely, one option is to face your rear end into the sun." "But with global warming, temperatures are increasingly higher than the norm, and then an exceptional high water temperature in summer can be as destructive as a storm." "An ominous white glow along the edge of the reef indicates it's under stress." "The corals have lost their colour." "This bleaching occurs because corals can only live in a narrow temperature range." "Healthy corals get their colour from microscopic blue green algae living in their tissues." "These manufacture food for the corals by photosynthesis, but when the temperature rises just two degrees above the normal summer maximum, the blue green algae cells are expelled because the coral can not maintain the best conditions for them to grow." "The bleaching effect is the white chalky skeleton showing through the coral's transparent tissues." "But they're not dead, not yet." "They can survive in this bleached state for several weeks." "If the temperature drops, the corals acquire new blue green algae from plankton floating by." "But if the warm water persists, the coral dies." "Coral bleaching hadn't been seen on the Great Barrier Reef before the 1980s." "Now, these hot weather events occur more often, due to global warming, bleaching's more common, and cyclones are too." "And there's something even more insidious." "Temperatures are rising because more and more carbon dioxide from human activity enters the atmosphere." "This dissolves in seawater turning the seas weakly acidic, which can stop coral growth." "If they can't build their chalky skeletons, reefs will start to crumble." "Something made worse by increased cyclone activity due to climate change." "With such threats, it's a wonder the reef has any future at all but it does have a chance, for the reef has a neat way to help itself recover and it's evident for just one week in spring." "Two to six days after a full moon, each hard coral species, along the entire reef, spawns at the same time, on the same night." "Eggs and sperm unite to form free-swimming larvae." "Smaller than a pinhead, a coral larva is a like a space capsule." "It floats away on the current and seeks a new place to grow." "Some larvae travel no more than a few metres." "Others drift thousands of kilometres across the ocean, depending on where the current takes them." "Attracted to settle by the smells coming from a reef, the larvae crawl around and search for a spot to call home." "It transforms into a coral polyp, like a miniature sea anemone anchored to the seabed," "and here it starts a brand new colony of coral." "This constant process of re-seeding may help ailing reefs to recover, as long as the damage is not too severe or too frequent." "Sunrise on Raine Island marks another mass movement of wildlife." "Seabirds are heading out to the open ocean to go fishing." "Both parents normally take turns to search for food, but as the chicks grow and become more demanding they both go to sea, leaving their offspring on its own." "Along with many other predators, the parents search far offshore for dense shoals of fish." "Here, sharks, tuna, and seabirds are competing for a tight ball of fish that'll last for just a few minutes." "The birds rely on the sharks and tuna to drive the smaller fish closer to the surface." "Their chicks' very survival depends on their success." "A hungry chick waits impatiently for its parents to return." "And this time it's lucky." "They've flown in with plenty of food." "By late summer," "Raine Island is the largest nursery on the Great Barrier Reef, with seabird chicks growing on top of the sand and turtle eggs developing underneath." "The very last birds to nest on the island are rufous night herons." "They haven't travelled far, just out from the mainland and they surely have the most unattractive chicks on the reef." "They've hatched late in the season for a very good reason, because their food is not out at sea, but right on the doorstep." "In late afternoon, the parent birds take their positions." "They scan the sand, alert to any movements." "As the sun sets, the temperature change triggers the start." "It's what the herons have been waiting for." "Each clutch of turtle eggs has been incubating under the sand for two months." "At the right moment, the hatchlings all emerge together." "And they must reach the water in the shortest possible time." "Baby turtles are food for baby herons, so the heron parents have timed the peak of their nesting to coincide with this mass emergence." "Even ripples in the sand slow the hatchling's headlong rush to the sea." "It could mean the difference between life and death." "But the herons have eaten their fill." "They couldn't eat another baby turtle." "It's been a narrow escape for this one." "The first wave has taken the brunt of the attacks, but the sacrifice of a few hundred ensures the following thousands have a better chance to get to the sea, but some babies go the wrong way, just like their parent." "The wall of rock's an even bigger obstacle for the hatchling than it was for its mother." "And there's something even more sinister up ahead." "Rock crabs, they like baby turtles too." "Their powerful pincers could tear a turtle limb from limb." "Another lucky escape, but reaching the sea is just the first step." "A whole new set of hazards is waiting for a tiny turtle in the vast ocean." "Where this little hatchling goes for the first five years of her life is a mystery." "She'll not return to Raine Island before her 20th birthday." "when she'll come back to lay eggs on the very beach she's leaving today." "That's if she survives in an uncertain and often hostile world." "Raine Island has the biggest concentration of wildlife on the Barrier Reef, but many of its animals are visitors and at departure time they leave behind the protection of one of the world's largest marine parks." "The migrants cross international borders travelling to places where animals are not protected." "It means their survival is linked very much to events in other parts of the world." "The Great Barrier Reef is still an amazing place." "It's a magical, underwater world, stunningly beautiful and a never-ending source of wonder." "But how will it be when our turtles return?" "The reef has proved to be resilient in the past, surviving great natural changes." "But nothing like the pace of man-made change today, especially the pace of climate change." "As we've seen the Great Barrier Reef is connected to the rest of the world, in many ways." "It means we're all, no matter how remote, involved in its future." "Only we can decide what that future will be."