"Off the east coast of Australia, up to 200 kilometres offshore, a line of surf rises out of the open ocean." "Beneath the surface, creating these breaking waves, is the most magical marine environment on Earth." "I'm Monty Halls and I'm a marine biologist and diver." "And I've always been fascinated by the sea." "For me, there's always been one place that is the epitome of everything that's wonderful about the marine environment." "It's a true global icon and is a Mecca for anyone who's ever heard the word coral..." "HE SHOUTS The Great Barrier Reef!" "Stretching for over 2,000km up the tropical coast of Australia, it is quite simply the largest living structure on the planet." "It's made up of almost 3,000 different reefs, each one with a different personality." "And it's so much more than just coral." "But the Great Barrier Reef is most famous for its underwater world." "Here, there is more life than almost anywhere else on Earth." "Many of the creatures are exquisitely beautiful but some seem straight out of science fiction." "While others can kill in an instant." "And just when you think you know the reef, it changes." "Every second, every hour, every day and every year." "It's a world of continual surprises." "This is the amazing story of an ever-changing natural miracle, the Great Barrier Reef." "The Great Barrier Reef is so large that it can be seen from space - quite an achievement, considering the size of the creatures that built it." "The vast system that is the Great Barrier Reef is created by animals that are tiny." "Although they might look like plants, actually they're animals called polyps." "Each polyp is like a tiny upside-down jellyfish sitting in a stony cup." "They live together in colonies, like underwater tower blocks." "They respond to touch, temperature, currents and the cycles of the sun and moon, and their tiny movements combine to give each colony a rhythm of its own." "The polyps can't build the reef alone." "For this they need partners that are even smaller." "Within each polyp's tentacles are millions of tiny brown dots." "Each is a microscopic plant which transforms sunlight into food and energy for the corals." "It's a miraculous partnership that allows the corals to turn minerals in the water into limestone, building their stony skeletons." "In this way, each colony grows." "If we were able to watch a reef over several years, we'd see a continually-growing marine metropolis." "Its intricate architecture provides homes for thousands of creatures." "Even though coral reefs cover less than one percent of the world's oceans, they contain a quarter of all known marine life." "And the Great Barrier Reef is the biggest of them all." "I still vividly remember my first dive on a coral reef over 20 years ago." "And the moment I put my head in the water," "I knew what I wanted to do for the rest of my life." "HE LAUGHS" "Even if I spent my entire life underwater," "I still couldn't hope to see all the species that live on this reef." "400 species of hard coral, 300 of soft coral, 1,600 species of fish," "134 species of shark and ray, 30 species of whale and dolphin, six species of turtle," "14 species of sea snake... ..3,000 species of mollusc," "1,300 species of crustacean..." "The sheer number of creatures that live here makes the reef as hectic as any human city, with a rhythm of almost perpetual rush hours." "Imagine all this activity, this constant ebb and flow of life, continuing for over 2,000km." "It's hard to believe that such a huge solid structure hasn't always been here, but compared with the rest of Australia this reef is very young." "20,000 years ago this exact spot where I'm standing, and indeed the entire length of the Great Barrier Reef, would have been high and dry." "Back then, in the middle of the last ice age, the sea level was 120 metres lower than it is today, and the coastline was about 30km in that direction." "But suppose I could go back those 20,000 years in an instant..." "What would it have looked like?" "Well, for a start, it wasn't even underwater." "This area would have been covered in hundreds and hundreds of miles of eucalyptus and paper-bark forest and there wouldn't have been a fish in sight." "It was a land abounding with the animals of the Australian plains." "Aboriginal people would've lived here too, hunting the wildlife in areas that are now deep under water." "Then at the end of the last ice age, ice at the poles melted and sea levels rose all around the world, flooding this low-lying coast." "This happened only 10,000 years ago." "In fact, stories of The Great Flood are still passed down in aboriginal culture." "As the sea rose, corals began to grow on the rocky fringes of the continental shelf, creating the Great Barrier Reef we see today." "These shallow tropical waters are clear and warm... perfect conditions for corals to thrive." "Sheltered behind this long strip of reef, a lagoon was born." "An area of protected water larger in size than the whole of Great Britain." "And a new coastline, too, with shallow sandy waters." "The rising sea also cut off areas of high ground, creating the 600 islands that dot the lagoon." "Some are little more than rocks." "Others substantial mountains covered in woodland." "The first British person to see that was Captain Cook, who stood here in 1770, and for the first time realised the scale of the Great Barrier Reef." "He also realised the scale of the problem that faced him." "For Cook and his men, without the benefit of modern charts and sonar, it represented nothing more than a deadly labyrinth." "But it's exactly this complexity and the sheer size of the reef that has created so many opportunities for life." "There are fish of almost every imaginable kind - coral eaters, plant eaters, plankton eaters, the hunters and the hunted." "Bluefin trevally - powerful predators that hunt in packs." "One minute apparently minding their own business, the next charging their prey with a sudden rush." "Working together, they create confusion." "The trevally depart as quickly as they arrived, and the colourful reef fish regroup again to feed." "Beyond them, silver baitfish never let down their guard." "They shimmer like a thousand tiny mirrors, swirling as one amorphous mass to confuse any attackers." "It is their only defence against this - a shark mackerel." "These are the greyhounds of the ocean." "They need all their speed and agility against this ghostly school that is here one moment and gone the next." "Despite the dangers, small fish have to risk leaving the protection of the reef to feed." "And perhaps none is bolder than this little wrasse." "This gigantic grouper is many thousands of times her size and an aggressive territorial predator with one of the largest mouths on the reef." "But she's not deterred from approaching." "What she's about to do seems almost suicidal." "She's feeding on tiny blood-sucking parasites, which is why the grouper allows this." "Nevertheless, the wrasse has to regularly vibrate her fins against the inside of his mouth just to remind him not to swallow." "A single wrasse can eat an incredible 1,200 parasites a day." "And she's not fussy where she finds them." "Without this decontamination the grouper would quickly become infested." "This service is so valuable that the grouper is a regular client, visiting several times a day." "But even he has his limits." "On the reef there seem to be an almost infinite variety of ways of feeding." "Some fish even cultivate their own food, so they're very protective of it." "There's one fish species that's so good at defending it's particular patch of reef that it's responsible for more attacks on divers than any other." "And that's this species here." "This is a white damsel, and it's a constant gardener, nurturing and caring for a patch of algae." "Much of the algae growing on the sandy seabed is edible, so the damsel farms it in green patches, encouraging some species, nipping down others, and gently transforming the landscape around her." "After all this hard work, she doesn't look kindly on trespassers." "Anything that that swims too close is chased away." "But is she brave enough to try to see me off?" "I'm just going to edge onto its patch a little bit more." "There could be trouble, I sense." "Oh-ho-ho!" "She can't scare me but she just doesn't give up." "Just listen to this." "AGGRESSIVE CLICKING" "This is one feisty little fish." "Even though I'm so much bigger than this damsel fish, it's still quite an intimidating sight." "Oh!" "That was close." "While damsels maintain areas of reef by nurturing, other fish destroy it." "Bumphead parrotfish, each the size of a small sheep." "They have a tough beak and gigantic jaw muscles that work like bolt cutters." "It might seem like a lot of hardware for a diet of soft algae and coral polyps, but the only way to get at them is by sheering off chunks of the coral rock." "A second set of internal teeth then grinds the coral into a fine paste." "A single bumphead can chew up to five tonnes of coral every year, which they excrete back onto the reef as sand." "As fast as the reef grows, parrotfish break it down again, creating a continually-changing environment." "The constant remodelling of the reef by ravenous parrotfish is nothing compared to the effect of the ocean." "This is the reef crest and it's the point where the waves generated by the open ocean impact the hard surface of the coral reef." "It's an unstoppable force hitting an immovable object." "I'm keenly aware that these are very small waves." "Believe me, in a big storm, to be here would be suicide." "But it's a vivid illustration of the power of the sea." "Oh-ho-ho!" "Tropical storms and cyclones regularly whip in from the Pacific Ocean." "The surf smashes into the reef, pulverising the coral below." "Waves and currents move the rubble and sand around and shape it into beautiful small islands..." "..known as coral cays." "Some come and go in a matter of days." "Others build and grow and can last for years, as plants take root and protect them from the elements." "At first glance it might seem that a sand cay is a pretty inhospitable environment but actually it can support life in abundance." "Today it's rare to find a cay free from humans and mainland predators, but this is one of the best " "Raine Island." "It's home to thousands of birds and in the centre of the island they jostle for the best available space." "Nowhere else on the reef are they found in such numbers, and such variety." "Frigate Birds, red-footed boobies and Caspian terns." "In fact, 84 different species are found here." "It's also the largest green turtle breeding ground in the world." "Having mated offshore, thousands of females haul themselves up the beach to lay their eggs," "The record is 26,000 turtles in a single night." "They turn a remote desert island into a crowded frenzy of nocturnal activity." "Wind and waves have created an island that, for the moment at least, has just the right conditions to support this incredible spectacle." "But the island still changes, every year, every season." "And the reef itself never stays the same for long." "Twice a day, life along the length of the Great Barrier Reef has to cope with complete upheaval, as the rhythm of the moon causes tides to flood and drain the reef." "During the lowest tides, the water runs off, and whole sections of living coral, that would normally be underwater, are exposed." "The level of water on the reef top decreases dramatically, getting ever shallower and going from a stable environment to an ever changing, hostile one." "It's almost as though some mighty force has lifted the reef right out of the water." "These are extreme conditions." "As the exposed corals start to heat up, the tiny polyps retract inside their stony skeletons for protection." "Then they secrete mucous." "It acts as a sunscreen, and there's no skimping on quantity." "An area no larger than the size of a coffee table can produce five litres of the stuff." "As the tide retreats to the edge of the reef, it cuts off pools of water." "For most animals, like these sea cucumbers and starfish, the best way to survive is by taking refuge in these rock pools." "With the tropical sun beating down, the exposed flat is one of the most hostile environments on the reef." "For a fish caught out here it should mean certain death." "But not for this one." "It's an epaulette shark." "It can't breathe out of water." "But it survives here by shutting down some parts of its brain and increasing the blood supply to others." "Not only that, it can walk on land." "These abilities make it a specialised reef-top hunter." "Manoeuvring around the confined space of a rock pool, it's master of all it surveys." "Nothing is safe." "Not even animals hiding under the sand." "It scans for smells, and for tiny electric signals given off by the bodies of hidden creatures." "It's detected a crab." "It doesn't need sharp teeth because it simply sucks up its prey." "The epaulette isn't the only shark able to adapt to a change of the tides." "When the sea returns, it brings cool oxygenated water back to the reef." "The relief, though, is short-lived." "Larger sharks gather at the reef's edge, waiting to get at the freshly accessible hunting grounds." "As the water rises, more and more creatures take the opportunity to feed in a new area." "For the sharks, fish that can normally outrun them are temporarily caught in the shallows, trapped between sand and surface, with few places to hide." "Lemon sharks hug the shore, surrounded by schooling bait fish," "But, for the moment, they seem to have little interest in all this food around them." "But they're simply waiting for help." "Young trevally." "They dart for the fish, causing them to panic and break ranks." "By putting themselves in the midst of the shoal, the sharks are in a perfect position to exploit the chaos." "For a hungry lemon shark, no water seems too shallow." "The sharks are able to use these special tactics to feed inshore because of the rhythm of the tides, which change conditions every few hours." "The cycle of day and night also has a dramatic effect on everything that lives on the reef." "There are many enduring mysteries about the Great Barrier Reef, particularly at night, but this is one of the more enchanting ones." "Watch this." "This coral is fluorescing under ultraviolet light." "It's this amazing light-show, and it covers pretty much the whole of the reef crest." "Look at this, Look at this!" "Vivid, vivid colours!" "Doesn't look real, looks like some bizarre galactic broccoli is what it looks like." "One theory is that these magical colours are in some way caused by the coral's natural sunscreen, but, despite lots of very clever people looking into it, no-one really knows why this happens." "And the same goes for much of the night-time activity here." "A coral reef during the day is a very different place from a coral reef at night." "The atmosphere changes completely." "The day shift has logged off and the night shift has gone to work." "It's mean and moody and the animals here mean business." "Just as in human cities, there's a new cast of characters after dark." "Octopus emerge from their holes to stalk their prey." "A tiny sole improves his chances of survival by mimicking a toxic flatworm." "Some of the strangest nocturnal creatures are relatives of the starfish, like this feather-mouthed sea cucumber, sifting food from the sand." "Another is the basket star." "It comes alive at night, throwing out thousands of arms to filter food from the plankton-rich current." "These animals can live for a very long time - up to 35 years." "There's records of these guys being seen night after night, in the same place, for over 15 years." "The day shift is still here, of course, it's just hidden." "But it's difficult to find somewhere big enough to hide if you are the size of turtle." "This green turtle has found himself a little ledge and is tucking in for the evening." "Hello, big fella." "He's a bit drowsy, and probably quite grumpy." "We all get like that occasionally, don't we?" "I'll leave him alone." "A lovely sight Sleep well big fella." "At night many fish sleep, resting in safe nooks and crannies on the reef." "Without eyelids, they enter a trance-like state, barely moving other than to keep water flowing through their gills," "But they do give off a scent, and that means they can be found." "This cone snail catches its prey in one of the most surprising ways imaginable, and specialises in killing fish." "It sniffs them out whilst they are sleeping." "The goatfish may be asleep but it's aware of its surroundings." "First, the snail must sneak close enough to bring its secret weapons into play." "The snail appears to release chemicals that paralyses its victim." "The goatfish seems powerless to escape, as it's swallowed alive." "Moments later, a venomous barb inside the snail puts the fish out of its misery." "In a place where even seashells are deadly, remaining undetected is essential if you want to survive the night." "And simply hiding is not enough, just a hint of your scent, wafting into the current, can attract trouble." "The parrotfish has come up with an elegant solution in the form of a sleeping bag made out of membrane." "It stops the smell spreading along the reef and being picked up." "It's a very good way, if you're a parrotfish, of ensuring you get a good night's sleep." "No-one has ever filmed a parrotfish making its cocoon before." "It starts by exuding a layer of mucous around its body." "It then draws water into its mouth and pushes it out of its gills, blowing up the mucous bubble around it." "Within half-an-hour, the fish is completely enveloped in a slimy cocoon, which traps its smell." "But not all the reef hunters are fooled." "White tipped reef sharks do use smell to find their prey, but if that doesn't work they have another sense." "They can detect an animal's bioelectrical energy, the tiny electrical signals given off by even the smallest of its twitching muscles." "This resting parrotfish risks discovery every time it moves a fin, or wafts its gills." "As the sharks pass, it holds its breath." "But sharks don't give up that easily." "White tips are the perfect size and shape for hunting in these cramped spaces." "Their snake-like bodies can even wriggle into crevices," "and they can fold their dorsal fin flat to squeeze through small holes." "If discovered, a sleeping parrotfish wouldn't stand a chance." "Once the first shark has caught its prey, the smell and commotion attracts others." "Every now and then you find yourself in moderately strange situations in life, and this is one of them." "I'm in a cave with FEEDING sharks." "They are really well designed to be in here and I'm not sure I am, quite frankly." "Because of their sophisticated senses and remarkable physique, white tips are great reef hunters." "At night they turn it into an art form." "Better than all other sharks." "We expect sharks to be hunters, but after dark, the reef itself becomes one giant predator." "By day corals may be passive and plant-like, but by night they become active hunters." "They snatch tiny animals from the current, immobilising them with stinging tentacles." "Just like their relatives the jellyfish." "And they're highly competitive." "Here, two individual polyps are fighting for possession of a single unfortunate creature." "At night the corals don't just hunt." "When territory is at stake, whole colonies go to war with each other." "Once a coral senses another is too close, it launches a barrage of stinging cells, and the enemy returns fire." "It's a war of numbers - the coral with the greatest firepower will win." "Eventually, the coral on the right forces its opponent to retreat." "Yet another small part of the reef has been changed." "Every night this goes on, from one end of the Great Barrier Reef to the other." "Day and night, the reef is always changing, but that's nothing compared to the impact of the seasons." "In summer, warmer water means more food, making it the best time to bring new life into the world." "A pair of noddy terns fly close together over an island on the reef." "These are two mates reaffirming their bond." "Noddies signal to each continually during courtship, both in the air and after landing." "Once in the trees, the performance changes." "They bow their heads repeatedly." "It's pretty clear how they got their name." "Once they've started courting they don't like any interruption." "BIRD SQUAWKS" "To reinforce their relationship the male has to collect nesting materials." "The females are famously fussy, so he chooses his present carefully." "A bent twig, it seems, is just the thing." "A few more of them and they have a nest, if you can call it that, ready for their single chick." "Noddies are devoted partners, and often remain together season after season." "Beneath the waves, the summer warmth triggers fish to court," "In bicolour parrotfish, the larger male leads a tender underwater ballet to woo a partner." "The urge to breed spreads across the reef." "These are surgeon fish." "Single females dart to the surface, pursued by groups of competing males." "As each female releases her eggs, the males race to fertilise them, with a puff of white sperm." "The rise in water temperature also sets the conditions for the most remarkable event on the reef." "On only a few summer nights each year, and triggered by a particular phase of the moon, the corals spawn." "This is the world's largest synchronised breeding event." "On any one of these nights, trillions of eggs and sperm are released, flooding the entire 2,000 kilometre length of the reef." "By spawning at this precise moment the corals have caught a lull in the tide, allowing sperm and eggs of the same species to meet and fertilise." "They form baby corals, as the tidal currents pick up, they're spread far and wide." "The young corals joins billions of other baby creatures floating in the plankton." "A baby grouper, only a centimetre in length, that will one day grow into a giant, over two meters long." "Most reef animals have tiny babies like these, carried at the mercy of the currents." "Many have some ability to direct themselves, and even the young corals can swim to a degree, propelling themselves with microscopic hairs." "Each one of these could grow into a whole colony and start a new coral reef." "This living soup represents the future of the reef." "Just one glass full has so much potential." "It's remarkable to think that such a structure as the Great Barrier Reef, large enough to be seen from space, and complex enough to support the interwoven lives of thousands of species, is dependent on minute fragile forms such as these." "The reef is a dynamic place, in constant flux, influenced by the forces of geology, weather, wind and waves, the rhythms of the sun and moon." "Yet its future depends on fragile young life forms that must survive in the face of these powerful natural forces, and find somewhere safe to settle and grow." "There can be few tales in the natural world quite so remarkable." "And this is just the start of what makes the Barrier Reef so great." "Only 7% of the ecosystem is coral." "The rest is covered with rainforest and rivers, lagoon, swamp, and shore." "These are home to some of the world's strangest creatures." "In the next programme we explore the fascinating worlds and wildlife beyond the reef, and discover their connections to this most magical place." "Subtitles by Red Bee Media Ltd" "Email subtitling@bbc.co.uk"