"Our planet is the greatest living puzzle in the universe." "A collection of worlds within worlds, each one a self-contained ecosystem bursting with life." "But how do they work?" "The intricate web of relationships and the influence of natural forces makes each microworld complex and unique." "So, to discover their secrets, we need to explore them one by one... ..untangle their interlocking pieces, and ultimately reveal the vital piece - the key to life itself - hidden deep within each of nature's microworlds." "Over 99% of space available for life on Earth is in the sea." "From top to bottom, the ocean contains a volume of water totalling 1.3 billion cubic kilometres." "It is the single largest ecosystem on our planet - far, far larger than any terrestrial ecosystem." "It's also the least explored." "We know more about the surface of Mars than we do about the majority of the marine environment." "This microworld is Earth's inner space." "Almost 90% of the ocean lies below a kilometre, and over 75% is deeper than three kilometres." "The very deepest part of the ocean lies at 11 kilometres and would easily engulf Mount Everest." "More people have walked on the moon than have visited the deepest part of the ocean." "Below the upper sunlit waters is a foreboding world of darkness and crushing pressures." "Despite this, it's home to countless living things." "Some of the most bizarre and unlikely creatures exist in the depths." "Even though they make up the majority of creatures upon our planet, we know very little about them." "The vast majority of these float or swim in the water column, a world without walls or sunlight." "But even at the very bottom of the ocean, there is life somehow making a living on a flat abyssal plain that spans the seabed between continents." "Recent discoveries in the deep sea have astounded the scientific community and given us an insight into another world." "So just how do creatures exist in the depths?" "What is the key that connects all these animals and allows them to survive in a cold, dark, hostile world?" "Our journey starts at the surface of the open ocean in the North Atlantic." "As the sun sets, a large mass of animals appear as if from nowhere to feed at the sea's surface..." "..millions upon millions of them." "A bizarre array of jellyfish trail stinging tentacles, or ingest food directly into their bodies." "Among the throng are animals that defy classification." "A small fish takes cover inside the body of this pelagic jelly for a very good reason." "Large shoals of squid arrive to hunt at the surface." "By coming at night, they can avoid the eyes of daytime predators." "But not all animals need eyes to hunt." "DOLPHINS SQUEAK" "Spotted dolphins use sonar in the darkness... ..targeting their prey with pinpoint accuracy." "So where do all these night-time animals come from?" "They ascend from a world with little or no sunlight, hundreds of metres down." "Every night, across the world's oceans, 100 million tonnes of these animals rise from the depths." "It's by far the largest migration of animal life on the planet." "They come to exploit the abundance of food in the surface waters." "An abundance that exists because of something that happens at a microscopic level - photosynthesis." "Tiny algae and plants known as phytoplankton use light from the sun to turn soluble carbon into organic matter." "This is known as primary production." "The by-product of photosynthesis is oxygen." "The waters up here are rich in this important element, which is essential for life." "50 billion tonnes of phytoplankton is produced in the upper oceans every year." "During spring in the North Atlantic, when conditions are right, their blooms can be seen from space." "It forms the basis of the marine food chain and is fundamental to all life in the ocean." "Phytoplankton is fed on by tiny animals known as zooplankton." "The most common are copepods." "These crustaceans are little more than a millimetre long but they are the most numerous animal in the ocean." "In the North Atlantic, a cubic metre of seawater may contain in excess of 100,000 of them." "Using their legs, they create currents which push the microscopic phytoplankton into their mouths." "It's the sheer numbers of phytoplankton and copepods that draw deep-sea animals up at night." "When dawn returns, the migration is reversed and the massive army of deep-sea creatures sink back down into the darkness." "If we're to follow them, we must use a deep-sea submersible capable of withstanding immense pressures." "Without one, we could not survive in their world." "As we descend, the sun's rays are absorbed and scattered." "At 200 metres, we leave the photic zone and enter the first layer of the deep sea - the twilight zone." "At this depth, there's less than 1% of the sunlight at the surface." "The pressure has increased twentyfold, and the temperature has dropped to four degrees, but we find a world of extraordinary beauty." "With nowhere to hide in the twilight zone, the best disguise is transparency." "Like this squid with a delicate glass-like body." "Almost nine centimetres long, this amphipod is a giant of its kind." "It's completely transparent, apart from its two enormous eyes and central nervous system." "Another peculiar crustacean lives like a hermit within the stolen body of a jellyfish." "This shell also houses her offspring." "Her habit of pushing this protective shell around has led to the nickname of pram bug." "The longest jellyfish of all are the giant siphonophores." "Their tentacles, lined with rows of stinging cells, can reach 40 metres long." "Of the countless billions of animals living below the photic zone, only a fraction migrate into shallower water to feed at night." "So what do these animals feed on?" "Looking out of the window of our submersible, we can see a constant rain of particles slowly drifting down around us." "Known as marine snow, this is a vital food source for everything living below 200 metres." "It rains down from the sunlit waters above." "But what exactly is it?" "The density and exuberance of life at the surface of the ocean far outweighs that of the deep sea." "This is where marine snow originates." "A pod of common dolphins prepare to hunt." "They're homing in on a shoal of mackerel swimming near the surface." "Waiting are flocks of shearwaters, hoping for the fish to be driven within range." "The mackerel swim in a tight ball for safety." "Working as a group, the dolphins drive the bait-ball upwards towards the waiting shearwaters." "Shearwaters dive deep to grab their share." "Caught between the birds and the dolphins, the mackerel have nowhere to go, and the frenzy builds." "The commotion attracts a school of yellowfin tuna." "These two-metre-long fish are capable of bursts of speed of 75 kilometres per hour." "This doesn't deter the shearwaters, which continue to feast on the shoal beneath." "Eventually, nothing remains of the huge shoal of mackerel, except for scraps of flesh and scales." "Leftovers, faeces, dead and dying plants and animals have only one way to go, and that's down." "This is marine snow." "Millions of tiny creatures, such as this sea spider, filter the snow." "Its feathery appendages gathering particles that are then drawn through its jaws." "As a result, the nutritional value of marine snow declines with depth." "Feeding activity also uses up precious reserves of dissolved oxygen." "Without photosynthesis to replace it, oxygen decreases." "So, as we descend, life begins to thin out." "But it becomes ever more extraordinary." "At 500 metres, it appears completely dark to human eyes." "The pressure is 50 times what it was at the surface and there's only the tiniest remnant of sunlight filtering through." "Animals here have adapted to cope with extreme pressure and their eyes have evolved to become disproportionately large." "The owner of THIS pair gazes ever upwards into the gloom, seeking the silhouette of its prey against the faint down-welling light." "To escape the attention of super-sensitive eyes, fish here have evolved light-transmitting cells on the underside of their body." "Using graphics, we can see how these exactly match the background light." "With this, their silhouette breaks up, making them appear almost invisible from below." "By 800 metres, the pressure is 80 times what it was at the surface and the temperature is below three degrees centigrade." "Oxygen levels have also decreased to less than 5% of what they were at the surface." "This is called the dead zone." "But something still manages to live here." "Vampyroteuthis infernalis - the vampire squid from hell." "Vampire squid have lived in the depths for 200 million years." "They share physical characteristics with octopus and squid and are thought to be a missing link." "Highly specialised blood cells allow them to live in this low-oxygen environment." "Despite its hellish name and fierce look, vampire squid are placid creatures, averaging only" "28 centimetres in length, and are completely harmless." "Get too close to a vampire squid and it puts on the most amazing light display." "Bioluminescent bacteria in its arms and on its body dazzle and confuse potential predators." "And with that, it disappears into the darkness." "Beyond 1,000 metres, we enter the dark zone, where not even the faintest remnant of sunlight can penetrate." "Yet when we switch off the submersible lights, we see bioluminescence." "There's life even here." "Furthermore, oxygen levels have risen and that's because we are now in a deep-water current called the Great Ocean Conveyer." "When surface water, enriched in oxygen by photosynthesis, meets polar ice, it sinks beyond the first 1,000 metres and flows towards the equator..." "..carrying oxygen to the very deepest corners of the abyss." "Here in the dark zone, we find the real monsters of the deep." "Oxygen may be plentiful here but food is scarce, and bioluminescence is the only light available." "Most of the flashing lights we can see come from deep-water copepods." "They provide food for other deep-sea animals." "To a hunting squid, this flashing light looks like food." "But in the darkness, nothing is what it seems." "Fooled by bioluminescent bacteria living in the antennae of an angler fish." "The angler fish can easily accommodate the squid in its extendable stomach." "Many deep-sea fish have disproportionately large stomachs." "In this sparse, cold world, it might be many days between meals." "Natural selection has produced lures of all shapes and sizes, each used to tempt prey to within easy reach." "This Wolftrap angler has a lure hanging amidst its formidable teeth." "In this dazzling battlefield, prey have developed some surprising methods of escape." "Deep-water shrimps confuse attackers by spinning and releasing a bioluminescent glue." "While the shrimp makes its escape, the glue sticks, leaving the attacker glowing in the face of its own enemies." "The shrimp's red colour is also its camouflage." "In the dark zone, the eyes of most predators are tuned to the blue or green of bioluminescent light." "So, to them, a red shrimp is almost invisible, but not to one deep-water resident - the dragonfish." "This predator has evolved red bioluminescent headlights below its eyes that are sensitive to red light." "Its target doesn't see it until it's too late." "Descending ever further, we eventually reach the sea floor, six kilometres down." "The pressure here is more than 600 times that of the surface, and temperatures are close to zero degrees." "It takes many weeks for marine snow to descend this far." "Here, it forms a vast blanket of soft sediment over a kilometre thick - the abyssal plain." "These plains cover a third of the Earth's surface." "By the time the snow gets here, it only contains a fraction of its original energy." "Under such extreme conditions, we would expect the abyssal plain to be lifeless." "But even here, we find life." "Deepwater sea urchins must sift large quantities of sediment to survive." "The strange balloon-like sacs on their backs may be filled with a noxious substance to deter predators." "Abyssal shrimps use their elongated antennae to feel for tiny particles of food floating around in the darkness." "Life here may be sparse, but because the abyssal plains cover such a large area of the Earth's surface, they are home to some of the most numerous animals on the planet." "Fish have been found living as far down as eight kilometres." "This rattail is one of the most common species." "The white spots around its eyes are pores, sensitive to the slightest movement - vital in the inky blackness of the abyssal plain." "Just occasionally, in this nutrient-poor environment, a feast arrives." "A dead tuna doesn't lie unnoticed for long." "This deep-sea conger eel has picked up the scent from far off." "Down here, a good sense of smell is a lifesaver." "The carcass also attracts the attention of one of the deep's most elusive creatures - the six-gilled shark." "These rarely seen sharks are active hunters and can grow to five metres in length." "This type of shark has been around for at least 200 million years." "There are many opportunistic scavengers on the seafloor." "They'll detect even the tiniest scent in the water and will move in from miles around." "Deep-sea crabs and scavenging arrowtooth eels are soon joined by giant isopods." "Related to woodlice, these strange-looking monsters are half a metre long." "Within hours, the carcass is stripped to nothing." "Even the bones are eaten." "Where food is at a premium, nothing is wasted." "When life was first discovered thriving in the abyss, it was a complete surprise, but the depths revealed another secret that shook the foundations of scientific thinking." "A secret that lay here on the mid-ocean ridges." "These undersea mountain ranges are the longest geological structure on Earth, part of a continuous 65,000 kilometre chain extending across the face of the planet." "This is the frontline of plate tectonics." "Here, the seafloor is parting at about two centimetres a year." "When submersibles first visited the ridges in the 1970s, they found towering basalt chimneys known as black smokers spewing hot water and hydrogen sulphide from deep inside the Earth's crust." "At 400 degrees centigrade, these smokers are hot enough to melt lead." "It was here that they made one of the most important scientific discoveries of the 20th century - dense populations of marine animals living in a toxic deep-sea environment without any reliance on energy from the sun." "In one location, they found swarms of deep-sea shrimps." "In another, they found bizarre polychaete worms tolerating water temperatures of 80 degrees centigrade." "No other animal on Earth is known to exist at such temperatures." "Clams, crabs and even fish live in large numbers around the vents." "How do all these animals survive in such densities, in total darkness, under conditions of scalding heat and intense pressure?" "The answer lies in mats of bacteria that coat the chimneys." "These bacteria are primary producers." "They substitute for phytoplankton in a world without sunlight." "Drawing chemical energy from the vent, they convert soluble carbon into organic matter - a process known as chemosynthesis." "It is this bacteria, not marine snow, that is the basis of the hydrothermal vent food chain." "But in order to exploit this resource, they've had to adapt to living with high levels of hydrogen sulphide - a poison as potent as cyanide." "These giant tube worms approach two metres long and they incorporate vent bacteria within their bodies." "The bacteria provide the worms directly with food and energy." "Despite the toxic environment, there's thousands of times more life at hydrothermal vents than at the abyssal plain." "The sheer scale and diversity of creatures here has opened up a whole new branch of research." "New species are being discovered on every single dive." "So, here, far away from the sun's rays, is a self-sufficient community thriving in the most hostile of environments." "This may be a glimpse of how life exists on other planets." "Our journey into the deep-sea microworld has revealed some of the most unusual life forms on Earth." "What we've found at every level is a diverse community of animals adopting unique ways of surviving." "Many of these creatures live well beyond the sun's rays." "But there's one thing that ties them all together wherever they dwell - the need for oxygen." "Even the bacteria that form the basis of hydrothermal communities need oxygen to make organic matter." "And oxygen can only be produced in one way - as a by-product of photosynthesis, a process driven by the sun." "Even though the majority of ocean creatures remain oblivious to its presence, ultimately it is the sun that allows life to exist in the depths of the abyss." "The sun connects all life, from the surface... ..to the deepest reaches of the sea." "We still know more about our neighbouring planets than we do the deep sea." "This inner space remains the last frontier on Earth - a frontier that continues to evoke awe and wonder." "Who knows what discoveries are yet to be made in the largest but least explored of all our microworlds?" "Subtitles by Red Bee Media Ltd"