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,
0:00:08 > 0:00:13each one a self-contained ecosystem bursting with life.
0:00:17 > 0:00:19But how do they work?
0:00:21 > 0:00:27The intricate web of relationships and the influence of natural forces
0:00:27 > 0:00:31makes each microworld complex and unique.
0:00:33 > 0:00:38So, to discover their secrets, we need to explore them one by one...
0:00:40 > 0:00:42..untangle their interlocking pieces,
0:00:42 > 0:00:49and ultimately reveal the vital piece - the key to life itself -
0:00:49 > 0:00:53hidden deep within each of nature's microworlds.
0:01:01 > 0:01:07Over 99% of space available for life on Earth is in the sea.
0:01:07 > 0:01:11From top to bottom, the ocean contains a volume of water
0:01:11 > 0:01:16totalling 1.3 billion cubic kilometres.
0:01:16 > 0:01:21It is the single largest ecosystem on our planet -
0:01:21 > 0:01:24far, far larger than any terrestrial ecosystem.
0:01:25 > 0:01:28It's also the least explored.
0:01:28 > 0:01:30We know more about the surface of Mars than
0:01:30 > 0:01:34we do about the majority of the marine environment.
0:01:34 > 0:01:37This microworld is Earth's inner space.
0:01:43 > 0:01:47Almost 90% of the ocean lies below a kilometre,
0:01:47 > 0:01:51and over 75% is deeper than three kilometres.
0:01:54 > 0:01:57The very deepest part of the ocean lies at 11 kilometres
0:01:57 > 0:02:00and would easily engulf Mount Everest.
0:02:02 > 0:02:05More people have walked on the moon than have visited
0:02:05 > 0:02:08the deepest part of the ocean.
0:02:08 > 0:02:13Below the upper sunlit waters is a foreboding world of darkness
0:02:13 > 0:02:15and crushing pressures.
0:02:15 > 0:02:19Despite this, it's home to countless living things.
0:02:21 > 0:02:26Some of the most bizarre and unlikely creatures exist in the depths.
0:02:26 > 0:02:30Even though they make up the majority of creatures
0:02:30 > 0:02:34upon our planet, we know very little about them.
0:02:35 > 0:02:39The vast majority of these float or swim in the water column,
0:02:39 > 0:02:42a world without walls or sunlight.
0:02:44 > 0:02:47But even at the very bottom of the ocean,
0:02:47 > 0:02:51there is life somehow making a living on a flat abyssal plain
0:02:51 > 0:02:54that spans the seabed between continents.
0:02:56 > 0:03:00Recent discoveries in the deep sea have astounded the scientific
0:03:00 > 0:03:05community and given us an insight into another world.
0:03:07 > 0:03:10So just how do creatures exist in the depths?
0:03:10 > 0:03:14What is the key that connects all these animals and allows them
0:03:14 > 0:03:18to survive in a cold, dark, hostile world?
0:03:23 > 0:03:26Our journey starts at the surface of the open ocean
0:03:26 > 0:03:28in the North Atlantic.
0:03:31 > 0:03:35As the sun sets, a large mass of animals appear as if from nowhere
0:03:35 > 0:03:38to feed at the sea's surface...
0:03:40 > 0:03:42..millions upon millions of them.
0:03:58 > 0:04:01A bizarre array of jellyfish trail stinging tentacles,
0:04:01 > 0:04:04or ingest food directly into their bodies.
0:04:10 > 0:04:14Among the throng are animals that defy classification.
0:04:17 > 0:04:22A small fish takes cover inside the body of this pelagic jelly
0:04:22 > 0:04:23for a very good reason.
0:04:27 > 0:04:30Large shoals of squid arrive to hunt at the surface.
0:04:35 > 0:04:38By coming at night, they can avoid the eyes of daytime predators.
0:04:40 > 0:04:43But not all animals need eyes to hunt.
0:04:43 > 0:04:45DOLPHINS SQUEAK
0:04:47 > 0:04:50Spotted dolphins use sonar in the darkness...
0:04:52 > 0:04:54..targeting their prey with pinpoint accuracy.
0:05:00 > 0:05:03So where do all these night-time animals come from?
0:05:05 > 0:05:08They ascend from a world with little or no sunlight,
0:05:08 > 0:05:10hundreds of metres down.
0:05:12 > 0:05:17Every night, across the world's oceans, 100 million tonnes of these
0:05:17 > 0:05:20animals rise from the depths.
0:05:20 > 0:05:24It's by far the largest migration of animal life on the planet.
0:05:28 > 0:05:31They come to exploit the abundance of food in the surface waters.
0:05:33 > 0:05:34An abundance that exists
0:05:34 > 0:05:39because of something that happens at a microscopic level -
0:05:39 > 0:05:41photosynthesis.
0:05:43 > 0:05:46Tiny algae and plants known as phytoplankton
0:05:46 > 0:05:50use light from the sun to turn soluble carbon into organic matter.
0:05:52 > 0:05:55This is known as primary production.
0:05:58 > 0:06:00The by-product of photosynthesis is oxygen.
0:06:01 > 0:06:04The waters up here are rich in this important element,
0:06:04 > 0:06:06which is essential for life.
0:06:08 > 0:06:1250 billion tonnes of phytoplankton is produced
0:06:12 > 0:06:14in the upper oceans every year.
0:06:15 > 0:06:17During spring in the North Atlantic,
0:06:17 > 0:06:21when conditions are right, their blooms can be seen from space.
0:06:24 > 0:06:27It forms the basis of the marine food chain
0:06:27 > 0:06:31and is fundamental to all life in the ocean.
0:06:34 > 0:06:39Phytoplankton is fed on by tiny animals known as zooplankton.
0:06:39 > 0:06:42The most common are copepods.
0:06:43 > 0:06:47These crustaceans are little more than a millimetre long
0:06:47 > 0:06:49but they are the most numerous animal in the ocean.
0:06:51 > 0:06:54In the North Atlantic, a cubic metre of seawater
0:06:54 > 0:06:58may contain in excess of 100,000 of them.
0:06:59 > 0:07:02Using their legs, they create currents which push
0:07:02 > 0:07:05the microscopic phytoplankton into their mouths.
0:07:07 > 0:07:10It's the sheer numbers of phytoplankton
0:07:10 > 0:07:14and copepods that draw deep-sea animals up at night.
0:07:17 > 0:07:21When dawn returns, the migration is reversed and the massive
0:07:21 > 0:07:26army of deep-sea creatures sink back down into the darkness.
0:07:29 > 0:07:33If we're to follow them, we must use a deep-sea submersible
0:07:33 > 0:07:36capable of withstanding immense pressures.
0:07:38 > 0:07:41Without one, we could not survive in their world.
0:07:45 > 0:07:49As we descend, the sun's rays are absorbed and scattered.
0:07:51 > 0:07:55At 200 metres, we leave the photic zone
0:07:55 > 0:07:58and enter the first layer of the deep sea -
0:07:58 > 0:08:00the twilight zone.
0:08:01 > 0:08:06At this depth, there's less than 1% of the sunlight at the surface.
0:08:07 > 0:08:11The pressure has increased twentyfold, and the temperature has
0:08:11 > 0:08:17dropped to four degrees, but we find a world of extraordinary beauty.
0:08:28 > 0:08:32With nowhere to hide in the twilight zone, the best disguise
0:08:32 > 0:08:34is transparency.
0:08:36 > 0:08:40Like this squid with a delicate glass-like body.
0:08:48 > 0:08:53Almost nine centimetres long, this amphipod is a giant of its kind.
0:08:55 > 0:08:59It's completely transparent, apart from its two enormous eyes
0:08:59 > 0:09:01and central nervous system.
0:09:04 > 0:09:08Another peculiar crustacean lives like a hermit within
0:09:08 > 0:09:10the stolen body of a jellyfish.
0:09:11 > 0:09:13This shell also houses her offspring.
0:09:15 > 0:09:18Her habit of pushing this protective shell around has led to
0:09:18 > 0:09:20the nickname of pram bug.
0:09:26 > 0:09:30The longest jellyfish of all are the giant siphonophores.
0:09:32 > 0:09:35Their tentacles, lined with rows of stinging cells,
0:09:35 > 0:09:37can reach 40 metres long.
0:09:41 > 0:09:44Of the countless billions of animals living below the photic zone,
0:09:44 > 0:09:48only a fraction migrate into shallower water to feed at night.
0:09:52 > 0:09:54So what do these animals feed on?
0:09:57 > 0:10:00Looking out of the window of our submersible, we can see
0:10:00 > 0:10:05a constant rain of particles slowly drifting down around us.
0:10:08 > 0:10:12Known as marine snow, this is a vital food source for everything
0:10:12 > 0:10:14living below 200 metres.
0:10:15 > 0:10:18It rains down from the sunlit waters above.
0:10:20 > 0:10:22But what exactly is it?
0:10:25 > 0:10:28The density and exuberance of life at the surface of the ocean
0:10:28 > 0:10:31far outweighs that of the deep sea.
0:10:32 > 0:10:35This is where marine snow originates.
0:10:38 > 0:10:41A pod of common dolphins prepare to hunt.
0:10:43 > 0:10:47They're homing in on a shoal of mackerel swimming near the surface.
0:10:50 > 0:10:53Waiting are flocks of shearwaters, hoping for the fish
0:10:53 > 0:10:55to be driven within range.
0:10:59 > 0:11:02The mackerel swim in a tight ball for safety.
0:11:02 > 0:11:06Working as a group, the dolphins drive the bait-ball upwards
0:11:06 > 0:11:08towards the waiting shearwaters.
0:11:09 > 0:11:12Shearwaters dive deep to grab their share.
0:11:14 > 0:11:16Caught between the birds and the dolphins,
0:11:16 > 0:11:20the mackerel have nowhere to go, and the frenzy builds.
0:11:25 > 0:11:28The commotion attracts a school of yellowfin tuna.
0:11:31 > 0:11:34These two-metre-long fish are capable of bursts of speed
0:11:34 > 0:11:37of 75 kilometres per hour.
0:11:45 > 0:11:48This doesn't deter the shearwaters, which continue to feast
0:11:48 > 0:11:50on the shoal beneath.
0:11:58 > 0:12:01Eventually, nothing remains of the huge shoal
0:12:01 > 0:12:05of mackerel, except for scraps of flesh and scales.
0:12:07 > 0:12:11Leftovers, faeces, dead and dying plants and animals
0:12:11 > 0:12:15have only one way to go, and that's down.
0:12:16 > 0:12:19This is marine snow.
0:12:23 > 0:12:28Millions of tiny creatures, such as this sea spider, filter the snow.
0:12:28 > 0:12:32Its feathery appendages gathering particles that are then drawn
0:12:32 > 0:12:34through its jaws.
0:12:39 > 0:12:43As a result, the nutritional value of marine snow declines with depth.
0:12:45 > 0:12:47Feeding activity also uses up
0:12:47 > 0:12:51precious reserves of dissolved oxygen.
0:12:51 > 0:12:55Without photosynthesis to replace it, oxygen decreases.
0:12:57 > 0:13:02So, as we descend, life begins to thin out.
0:13:02 > 0:13:05But it becomes ever more extraordinary.
0:13:08 > 0:13:12At 500 metres, it appears completely dark to human eyes.
0:13:14 > 0:13:16The pressure is 50 times what it was at the surface
0:13:16 > 0:13:21and there's only the tiniest remnant of sunlight filtering through.
0:13:21 > 0:13:24Animals here have adapted to cope with extreme pressure
0:13:24 > 0:13:28and their eyes have evolved to become disproportionately large.
0:13:30 > 0:13:34The owner of THIS pair gazes ever upwards into the gloom,
0:13:34 > 0:13:39seeking the silhouette of its prey against the faint down-welling light.
0:13:43 > 0:13:47To escape the attention of super-sensitive eyes, fish here have
0:13:47 > 0:13:51evolved light-transmitting cells on the underside of their body.
0:13:53 > 0:13:57Using graphics, we can see how these exactly match the background light.
0:13:59 > 0:14:02With this, their silhouette breaks up, making them
0:14:02 > 0:14:04appear almost invisible from below.
0:14:09 > 0:14:14By 800 metres, the pressure is 80 times what it was at the surface
0:14:14 > 0:14:18and the temperature is below three degrees centigrade.
0:14:20 > 0:14:23Oxygen levels have also decreased to less than 5%
0:14:23 > 0:14:25of what they were at the surface.
0:14:25 > 0:14:28This is called the dead zone.
0:14:29 > 0:14:32But something still manages to live here.
0:14:37 > 0:14:42Vampyroteuthis infernalis - the vampire squid from hell.
0:14:44 > 0:14:49Vampire squid have lived in the depths for 200 million years.
0:14:49 > 0:14:52They share physical characteristics with octopus and squid
0:14:52 > 0:14:55and are thought to be a missing link.
0:14:57 > 0:14:59Highly specialised blood cells allow them
0:14:59 > 0:15:02to live in this low-oxygen environment.
0:15:03 > 0:15:06Despite its hellish name and fierce look,
0:15:06 > 0:15:09vampire squid are placid creatures, averaging only
0:15:09 > 0:15:1328 centimetres in length, and are completely harmless.
0:15:17 > 0:15:19Get too close to a vampire squid
0:15:19 > 0:15:22and it puts on the most amazing light display.
0:15:25 > 0:15:28Bioluminescent bacteria in its arms and on its body
0:15:28 > 0:15:31dazzle and confuse potential predators.
0:15:34 > 0:15:37And with that, it disappears into the darkness.
0:15:43 > 0:15:47Beyond 1,000 metres, we enter the dark zone, where not even
0:15:47 > 0:15:50the faintest remnant of sunlight can penetrate.
0:15:50 > 0:15:53Yet when we switch off the submersible lights,
0:15:53 > 0:15:56we see bioluminescence.
0:15:56 > 0:15:58There's life even here.
0:16:00 > 0:16:05Furthermore, oxygen levels have risen and that's because we are now
0:16:05 > 0:16:09in a deep-water current called the Great Ocean Conveyer.
0:16:12 > 0:16:15When surface water, enriched in oxygen by photosynthesis,
0:16:15 > 0:16:21meets polar ice, it sinks beyond the first 1,000 metres
0:16:21 > 0:16:23and flows towards the equator...
0:16:25 > 0:16:30..carrying oxygen to the very deepest corners of the abyss.
0:16:32 > 0:16:36Here in the dark zone, we find the real monsters of the deep.
0:16:43 > 0:16:47Oxygen may be plentiful here but food is scarce,
0:16:47 > 0:16:50and bioluminescence is the only light available.
0:16:53 > 0:16:57Most of the flashing lights we can see come from deep-water copepods.
0:16:57 > 0:17:00They provide food for other deep-sea animals.
0:17:04 > 0:17:09To a hunting squid, this flashing light looks like food.
0:17:09 > 0:17:12But in the darkness, nothing is what it seems.
0:17:16 > 0:17:20Fooled by bioluminescent bacteria living in the antennae
0:17:20 > 0:17:21of an angler fish.
0:17:23 > 0:17:25The angler fish can easily accommodate
0:17:25 > 0:17:27the squid in its extendable stomach.
0:17:30 > 0:17:33Many deep-sea fish have disproportionately large stomachs.
0:17:35 > 0:17:40In this sparse, cold world, it might be many days between meals.
0:17:44 > 0:17:48Natural selection has produced lures of all shapes and sizes,
0:17:48 > 0:17:52each used to tempt prey to within easy reach.
0:17:56 > 0:18:01This Wolftrap angler has a lure hanging amidst its formidable teeth.
0:18:03 > 0:18:06In this dazzling battlefield, prey have developed
0:18:06 > 0:18:09some surprising methods of escape.
0:18:10 > 0:18:13Deep-water shrimps confuse attackers by spinning
0:18:13 > 0:18:16and releasing a bioluminescent glue.
0:18:19 > 0:18:23While the shrimp makes its escape, the glue sticks,
0:18:23 > 0:18:26leaving the attacker glowing in the face of its own enemies.
0:18:29 > 0:18:33The shrimp's red colour is also its camouflage.
0:18:35 > 0:18:37In the dark zone, the eyes of most predators
0:18:37 > 0:18:41are tuned to the blue or green of bioluminescent light.
0:18:42 > 0:18:47So, to them, a red shrimp is almost invisible,
0:18:47 > 0:18:50but not to one deep-water resident -
0:18:50 > 0:18:53the dragonfish.
0:18:54 > 0:18:57This predator has evolved red bioluminescent headlights
0:18:57 > 0:19:01below its eyes that are sensitive to red light.
0:19:03 > 0:19:07Its target doesn't see it until it's too late.
0:19:12 > 0:19:16Descending ever further, we eventually reach the sea floor,
0:19:16 > 0:19:18six kilometres down.
0:19:21 > 0:19:25The pressure here is more than 600 times that of the surface,
0:19:25 > 0:19:28and temperatures are close to zero degrees.
0:19:31 > 0:19:35It takes many weeks for marine snow to descend this far.
0:19:37 > 0:19:41Here, it forms a vast blanket of soft sediment over a kilometre thick -
0:19:41 > 0:19:43the abyssal plain.
0:19:45 > 0:19:48These plains cover a third of the Earth's surface.
0:19:49 > 0:19:52By the time the snow gets here, it only contains
0:19:52 > 0:19:54a fraction of its original energy.
0:19:57 > 0:20:00Under such extreme conditions, we would expect the abyssal plain
0:20:00 > 0:20:02to be lifeless.
0:20:03 > 0:20:06But even here, we find life.
0:20:17 > 0:20:22Deepwater sea urchins must sift large quantities of sediment
0:20:22 > 0:20:23to survive.
0:20:23 > 0:20:27The strange balloon-like sacs on their backs may be filled
0:20:27 > 0:20:30with a noxious substance to deter predators.
0:20:30 > 0:20:35Abyssal shrimps use their elongated antennae to feel for tiny
0:20:35 > 0:20:38particles of food floating around in the darkness.
0:20:41 > 0:20:45Life here may be sparse, but because the abyssal plains cover
0:20:45 > 0:20:47such a large area of the Earth's surface,
0:20:47 > 0:20:51they are home to some of the most numerous animals on the planet.
0:20:53 > 0:20:59Fish have been found living as far down as eight kilometres.
0:20:59 > 0:21:02This rattail is one of the most common species.
0:21:03 > 0:21:05The white spots around its eyes are pores,
0:21:05 > 0:21:08sensitive to the slightest movement -
0:21:08 > 0:21:11vital in the inky blackness of the abyssal plain.
0:21:15 > 0:21:18Just occasionally, in this nutrient-poor environment,
0:21:18 > 0:21:20a feast arrives.
0:21:20 > 0:21:23A dead tuna doesn't lie unnoticed for long.
0:21:25 > 0:21:28This deep-sea conger eel has picked up the scent from far off.
0:21:31 > 0:21:34Down here, a good sense of smell is a lifesaver.
0:21:38 > 0:21:41The carcass also attracts the attention of one of the deep's
0:21:41 > 0:21:44most elusive creatures -
0:21:44 > 0:21:45the six-gilled shark.
0:21:47 > 0:21:50These rarely seen sharks are active hunters
0:21:50 > 0:21:52and can grow to five metres in length.
0:21:54 > 0:21:59This type of shark has been around for at least 200 million years.
0:22:02 > 0:22:06There are many opportunistic scavengers on the seafloor.
0:22:06 > 0:22:08They'll detect even the tiniest scent in the water
0:22:08 > 0:22:10and will move in from miles around.
0:22:12 > 0:22:16Deep-sea crabs and scavenging arrowtooth eels
0:22:16 > 0:22:18are soon joined by giant isopods.
0:22:19 > 0:22:22Related to woodlice, these strange-looking monsters
0:22:22 > 0:22:24are half a metre long.
0:22:27 > 0:22:31Within hours, the carcass is stripped to nothing.
0:22:32 > 0:22:34Even the bones are eaten.
0:22:36 > 0:22:38Where food is at a premium, nothing is wasted.
0:22:45 > 0:22:49When life was first discovered thriving in the abyss, it was
0:22:49 > 0:22:54a complete surprise, but the depths revealed another secret
0:22:54 > 0:22:57that shook the foundations of scientific thinking.
0:23:00 > 0:23:04A secret that lay here on the mid-ocean ridges.
0:23:06 > 0:23:09These undersea mountain ranges are the longest geological
0:23:09 > 0:23:15structure on Earth, part of a continuous 65,000 kilometre chain
0:23:15 > 0:23:17extending across the face of the planet.
0:23:19 > 0:23:22This is the frontline of plate tectonics.
0:23:23 > 0:23:27Here, the seafloor is parting at about two centimetres a year.
0:23:30 > 0:23:34When submersibles first visited the ridges in the 1970s,
0:23:34 > 0:23:38they found towering basalt chimneys known as black smokers
0:23:38 > 0:23:41spewing hot water and hydrogen sulphide
0:23:41 > 0:23:44from deep inside the Earth's crust.
0:23:44 > 0:23:50At 400 degrees centigrade, these smokers are hot enough to melt lead.
0:23:51 > 0:23:52It was here that they made
0:23:52 > 0:23:57one of the most important scientific discoveries of the 20th century -
0:23:57 > 0:23:59dense populations of marine animals
0:23:59 > 0:24:03living in a toxic deep-sea environment
0:24:03 > 0:24:07without any reliance on energy from the sun.
0:24:10 > 0:24:14In one location, they found swarms of deep-sea shrimps.
0:24:16 > 0:24:20In another, they found bizarre polychaete worms tolerating
0:24:20 > 0:24:23water temperatures of 80 degrees centigrade.
0:24:24 > 0:24:29No other animal on Earth is known to exist at such temperatures.
0:24:30 > 0:24:36Clams, crabs and even fish live in large numbers around the vents.
0:24:39 > 0:24:42How do all these animals survive in such densities,
0:24:42 > 0:24:44in total darkness, under conditions
0:24:44 > 0:24:47of scalding heat and intense pressure?
0:24:52 > 0:24:57The answer lies in mats of bacteria that coat the chimneys.
0:24:58 > 0:25:01These bacteria are primary producers.
0:25:02 > 0:25:07They substitute for phytoplankton in a world without sunlight.
0:25:07 > 0:25:09Drawing chemical energy from the vent,
0:25:09 > 0:25:13they convert soluble carbon into organic matter -
0:25:13 > 0:25:15a process known as chemosynthesis.
0:25:19 > 0:25:22It is this bacteria, not marine snow, that is
0:25:22 > 0:25:25the basis of the hydrothermal vent food chain.
0:25:27 > 0:25:30But in order to exploit this resource,
0:25:30 > 0:25:34they've had to adapt to living with high levels of hydrogen sulphide -
0:25:34 > 0:25:36a poison as potent as cyanide.
0:25:40 > 0:25:43These giant tube worms approach two metres long
0:25:43 > 0:25:47and they incorporate vent bacteria within their bodies.
0:25:49 > 0:25:53The bacteria provide the worms directly with food and energy.
0:25:56 > 0:25:59Despite the toxic environment,
0:25:59 > 0:26:02there's thousands of times more life at hydrothermal vents
0:26:02 > 0:26:04than at the abyssal plain.
0:26:09 > 0:26:13The sheer scale and diversity of creatures here has opened up
0:26:13 > 0:26:16a whole new branch of research.
0:26:16 > 0:26:20New species are being discovered on every single dive.
0:26:22 > 0:26:26So, here, far away from the sun's rays, is a self-sufficient
0:26:26 > 0:26:29community thriving in the most hostile of environments.
0:26:31 > 0:26:37This may be a glimpse of how life exists on other planets.
0:26:42 > 0:26:45Our journey into the deep-sea microworld has revealed
0:26:45 > 0:26:48some of the most unusual life forms on Earth.
0:26:50 > 0:26:54What we've found at every level is a diverse community of animals
0:26:54 > 0:26:56adopting unique ways of surviving.
0:26:59 > 0:27:02Many of these creatures live well beyond the sun's rays.
0:27:06 > 0:27:11But there's one thing that ties them all together wherever they dwell -
0:27:11 > 0:27:13the need for oxygen.
0:27:15 > 0:27:19Even the bacteria that form the basis of hydrothermal communities
0:27:19 > 0:27:22need oxygen to make organic matter.
0:27:23 > 0:27:28And oxygen can only be produced in one way -
0:27:28 > 0:27:31as a by-product of photosynthesis,
0:27:31 > 0:27:34a process driven by the sun.
0:27:42 > 0:27:45Even though the majority of ocean creatures remain oblivious
0:27:45 > 0:27:48to its presence, ultimately it is the sun
0:27:48 > 0:27:52that allows life to exist in the depths of the abyss.
0:27:54 > 0:27:58The sun connects all life, from the surface...
0:28:00 > 0:28:03..to the deepest reaches of the sea.
0:28:04 > 0:28:07We still know more about our neighbouring planets than
0:28:07 > 0:28:09we do the deep sea.
0:28:10 > 0:28:14This inner space remains the last frontier on Earth -
0:28:14 > 0:28:18a frontier that continues to evoke awe and wonder.
0:28:20 > 0:28:24Who knows what discoveries are yet to be made in the largest
0:28:24 > 0:28:28but least explored of all our microworlds?
0:28:53 > 0:28:56Subtitles by Red Bee Media Ltd