1 00:00:29,400 --> 00:00:32,870 Away from all land... 2 00:00:32,960 --> 00:00:34,710 the ocean. 3 00:00:35,840 --> 00:00:41,310 It covers more than half the surface of our planet and yet, for the most part, 4 00:00:41,400 --> 00:00:44,310 it is beyond our reach. 5 00:00:47,200 --> 00:00:52,910 Much of it is virtually empty, a watery desert. 6 00:00:58,840 --> 00:01:04,870 All life that is here is locked in a constant search to find food. 7 00:01:06,400 --> 00:01:11,550 A struggle to conserve precious energy in the open ocean. 8 00:01:31,360 --> 00:01:34,350 The biggest of all fish. 9 00:01:34,440 --> 00:01:38,990 30 tons in weight, 12 metres long, 10 00:01:39,080 --> 00:01:41,430 a whale shark. 11 00:01:42,320 --> 00:01:48,070 Its huge bulk is sustained by near microscopic creatures of the sea, 12 00:01:49,040 --> 00:01:50,910 plankton. 13 00:01:52,440 --> 00:01:56,750 Whale sharks cruise on regular, habitual routes 14 00:01:56,840 --> 00:01:59,790 between the best feeding grounds. 15 00:02:03,680 --> 00:02:09,870 In February, that takes them to the surface waters far from the coast of Venezuela. 16 00:02:13,280 --> 00:02:16,030 Others are already here. 17 00:02:16,120 --> 00:02:20,710 Bait fish have come for the same reason, to feed on the plankton. 18 00:02:21,720 --> 00:02:26,310 The whale shark has timed its arrival exactly right. 19 00:02:27,800 --> 00:02:31,470 Oddly, the tiny fish swarm around it. 20 00:02:39,200 --> 00:02:41,870 They're using it as a shield. 21 00:02:45,600 --> 00:02:49,230 Other predatory fish are lurking nearby. 22 00:02:52,240 --> 00:02:56,750 Yellowfin tuna, they seem wary of the giant. 23 00:03:03,480 --> 00:03:07,870 The shark dives, as if to escape from such overcrowding. 24 00:03:12,960 --> 00:03:17,870 Now the tuna have a chance to attack the unprotected bait fish. 25 00:03:29,520 --> 00:03:32,950 But then back comes the giant! 26 00:03:39,680 --> 00:03:43,630 It has taken a vast mouthful of the bait fish itself. 27 00:03:43,800 --> 00:03:49,310 Plankton, it seems, is not the only food for a whale shark. 28 00:03:59,400 --> 00:04:03,270 Both shark and tuna feast together. 29 00:04:17,960 --> 00:04:20,710 But the tuna must beware. 30 00:04:24,400 --> 00:04:28,150 Even they can end up in the whale shark's stomach. 31 00:04:36,560 --> 00:04:40,790 Predators here must grab what they can, when they can, 32 00:04:40,880 --> 00:04:43,630 for such events do not last long. 33 00:04:46,680 --> 00:04:52,710 The dense shoals on which so many depend gather only when water conditions are perfect. 34 00:05:06,640 --> 00:05:12,870 Many predators spend much of their time cruising the open ocean, endlessly searching. 35 00:05:18,080 --> 00:05:23,150 Plankton-feeding rays do so, gliding with minimum effort. 36 00:05:37,520 --> 00:05:40,190 The Oceanic whitetip shark, 37 00:05:42,600 --> 00:05:45,910 another energy-efficient traveller. 38 00:05:47,920 --> 00:05:53,470 It specialises in locating prey in the emptiest areas of the open ocean, 39 00:05:53,560 --> 00:05:56,390 patrolling the top 100 metres of water. 40 00:06:05,280 --> 00:06:10,070 Taste in water is the equivalent of smell in the air. 41 00:06:10,160 --> 00:06:14,990 An Oceanic whitetip is able to detect even the faintest trace. 42 00:06:18,720 --> 00:06:21,190 Small pilot fish swim with it. 43 00:06:21,280 --> 00:06:24,750 The shark can find prey far more easily than they can 44 00:06:24,840 --> 00:06:28,790 and they'll be able to collect the scraps from its meals. 45 00:06:33,200 --> 00:06:38,830 Its long, fixed pectoral fins enable it to soar through the water 46 00:06:38,920 --> 00:06:42,150 with the least expenditure of energy. 47 00:06:46,560 --> 00:06:51,070 This shark has found a school of rainbow runners. 48 00:06:51,160 --> 00:06:54,030 It would eat one, given the chance. 49 00:06:54,120 --> 00:06:58,430 But rainbow runners are swift and agile and not easily caught. 50 00:07:01,400 --> 00:07:04,190 So, it bides its time. 51 00:07:04,320 --> 00:07:09,750 There's a chance that, eventually, it may spot a weakened fish that's catchable. 52 00:07:13,440 --> 00:07:16,870 The hunter, endlessly waiting. 53 00:07:27,680 --> 00:07:31,230 Excitement far from land. 54 00:07:39,200 --> 00:07:42,750 A school of dolphin, 500 strong! 55 00:07:57,880 --> 00:08:02,390 They've sensed there's food around and they're racing to catch up with it. 56 00:08:10,760 --> 00:08:16,030 The news has spread. Now a number of schools are on their way. 57 00:08:28,880 --> 00:08:31,430 They're heading towards the Azores, 58 00:08:31,520 --> 00:08:35,310 volcanic islands 1, 000 miles west of Portugal. 59 00:08:45,920 --> 00:08:51,150 The dolphins scan the water ahead with their sonar. They're close to their target. 60 00:08:59,720 --> 00:09:02,950 This is it, scad mackerel. 61 00:09:06,320 --> 00:09:10,510 It's difficult for a single dolphin to catch the fish. 62 00:09:11,640 --> 00:09:15,310 To avoid wasting energy, they work as a group. 63 00:09:22,160 --> 00:09:26,310 They drive the fish upwards, trapping them against the surface. 64 00:09:28,520 --> 00:09:31,910 And there, other predators await them. 65 00:09:34,840 --> 00:09:37,830 Cory shearwaters. 66 00:09:37,920 --> 00:09:42,670 They're waiting for the dolphin to drive the prey closer to the surface. 67 00:09:52,440 --> 00:09:58,070 Now the shearwaters can dive down on them, descending to 20 metres or more. 68 00:10:02,520 --> 00:10:06,070 And the dolphins block the bait ball's retreat. 69 00:10:36,200 --> 00:10:40,270 The dolphins leave as soon as they've had their fill. 70 00:10:44,560 --> 00:10:49,910 And, at last, the mackerel sink below the diving range of the birds. 71 00:11:13,280 --> 00:11:18,470 As the sun disappears, a profound change takes place in the ocean. 72 00:11:20,080 --> 00:11:23,630 Deepwater plankton start to rise from the depths 73 00:11:23,720 --> 00:11:28,550 and another hungry army prepares to receive it. 74 00:11:52,040 --> 00:11:57,990 Every night, wherever conditions are right, countless millions of creatures from the deep 75 00:11:58,080 --> 00:12:01,630 migrate to the surface seeking food. 76 00:12:10,360 --> 00:12:16,430 A baby sailfish, 15 centimetres long, snaps up everything in its path. 77 00:12:17,480 --> 00:12:24,150 In three years' time, it will be one of the ocean's most formidable hunters, weighing 60 kilos. 78 00:12:25,280 --> 00:12:28,990 Just now, however, it's very vulnerable. 79 00:13:09,880 --> 00:13:13,030 These manta rays are giants. 80 00:13:13,640 --> 00:13:17,630 Eight metres across and weighing over two tons. 81 00:13:21,200 --> 00:13:24,270 The blade-like projections on either side of the head 82 00:13:24,360 --> 00:13:28,150 help to steer plankton into the manta's mouth. 83 00:13:47,200 --> 00:13:51,670 Dawn returns and the plankton sinks back into the depths. 84 00:13:54,120 --> 00:13:58,550 If we are to follow, we must use a submarine. 85 00:14:01,440 --> 00:14:07,590 As we descend into the darkness, the pressure builds, the temperature falls. 86 00:14:11,560 --> 00:14:15,830 Below 500 metres, new mysterious animals appear. 87 00:14:21,640 --> 00:14:27,750 Their bizarre shapes help them to remain suspended in the dark space. 88 00:14:31,000 --> 00:14:35,230 Some resemble creatures familiar from shallower waters. 89 00:14:43,440 --> 00:14:46,590 Others defy classification. 90 00:14:52,120 --> 00:14:56,990 All around, organic particles drift downwards. 91 00:14:58,720 --> 00:15:04,830 Marine snow, detritus from the creatures swarming in the sunlit waters above. 92 00:15:07,280 --> 00:15:10,350 The snow is food for many animals here, 93 00:15:10,440 --> 00:15:15,350 like the sea spider, a small relative of shrimps and crabs. 94 00:15:27,240 --> 00:15:33,590 Those strange leg-like appendages are feathered to stop it from sinking. 95 00:15:35,600 --> 00:15:42,390 They can also enmesh marine snow, which it wipes carefully into its jaws. 96 00:15:57,880 --> 00:16:02,670 A sawtooth eel hangs upright and motionless. 97 00:16:05,840 --> 00:16:07,390 Gazing ever upwards, 98 00:16:07,520 --> 00:16:13,710 it watches for prey silhouetted against the faint glimmerings of light from the surface. 99 00:16:19,640 --> 00:16:25,310 Days may pass before prey swims close enough for it to strike. 100 00:16:30,400 --> 00:16:34,030 Farther down still, the blackness is complete. 101 00:16:35,000 --> 00:16:39,830 No vestige of sunlight can penetrate as far as this. 102 00:16:43,200 --> 00:16:48,870 Food is very scarce and nothing can afford to waste any energy. 103 00:16:52,000 --> 00:16:56,190 A dumbo octopus simply flaps a fin. 104 00:16:56,280 --> 00:17:01,070 No need for the jet propulsion used by its shallow water relatives above. 105 00:17:31,560 --> 00:17:35,230 The weirdest in this world of the strange, 106 00:17:35,320 --> 00:17:40,470 Vampyroteuthis, the vampire squid from hell. 107 00:17:45,160 --> 00:17:50,190 Disturb it and it only retreats a little distance. 108 00:17:53,120 --> 00:17:57,270 Go after it and it has a special defence. 109 00:18:01,000 --> 00:18:04,590 To see what it does, you must switch off the lights. 110 00:18:07,360 --> 00:18:10,910 The vampire squid has lights of its own. 111 00:18:13,600 --> 00:18:20,070 Bioluminescent bacteria shine from pockets on its arms to confuse its predators. 112 00:18:21,120 --> 00:18:23,390 Are those eyes? 113 00:18:27,400 --> 00:18:33,070 In fact, they're spots at the end of its mantle. A bite there would leave the head unscathed. 114 00:18:35,680 --> 00:18:42,350 The threat diminishes and Vampyroteuthis disappears into the blackness. 115 00:18:57,960 --> 00:19:00,590 At last, the sea floor. 116 00:19:00,680 --> 00:19:03,350 Over two miles down, 117 00:19:03,440 --> 00:19:08,350 the pressure here is 300 times that at the surface. 118 00:19:08,440 --> 00:19:13,990 It takes several months for marine snow to drift down as far as this. 119 00:19:18,440 --> 00:19:22,390 As you travel away from the rocky margins of the continents, 120 00:19:22,480 --> 00:19:26,270 an immense plain stretches ahead. 121 00:19:28,600 --> 00:19:33,950 It extends for thousands of miles, gradually sinking downwards. 122 00:19:41,040 --> 00:19:47,390 There are faint trails in the ooze, signs that even here, there is life. 123 00:19:54,760 --> 00:19:57,710 These are what made some of them. 124 00:19:57,800 --> 00:20:02,230 Sea urchins sifting the accumulating drifts. 125 00:20:06,600 --> 00:20:13,190 Shrimps, standing on elegant tiptoe, fastidiously select the particles that appeal to them. 126 00:20:21,320 --> 00:20:28,110 But in the deep sea, as everywhere else, if there are grazers, there are hunters. 127 00:20:37,480 --> 00:20:43,110 A monkfish, almost indistinguishable from the sand on which it lies. 128 00:20:47,760 --> 00:20:54,070 Why waste energy chasing around if you can attract prey towards you with a lure? 129 00:21:12,240 --> 00:21:15,430 Maybe that one was a bit big. 130 00:21:18,040 --> 00:21:21,550 The monkfish can wait for days, if necessary, 131 00:21:21,640 --> 00:21:25,150 until the right-sized meal turns up. 132 00:21:28,280 --> 00:21:33,510 Scavengers, on the other hand, have to move around to find their food. 133 00:21:36,000 --> 00:21:40,630 Crabs can detect the faintest of tastes in the water and that helps them locate 134 00:21:40,720 --> 00:21:44,670 the latest body to drift down from above. 135 00:21:45,840 --> 00:21:49,190 Eels are already feeding on the corpse. 136 00:21:55,880 --> 00:22:00,430 Isopods, like giant marine woodlice a third of a metre long, 137 00:22:00,520 --> 00:22:03,510 are ripping into the rotting flesh. 138 00:22:04,280 --> 00:22:07,670 Over the next few hours, there'll be frenzied competition 139 00:22:07,760 --> 00:22:11,990 between scavengers of all kinds to grab a share. 140 00:22:40,560 --> 00:22:45,310 Just occasionally, there is a gigantic bonanza. 141 00:22:53,920 --> 00:22:57,350 The remains of a sperm whale. 142 00:22:57,440 --> 00:23:00,550 It died five months or so ago. 143 00:23:00,640 --> 00:23:04,990 There is little left but fatty blubber clinging to its bones. 144 00:23:08,040 --> 00:23:14,070 Its flesh has nourished life for miles around, but now, the feast is almost over. 145 00:23:19,320 --> 00:23:25,190 Spider crabs, a metre across, still pick at the last putrid remains. 146 00:23:32,880 --> 00:23:37,990 A few weeks more and nothing will be left but bare bones. 147 00:23:38,800 --> 00:23:43,830 The crabs will have to fast until the next carcass drifts down. 148 00:23:50,440 --> 00:23:54,750 But not all food comes from the sunlit world above. 149 00:23:56,040 --> 00:24:02,550 The floor of the Atlantic Ocean is split in two by an immense volcanic mountain chain 150 00:24:02,640 --> 00:24:07,510 that winds unbroken for 45, 000 miles around the globe. 151 00:24:11,640 --> 00:24:17,270 In places it's riven by great fissures from which superheated water, 152 00:24:17,360 --> 00:24:22,150 loaded with dissolved minerals, blasts into the icy depths. 153 00:24:33,200 --> 00:24:39,950 Clouds of sulphides solidify into towering chimneys as tall as a three-storey house. 154 00:24:44,480 --> 00:24:48,750 At 400 degrees, this scalding cocktail of chemicals 155 00:24:48,840 --> 00:24:53,390 would be lethally toxic to most forms of life. 156 00:24:54,440 --> 00:24:59,750 But, astoundingly, a particular kind of bacteria thrives here. 157 00:24:59,840 --> 00:25:04,510 And feeding on the bacteria, vast numbers of shrimps. 158 00:25:14,520 --> 00:25:18,030 So, beyond the farthest reach of the sun's power, 159 00:25:18,120 --> 00:25:22,630 a rich independent community exists that draws all its energy 160 00:25:22,720 --> 00:25:25,470 directly from the earth's molten core. 161 00:25:40,880 --> 00:25:45,910 On the other side of the planet, in the western Pacific bordering Japan, 162 00:25:47,000 --> 00:25:49,070 the dragon chimneys. 163 00:25:49,160 --> 00:25:52,830 Another series of hot vents erupting in the darkness. 164 00:26:00,560 --> 00:26:06,350 Here, more, but different, bacteria thrive in a similar way. 165 00:26:11,520 --> 00:26:15,830 And here, too, more crustaceans, but quite different species 166 00:26:15,920 --> 00:26:19,630 from those around the hot vents in the Atlantic. 167 00:26:25,000 --> 00:26:29,710 These are squat lobsters clad in furry armour, 168 00:26:29,800 --> 00:26:33,910 jostling with one another beside the jets of superheated water 169 00:26:34,000 --> 00:26:38,190 for the best places from which to graze on bacteria. 170 00:26:44,080 --> 00:26:49,430 These vents, too, like those in the Atlantic, are isolated oases, 171 00:26:49,520 --> 00:26:54,270 so widely separated, that each community is unique. 172 00:27:04,600 --> 00:27:10,190 Cross to the other side of the Pacific, to the deep near the Galapagos Islands, 173 00:27:10,280 --> 00:27:14,390 and there are yet other fissures venting superheated water. 174 00:27:20,200 --> 00:27:25,070 One and a half miles down, at a site known as Nine North, 175 00:27:25,200 --> 00:27:30,230 towering chimneys support a spectacular display of giant tubeworms. 176 00:27:36,960 --> 00:27:43,750 These vents give off so much energy that some of the worms reach three metres in length. 177 00:27:43,840 --> 00:27:47,830 They're the fastest-growing marine invertebrates known. 178 00:27:52,480 --> 00:27:58,430 All told, over 50 different species have so far been found living here. 179 00:28:02,760 --> 00:28:07,750 The inhabitants of these bustling communities may grow at speed, 180 00:28:07,840 --> 00:28:13,590 but their existence can also be short, for the vents do not erupt indefinitely. 181 00:28:14,400 --> 00:28:18,070 Suddenly, unpredictably, they may become inactive. 182 00:28:23,320 --> 00:28:26,990 Nine months have passed at Nine North. 183 00:28:27,080 --> 00:28:30,550 What were only recently chimneys teeming with life 184 00:28:30,640 --> 00:28:35,950 have turned into cold, sterile, mineral monuments. 185 00:28:38,760 --> 00:28:45,430 Some eddy, deep in the earth's crust, diverted the volcanic energy elsewhere 186 00:28:45,520 --> 00:28:49,790 and an entire micro-world was extinguished. 187 00:28:57,880 --> 00:29:03,110 In places, volcanoes have erupted to build great submarine mountains. 188 00:29:04,120 --> 00:29:08,510 There are thought to be around 30, 000 such volcanoes. 189 00:29:08,600 --> 00:29:13,830 Some measured from the sea floor are taller than Everest. 190 00:29:19,440 --> 00:29:24,110 Sheer cliffs soaring to drowned volcanic peaks. 191 00:29:35,920 --> 00:29:39,270 Powerful currents sweep up the mountain's flanks, 192 00:29:39,360 --> 00:29:43,750 transporting nutrients from deep water towards the summits. 193 00:29:46,720 --> 00:29:49,590 The hard rock provides excellent anchorage 194 00:29:49,680 --> 00:29:53,910 for communities of great variety and stunning colour. 195 00:29:56,120 --> 00:30:03,030 Soft corals, several metres across, collect the marine snow as it drifts past. 196 00:30:05,920 --> 00:30:09,390 Whip corals stretch out into the current. 197 00:30:12,000 --> 00:30:16,350 Giant sponges filter nourishment from the cold water. 198 00:30:30,960 --> 00:30:34,150 A richly varied community flourishes here, 199 00:30:34,280 --> 00:30:40,350 sustained by the nutrients and detritus in the icy currents that flow around the peak. 200 00:30:45,800 --> 00:30:52,510 Yet, it is all blossoming on an extinct volcano a mile below the reach of the sun. 201 00:31:17,800 --> 00:31:22,630 A nautilus. It spends its days hiding 400 metres down, 202 00:31:22,720 --> 00:31:27,270 but as night falls, it ascends up to the reefs to look for food. 203 00:31:33,120 --> 00:31:39,430 Its graceful shell contains gas-filled floatation chambers that control its depth. 204 00:31:42,520 --> 00:31:46,590 It's powered by a jet of water squirting from a siphon. 205 00:31:49,240 --> 00:31:54,630 But it travels shell-first, so it can't see exactly where it's going. 206 00:31:59,800 --> 00:32:03,270 Its nearest living relatives are squid and octopus 207 00:32:03,360 --> 00:32:07,990 which, over evolutionary time, have both lost their shells. 208 00:32:08,080 --> 00:32:13,110 And the octopus has become one of the nautilus's major predators. 209 00:32:13,200 --> 00:32:15,990 It's a master of disguise. 210 00:32:22,160 --> 00:32:24,870 The nautilus keeps well clear of them. 211 00:32:27,640 --> 00:32:32,110 Its small tentacles carry highly-developed chemical sensors 212 00:32:32,200 --> 00:32:36,230 which can detect traces of both predators and prey. 213 00:32:42,120 --> 00:32:46,110 It uses its water jet to dig in the sand. 214 00:32:51,320 --> 00:32:56,670 Because it devotes so little energy to swimming, it only needs a meal once a month. 215 00:33:02,240 --> 00:33:05,550 Got something, and just as well. 216 00:33:05,680 --> 00:33:10,430 Dawn is approaching and it has to puff its way back to deeper waters. 217 00:33:32,520 --> 00:33:37,990 Thirty miles away, shoals of squid are jetting upwards towards the surface. 218 00:33:39,760 --> 00:33:44,830 By night, they seek small fish among the plankton, but they're cautious. 219 00:33:52,240 --> 00:33:54,190 Pacific spotted dolphin. 220 00:33:58,560 --> 00:34:01,310 They're guided by their sonar. 221 00:34:05,800 --> 00:34:09,310 The dolphin, as so often, are working as a team, 222 00:34:09,400 --> 00:34:13,030 synchronising their attacks to confuse their prey. 223 00:34:31,640 --> 00:34:33,110 As dawn approaches, 224 00:34:33,200 --> 00:34:39,270 squid and fish and plankton retreat downwards to shelter in the darkness. 225 00:35:05,240 --> 00:35:11,510 Some of these isolated volcanoes rise as much as 9, 000 metres from the sea floor, 226 00:35:11,600 --> 00:35:14,030 reaching close to the surface. 227 00:35:17,840 --> 00:35:21,510 Around these peaks, invigorated by daily sunshine, 228 00:35:21,600 --> 00:35:26,150 marine life flourishes in spectacular abundance. 229 00:35:35,200 --> 00:35:40,550 Fish crowd here because the volcano forces nutrients to the surface, 230 00:35:40,640 --> 00:35:43,590 encouraging the plankton to bloom. 231 00:36:02,120 --> 00:36:05,110 An oceanic wanderer, a Mola mola, 232 00:36:05,200 --> 00:36:10,110 stops by to be cleaned by reef fish at the seamount edge. 233 00:36:18,000 --> 00:36:22,790 Butterfly fish pluck string-like parasites from its flanks. 234 00:36:29,800 --> 00:36:34,150 The huge fish lives on jellyfish over 1, 000 metres down, 235 00:36:34,240 --> 00:36:37,230 where the water is 20 degrees colder. 236 00:36:38,160 --> 00:36:40,710 So, a brush-up near the surface 237 00:36:40,800 --> 00:36:45,910 allows it to warm up before making more deep-water forays. 238 00:36:53,640 --> 00:36:58,950 The summit of this volcanic mountain rises above the surface of the sea. 239 00:37:00,280 --> 00:37:06,070 It's Ascension Island, 800 miles from any other land. 240 00:37:06,160 --> 00:37:10,750 A welcome, vital haven for long-distance travellers. 241 00:37:15,560 --> 00:37:20,070 Frigate birds spend months continuously airborne at sea, 242 00:37:20,200 --> 00:37:24,870 but at nesting time, they come to Ascension from all over the ocean. 243 00:37:29,800 --> 00:37:33,630 The island's barren slopes of volcanic ash and lava 244 00:37:33,720 --> 00:37:37,590 might seem to offer perfectly good sites for a nest, 245 00:37:39,400 --> 00:37:43,470 but the frigates choose an even more isolated site. 246 00:37:45,280 --> 00:37:50,590 Boatswainbird Island, a lonely pillar just off Ascension's coast. 247 00:37:59,280 --> 00:38:03,910 Frigates are the world's lightest bird relative to their wingspan 248 00:38:04,000 --> 00:38:07,550 and they can soar for weeks on end with minimal effort. 249 00:38:12,440 --> 00:38:17,630 They seem much more at home in the skies than in a crowded colony on land. 250 00:38:17,720 --> 00:38:19,550 But nest they must. 251 00:38:21,960 --> 00:38:26,350 They come from all over the Atlantic to this, their only colony. 252 00:38:30,080 --> 00:38:32,150 There are boobies here, too. 253 00:38:37,280 --> 00:38:42,950 To raise their young, seabirds worldwide seek such remote islands. 254 00:38:53,920 --> 00:38:57,350 Swimmers also come to Ascension to breed. 255 00:38:59,000 --> 00:39:02,630 A female green turtle approaches the coast. 256 00:39:05,760 --> 00:39:09,270 She has not eaten once in two months. 257 00:39:10,520 --> 00:39:14,630 She may have travelled 1, 000 miles from her feeding grounds. 258 00:39:14,720 --> 00:39:17,110 The greatest journey of her kind. 259 00:39:24,760 --> 00:39:30,630 Many others are here, too, resting on the sandy sea floor awaiting the darkness of night 260 00:39:30,720 --> 00:39:33,550 when it will be safer to visit the beaches. 261 00:39:39,960 --> 00:39:45,550 Eggs that were laid a few weeks ago at the start of the season are beginning to hatch. 262 00:39:58,400 --> 00:40:00,670 Most hatchings happen at night. 263 00:40:01,640 --> 00:40:05,950 Now, in the light of day, the young are extremely vulnerable. 264 00:40:12,240 --> 00:40:15,190 They must get to the sea as soon as possible. 265 00:40:17,680 --> 00:40:20,710 But their trials have only just begun. 266 00:40:33,640 --> 00:40:37,590 Many will drown in the pounding waves. 267 00:41:01,880 --> 00:41:07,390 During the next 20 years, the vast majority will inevitably die. 268 00:41:07,480 --> 00:41:11,830 But those that survive will, eventually, as their mothers did before them, 269 00:41:11,920 --> 00:41:15,350 return to the very same beach where they were hatched. 270 00:41:18,080 --> 00:41:22,470 How they find their way back, across thousands of miles of open ocean, 271 00:41:22,560 --> 00:41:24,830 we still have no idea. 272 00:41:33,400 --> 00:41:35,630 A frigate soars. 273 00:41:37,160 --> 00:41:42,310 Somewhere beneath the surface below there is the food it must have. 274 00:41:43,360 --> 00:41:44,990 But where? 275 00:41:46,920 --> 00:41:52,510 Those that fly above the ocean must be able to read the signs of fresh supplies 276 00:41:52,600 --> 00:41:54,150 or perish. 277 00:41:57,120 --> 00:42:02,270 A hundred miles from the Mexican coast and keen eyes have spotted movement. 278 00:42:07,520 --> 00:42:12,630 Sailfish, three metres long, are closing in on prey. 279 00:42:18,480 --> 00:42:23,390 They will only use just enough energy to make their kill, 280 00:42:23,480 --> 00:42:25,630 never wasting a fin stroke. 281 00:42:30,520 --> 00:42:34,910 Nearly a hundred sailfish have surrounded a single school of bait fish. 282 00:42:35,000 --> 00:42:39,310 It's very rare to see so many of these hunters in one place. 283 00:42:42,480 --> 00:42:47,270 To herd their prey, the predators raise their huge dorsal fins. 284 00:43:05,040 --> 00:43:09,470 A mistimed strike by one sailfish could fatally damage another. 285 00:43:09,560 --> 00:43:14,870 But each continually changes its colour from blue, to striped, to black. 286 00:43:15,000 --> 00:43:20,270 That warns its companions of its intentions and also confuses the prey. 287 00:43:23,360 --> 00:43:28,550 As the shoal is driven nearer the surface, it comes within the range of the seabirds. 288 00:43:43,480 --> 00:43:48,630 Out here in the open ocean, there is nowhere for the bait fish to hide. 289 00:44:31,040 --> 00:44:34,430 Sailfish live a high-octane life. 290 00:44:35,160 --> 00:44:39,030 To survive, they must find prey daily. 291 00:44:39,120 --> 00:44:43,190 So their entire existence will be spent on the move. 292 00:44:54,640 --> 00:45:00,950 Over 90% of the living space for life on our planet is in the oceans. 293 00:45:02,280 --> 00:45:07,870 Home to the biggest animal that exists or has ever existed. 294 00:45:10,040 --> 00:45:12,110 The blue whale. 295 00:45:19,720 --> 00:45:26,630 Some weigh nearly 200 tons, twice the size of the largest dinosaur. 296 00:45:31,560 --> 00:45:33,510 Despite their great size, 297 00:45:33,600 --> 00:45:38,510 we still have little idea of where they travel in the vast oceans 298 00:45:38,600 --> 00:45:41,870 and none at all of where they go to breed. 299 00:46:11,120 --> 00:46:15,630 The largest animal on Earth feeds almost exclusively 300 00:46:15,720 --> 00:46:20,790 on one of the smallest, krill, shrimp-like crustaceans. 301 00:46:22,920 --> 00:46:28,550 They take many tons of water into their ballooning throats in a single gulp 302 00:46:28,640 --> 00:46:31,830 and sieve out what it contains. 303 00:46:38,320 --> 00:46:44,110 Every day, each one swallows some four million krill. 304 00:46:47,080 --> 00:46:53,230 Such gargantuan harvests depend on the continuing fertility of the oceans. 305 00:46:56,160 --> 00:47:00,710 But global changes now threaten the great blooms of plankton 306 00:47:00,800 --> 00:47:02,870 on which the whales depend. 307 00:47:05,520 --> 00:47:08,590 Once, and not so long ago, 308 00:47:08,680 --> 00:47:12,510 300,000 blue whales roamed the oceans. 309 00:47:13,480 --> 00:47:17,350 Now, less than 3% of that number remains. 310 00:47:22,160 --> 00:47:25,950 Our planet is still full of wonders. 311 00:47:26,080 --> 00:47:32,070 As we explore them, so we gain not only understanding, but power. 312 00:47:34,200 --> 00:47:39,110 It's not just the future of the whale that today lies in our hands. 313 00:47:39,200 --> 00:47:44,830 It's the survival of the natural world in all parts of the living planet. 314 00:47:46,480 --> 00:47:49,950 We can now destroy or we can cherish. 315 00:47:53,480 --> 00:47:56,630 The choice is ours.