1 00:00:35,900 --> 00:00:38,640 This is the entrance 2 00:00:38,640 --> 00:00:42,000 to one of the greatest of natural wonders. 3 00:00:59,000 --> 00:01:03,000 Hang Son Doong, in Vietnam. 4 00:01:09,400 --> 00:01:13,080 It's thought to be the largest cave on Earth. 5 00:01:23,200 --> 00:01:26,520 Here, 200 metres underground, 6 00:01:26,520 --> 00:01:29,600 for just a few weeks a year, 7 00:01:29,600 --> 00:01:34,840 sunbeams cast light through a hole in the cave's roof. 8 00:01:52,280 --> 00:01:54,440 There is just enough light 9 00:01:54,440 --> 00:01:57,120 to enable plants to grow. 10 00:01:59,760 --> 00:02:01,760 But travel deeper, 11 00:02:01,760 --> 00:02:03,960 and darkness returns. 12 00:02:07,040 --> 00:02:09,440 And you reach the limit 13 00:02:09,440 --> 00:02:11,560 of where life can exist. 14 00:02:32,120 --> 00:02:35,320 These most extreme places on our planet 15 00:02:35,320 --> 00:02:39,160 are often hard for us to reach. 16 00:02:41,080 --> 00:02:45,160 Indeed, this cave was only recently discovered. 17 00:02:48,240 --> 00:02:52,520 And it contains many surprises. 18 00:02:59,040 --> 00:03:01,480 Cave pearls. 19 00:03:01,480 --> 00:03:03,720 Once tiny sand grains, 20 00:03:03,720 --> 00:03:06,800 now spheres the size of tennis balls. 21 00:03:16,560 --> 00:03:19,480 And giant stalagmites... 22 00:03:20,920 --> 00:03:22,880 ..formed over centuries 23 00:03:22,880 --> 00:03:26,080 by billions of individual drops of water. 24 00:03:34,080 --> 00:03:36,400 Underground rivers have carved 25 00:03:36,400 --> 00:03:39,560 this great passage through the limestone. 26 00:03:56,000 --> 00:03:58,600 Over five miles long 27 00:03:58,600 --> 00:04:01,000 and wide enough in places 28 00:04:01,000 --> 00:04:03,480 to accommodate a jumbo jet. 29 00:04:15,480 --> 00:04:17,440 And it is here, 30 00:04:17,440 --> 00:04:20,320 in the cave's furthest chamber, 31 00:04:20,320 --> 00:04:22,680 that life can be found. 32 00:04:29,760 --> 00:04:32,360 Translucent cave fish... 33 00:04:34,200 --> 00:04:35,840 ..and shrimps. 34 00:04:37,640 --> 00:04:40,160 Isolated for millennia, 35 00:04:40,160 --> 00:04:44,800 feeding on little more than the nutrients from the jungle above. 36 00:04:51,680 --> 00:04:55,040 The fish are completely blind 37 00:04:55,040 --> 00:04:58,920 and find their way about by touch alone. 38 00:05:06,640 --> 00:05:10,640 These underground worlds are at the very limit 39 00:05:10,640 --> 00:05:12,600 of where life can exist. 40 00:05:21,440 --> 00:05:23,640 But above ground, 41 00:05:23,640 --> 00:05:26,040 our planet's most extreme conditions 42 00:05:26,040 --> 00:05:28,520 are found at the poles. 43 00:05:28,520 --> 00:05:30,640 WIND WHISTLES 44 00:05:33,800 --> 00:05:35,880 Here in the north, 45 00:05:35,880 --> 00:05:40,040 winter temperatures can drop to -50 degrees Celsius. 46 00:05:43,320 --> 00:05:45,680 For four months of the year, 47 00:05:45,680 --> 00:05:48,440 there is barely a glimmer of sunlight. 48 00:05:54,760 --> 00:05:57,160 But here, too, 49 00:05:57,160 --> 00:05:59,160 life finds a way to survive. 50 00:06:03,720 --> 00:06:07,000 A pack of Arctic wolves. 51 00:06:12,480 --> 00:06:15,440 Their dense fur has protected them 52 00:06:15,440 --> 00:06:18,200 through months of bitter cold. 53 00:06:24,560 --> 00:06:28,280 But now the sun is returning. 54 00:06:40,920 --> 00:06:44,200 This is Ellesmere Island. 55 00:06:51,080 --> 00:06:56,480 The short summer gives the wolves a chance to rebuild their strength. 56 00:07:10,560 --> 00:07:12,160 But here in the Arctic... 57 00:07:13,800 --> 00:07:16,440 ..life is never easy. 58 00:07:28,480 --> 00:07:32,360 This pack is led by an experienced female. 59 00:07:40,880 --> 00:07:43,720 If she can't lead them to food, 60 00:07:43,720 --> 00:07:47,000 the coming winter could be their last. 61 00:07:58,400 --> 00:08:01,360 Prey is spread over such a vast area 62 00:08:01,360 --> 00:08:04,520 that they must be constantly on the move... 63 00:08:08,760 --> 00:08:10,600 ..in search of it. 64 00:08:24,760 --> 00:08:28,240 But finding prey is only the beginning. 65 00:08:37,560 --> 00:08:40,160 Musk oxen. 66 00:08:40,160 --> 00:08:43,640 They are five times the size of a wolf. 67 00:08:58,760 --> 00:09:01,560 OXEN SNORT 68 00:09:05,720 --> 00:09:10,880 The conflict between wolf and oxen is an ancient one, 69 00:09:10,880 --> 00:09:14,040 and has been waged since the Ice Age. 70 00:09:18,960 --> 00:09:21,480 OXEN GRUNT 71 00:09:24,560 --> 00:09:28,480 Each side knows the other's tactics. 72 00:09:43,120 --> 00:09:45,960 The wolves try to isolate 73 00:09:45,960 --> 00:09:48,320 the weak or the young. 74 00:09:53,680 --> 00:09:56,360 A calf stumbles... 75 00:09:56,360 --> 00:09:59,960 ..and the pack leader immediately closes in. 76 00:10:05,680 --> 00:10:07,040 WOLF YELPS 77 00:10:11,560 --> 00:10:14,080 But the herd regroups, 78 00:10:14,080 --> 00:10:16,240 horns at the ready. 79 00:10:20,200 --> 00:10:22,600 OXEN GRUNT 80 00:10:25,400 --> 00:10:27,280 Stalemate. 81 00:10:28,800 --> 00:10:31,720 But it can only be brief. 82 00:10:55,920 --> 00:10:59,760 This time, the herd race to higher ground. 83 00:11:12,600 --> 00:11:15,480 As they reach the top of the ridge, 84 00:11:15,480 --> 00:11:18,360 the males turn to defend themselves. 85 00:11:18,360 --> 00:11:20,160 OXEN GROWL 86 00:11:21,720 --> 00:11:24,080 Stalemate... 87 00:11:24,080 --> 00:11:25,360 ..again. 88 00:11:53,640 --> 00:11:56,480 Every unsuccessful hunt 89 00:11:56,480 --> 00:11:59,240 leaves the pack weakened. 90 00:12:14,240 --> 00:12:17,400 This could be an easier meal. 91 00:12:19,920 --> 00:12:21,480 WOLF SNIFFS 92 00:12:21,480 --> 00:12:24,000 But something is not right. 93 00:12:27,440 --> 00:12:31,400 This musk ox has died from a disease. 94 00:12:31,400 --> 00:12:33,200 WOLVES SNIFF 95 00:12:33,200 --> 00:12:37,040 Outbreaks like this are becoming increasingly common 96 00:12:37,040 --> 00:12:40,280 as the climate warms across the Arctic. 97 00:12:46,760 --> 00:12:48,960 Despite their hunger, 98 00:12:48,960 --> 00:12:51,880 the pack don't eat this carcass. 99 00:13:15,760 --> 00:13:18,600 Another herd. 100 00:13:18,600 --> 00:13:21,000 Another chance for the pack. 101 00:13:39,120 --> 00:13:42,800 This time, the wolves manage to split the herd. 102 00:13:44,400 --> 00:13:47,080 That may improve their chances. 103 00:14:05,000 --> 00:14:07,960 The lead female makes sure that the oxen 104 00:14:07,960 --> 00:14:10,480 do not reach the ridge again. 105 00:14:17,160 --> 00:14:21,760 With one less defender, the calf is more vulnerable. 106 00:14:30,400 --> 00:14:32,800 OXEN CALF CALLS 107 00:14:38,760 --> 00:14:41,560 A much-needed meal. 108 00:14:53,960 --> 00:14:56,880 The wolves must continue to make the most 109 00:14:56,880 --> 00:14:59,520 of the remaining weeks of summer... 110 00:15:02,800 --> 00:15:06,720 ..so when winter returns 111 00:15:06,720 --> 00:15:09,600 ..they're ready to endure 112 00:15:09,600 --> 00:15:12,000 some of the harshest conditions 113 00:15:12,000 --> 00:15:15,480 that exist on planet Earth. 114 00:15:37,440 --> 00:15:40,680 Such inhospitable extremes 115 00:15:40,680 --> 00:15:43,360 are not only found at the poles. 116 00:15:50,080 --> 00:15:52,960 As altitude increases... 117 00:15:52,960 --> 00:15:55,160 ..temperatures fall. 118 00:16:01,200 --> 00:16:06,280 Here, too, animals must make the most of the seasonal change. 119 00:16:14,040 --> 00:16:17,720 Particularly if you're ready to mate. 120 00:16:26,280 --> 00:16:28,960 A male common frog 121 00:16:28,960 --> 00:16:31,560 in the French Alps. 122 00:16:38,040 --> 00:16:43,480 He has spent all winter hibernating under the ice. 123 00:16:46,800 --> 00:16:49,800 But now it's time to wake up. 124 00:16:51,760 --> 00:16:56,160 Being cold-blooded, he needs the sun to warm his body. 125 00:16:58,320 --> 00:17:01,440 But once it sets in a few hours' time, 126 00:17:01,440 --> 00:17:03,960 he could freeze to death. 127 00:17:05,960 --> 00:17:09,800 So finding a mate is urgent. 128 00:17:14,680 --> 00:17:16,960 The weather is good. 129 00:17:20,560 --> 00:17:23,360 And he starts to sledge. 130 00:17:26,200 --> 00:17:30,400 But this is not a one-frog race. 131 00:17:31,920 --> 00:17:35,120 Dozens are rushing to the breeding pool. 132 00:17:39,880 --> 00:17:43,080 This many frogs out on the slopes... 133 00:17:43,080 --> 00:17:45,280 ..doesn't go unnoticed. 134 00:17:49,160 --> 00:17:51,440 You can't afford any slips. 135 00:17:59,840 --> 00:18:02,320 It was all going so well. 136 00:18:16,000 --> 00:18:17,880 Made it. 137 00:18:17,880 --> 00:18:19,760 FROG CROAKS 138 00:18:22,400 --> 00:18:25,400 But all the females have already got partners. 139 00:18:28,440 --> 00:18:31,520 There's only one thing for it. 140 00:18:34,840 --> 00:18:36,440 Wrestling. 141 00:18:52,800 --> 00:18:54,720 Not this time. 142 00:19:02,120 --> 00:19:04,040 Another latecomer. 143 00:19:05,600 --> 00:19:07,480 A female. 144 00:19:10,840 --> 00:19:12,880 Could she be the one? 145 00:19:23,080 --> 00:19:26,800 Sometimes being late can pay off. 146 00:19:50,560 --> 00:19:55,080 After all his efforts, the male is not going to let this chance 147 00:19:55,080 --> 00:19:57,480 slip through his fingers. 148 00:20:16,840 --> 00:20:22,040 This is the cedar forest of the Atlas Mountains, in North Africa. 149 00:20:28,040 --> 00:20:32,320 These Barbary macaques enjoyed a summer of plenty, 150 00:20:32,320 --> 00:20:34,360 but now they're paying the price. 151 00:20:38,600 --> 00:20:42,240 Winter here can be brutal. 152 00:20:48,440 --> 00:20:51,840 In some years, over half the group perish. 153 00:20:56,880 --> 00:20:58,920 But in this macaque society, 154 00:20:58,920 --> 00:21:02,160 the more social the monkey, 155 00:21:02,160 --> 00:21:05,000 the more likely it is to survive. 156 00:21:07,320 --> 00:21:11,960 Keeping warm by huddling can make all the difference. 157 00:21:16,200 --> 00:21:19,040 But they can't huddle all day. 158 00:21:22,880 --> 00:21:26,680 They must, eventually, break apart to feed. 159 00:21:38,120 --> 00:21:41,720 They spread out, searching for food. 160 00:22:00,360 --> 00:22:06,200 But each night, the group return to huddle in the cedar trees. 161 00:22:19,600 --> 00:22:22,080 No-one wants to be left out. 162 00:22:24,200 --> 00:22:27,080 Especially the infants. 163 00:22:28,920 --> 00:22:31,000 MACAQUE SQUEAKS 164 00:22:37,320 --> 00:22:41,320 This baby, somehow, has got separated. 165 00:22:57,080 --> 00:23:00,280 He must get back to the life-saving warmth of the huddle, 166 00:23:00,280 --> 00:23:02,400 and soon. 167 00:23:28,760 --> 00:23:31,520 But there is an obstacle. 168 00:23:36,040 --> 00:23:41,000 A grumpy male is giving the baby a rather cold shoulder. 169 00:23:44,480 --> 00:23:46,480 ADULT MACAQUE GRUNTS 170 00:23:48,000 --> 00:23:50,040 INFANT MACAQUE CHITTERS 171 00:23:52,000 --> 00:23:56,080 Maybe a more subtle approach. 172 00:24:03,760 --> 00:24:07,800 The male decides to leave. 173 00:24:22,760 --> 00:24:25,280 At last... 174 00:24:25,280 --> 00:24:28,360 ..an enthusiastic reception. 175 00:24:37,520 --> 00:24:40,360 The male has had less luck. 176 00:24:41,560 --> 00:24:44,440 It could be a long night. 177 00:24:49,600 --> 00:24:54,040 Huddling might seem a simple expression of affection, 178 00:24:54,040 --> 00:24:57,600 but among these macaques it's essential for survival. 179 00:25:01,280 --> 00:25:03,440 And not just here. 180 00:25:08,280 --> 00:25:12,520 5,000 miles away, in the forests of Mexico, 181 00:25:12,520 --> 00:25:15,480 monarch butterflies, in millions, 182 00:25:15,480 --> 00:25:18,840 also huddle for survival. 183 00:25:23,440 --> 00:25:26,760 All of them have travelled across North America 184 00:25:26,760 --> 00:25:30,320 to escape the winter that is coming there. 185 00:25:33,280 --> 00:25:37,080 This forest, at an altitude of 3,000 metres, 186 00:25:37,080 --> 00:25:40,200 might seem to offer little comfort... 187 00:25:42,280 --> 00:25:46,280 ..for temperatures here can drop below freezing, 188 00:25:46,280 --> 00:25:49,760 and that could kill the butterflies. 189 00:25:54,720 --> 00:25:57,920 But in fact, a thin layer of warm air 190 00:25:57,920 --> 00:26:01,440 trapped by the tree canopy protects them, 191 00:26:01,440 --> 00:26:04,120 and will do so throughout the winter... 192 00:26:05,920 --> 00:26:10,280 ..provided that the forest remains undisturbed. 193 00:26:16,520 --> 00:26:19,520 Unpredictable extremes of weather, however, 194 00:26:19,520 --> 00:26:23,320 can disrupt this delicate balance. 195 00:26:26,760 --> 00:26:31,520 Strong winds can push deep into the forest, chilling the air. 196 00:26:32,920 --> 00:26:35,880 WIND GUSTS 197 00:26:37,400 --> 00:26:39,920 Hundreds of thousands of butterflies 198 00:26:39,920 --> 00:26:42,480 may be blown from the branches. 199 00:27:07,880 --> 00:27:14,560 In some years, storms kill 75% of hibernating monarchs. 200 00:27:31,120 --> 00:27:33,480 This one has survived... 201 00:27:33,480 --> 00:27:36,640 ..but on the ground. 202 00:27:39,920 --> 00:27:42,640 And night-time frosts are fatal. 203 00:27:44,080 --> 00:27:47,880 It can't stay here and live. 204 00:27:52,360 --> 00:27:55,560 It is too cold to fly, 205 00:27:55,560 --> 00:27:59,000 but it must get back into the tree branches somehow. 206 00:28:02,000 --> 00:28:04,560 So it starts to climb. 207 00:28:13,200 --> 00:28:18,080 It's the equivalent of a human climbing up a skyscraper. 208 00:28:49,160 --> 00:28:52,280 Safety, at last. 209 00:28:57,640 --> 00:29:00,800 As spring returns, 210 00:29:00,800 --> 00:29:04,480 the monarchs wake from their long slumber. 211 00:29:20,760 --> 00:29:26,360 And now starts one of the Earth's greatest natural spectacles. 212 00:29:58,360 --> 00:30:02,320 But this mass awakening is under threat. 213 00:30:04,560 --> 00:30:09,280 The changing climate is creating more extreme weather, 214 00:30:09,280 --> 00:30:12,280 and storms are becoming more frequent. 215 00:30:14,720 --> 00:30:19,560 But, for now, this miracle of nature continues. 216 00:30:29,560 --> 00:30:31,000 Fire. 217 00:30:34,920 --> 00:30:38,280 Unpredictable and destructive. 218 00:30:45,600 --> 00:30:49,720 This is South Africa's fynbos. 219 00:30:49,720 --> 00:30:53,320 Botanically one of the most diverse habitats on Earth... 220 00:30:54,600 --> 00:30:57,720 ..with over 8,000 different plant species. 221 00:31:00,760 --> 00:31:03,080 Yet, every two decades, 222 00:31:03,080 --> 00:31:06,240 fire reduces the vegetation to ashes. 223 00:31:13,960 --> 00:31:16,080 This may seem catastrophic. 224 00:31:17,360 --> 00:31:21,880 But just days later, plants burst into life... 225 00:31:21,880 --> 00:31:24,480 ..and transform the landscape. 226 00:31:28,400 --> 00:31:32,720 Seeds and bulbs have been waiting for this event. 227 00:31:40,320 --> 00:31:44,560 90 million years of evolution alongside fire 228 00:31:44,560 --> 00:31:48,040 has made some plants so closely adapted 229 00:31:48,040 --> 00:31:52,240 that they can't germinate without the presence of smoke. 230 00:31:59,520 --> 00:32:05,000 Remarkably, around 50% of planet Earth's terrestrial habitats 231 00:32:05,000 --> 00:32:08,360 depend on fire to remain healthy. 232 00:32:15,280 --> 00:32:19,920 In Northern Australia, these savannahs need seasonal fires 233 00:32:19,920 --> 00:32:24,160 to keep them open and prevent the spread of the nearby forest. 234 00:32:28,200 --> 00:32:31,280 And they are a vital habitat 235 00:32:31,280 --> 00:32:33,960 for this colourful resident. 236 00:32:39,640 --> 00:32:43,320 The rare golden-shouldered parrot. 237 00:32:45,600 --> 00:32:48,920 He and his lifelong partner.. 238 00:32:48,920 --> 00:32:53,760 rely on grassland for both food... 239 00:32:53,760 --> 00:32:56,080 ..and a nest site. 240 00:32:57,680 --> 00:33:02,280 As usual, they've laid their eggs in a hole in a termite mound. 241 00:33:02,280 --> 00:33:04,200 CHIRPING 242 00:33:05,880 --> 00:33:09,320 And now there are four chicks. 243 00:33:11,960 --> 00:33:16,560 The thick mud walls don't only protect chicks from predators, 244 00:33:16,560 --> 00:33:20,760 but insulate them from the heat of the scorching sun. 245 00:33:27,240 --> 00:33:30,440 With fledging just days away, 246 00:33:30,440 --> 00:33:33,000 the parents are working overtime. 247 00:33:35,840 --> 00:33:39,440 They collect thousands of seeds every day. 248 00:34:01,480 --> 00:34:04,160 But they have a problem. 249 00:34:12,240 --> 00:34:16,280 The fire season is becoming more irregular. 250 00:34:34,440 --> 00:34:37,560 If they are not ready to fly, 251 00:34:37,560 --> 00:34:40,520 chicks have no escape. 252 00:35:04,400 --> 00:35:09,080 But choosing to nest in a termite mound has a surprise benefit. 253 00:35:10,920 --> 00:35:14,720 Their home is fireproof. 254 00:35:25,480 --> 00:35:31,480 Concrete-like walls keep chicks safe from the flames outside. 255 00:35:31,480 --> 00:35:33,920 CHICKS CHIRP 256 00:35:40,520 --> 00:35:44,200 The seeds of fire grass are now easier for the parrots 257 00:35:44,200 --> 00:35:47,760 to find on the blackened earth... 258 00:35:47,760 --> 00:35:50,880 ..and they are the parrots' favourite food. 259 00:35:59,160 --> 00:36:03,840 So, for both adults and their fledgling chicks... 260 00:36:05,040 --> 00:36:08,320 ..fires are essential for survival. 261 00:36:15,840 --> 00:36:18,880 But their grassland home is shrinking, 262 00:36:18,880 --> 00:36:23,480 and only about 1,000 of these lovely parrots still survive. 263 00:36:25,480 --> 00:36:29,880 It's through the active management of fire by humans 264 00:36:29,880 --> 00:36:33,520 that their world now exists at all. 265 00:36:39,080 --> 00:36:43,760 Humans are affecting habitats across planet Earth. 266 00:36:43,760 --> 00:36:46,160 For wildlife, all too often, 267 00:36:46,160 --> 00:36:49,400 the consequences have been disastrous. 268 00:36:52,560 --> 00:36:55,840 ELEPHANT CALLS 269 00:36:58,120 --> 00:37:01,960 Elephants are among the most tolerant of animals. 270 00:37:01,960 --> 00:37:04,960 ELEPHANT RUMBLES 271 00:37:04,960 --> 00:37:07,640 They are very long-lived, 272 00:37:07,640 --> 00:37:12,240 and knowledge passed between generations enables them 273 00:37:12,240 --> 00:37:15,520 to find food and little-used sources of water in times 274 00:37:15,520 --> 00:37:18,160 of great environmental stress. 275 00:37:20,280 --> 00:37:22,680 But even among elephants, 276 00:37:22,680 --> 00:37:25,840 survival strategies are now beginning to fail. 277 00:37:33,000 --> 00:37:37,240 In Amboseli, Kenya, years of extreme weather, 278 00:37:37,240 --> 00:37:40,840 combined with overgrazing by domestic cattle, 279 00:37:40,840 --> 00:37:45,000 are turning these grasslands to dust. 280 00:38:13,640 --> 00:38:16,960 This matriarch is over 40 years old... 281 00:38:16,960 --> 00:38:18,960 ELEPHANT RUMBLES 282 00:38:18,960 --> 00:38:20,880 ..and she's already survived 283 00:38:20,880 --> 00:38:23,840 some of East Africa's worst droughts. 284 00:38:27,040 --> 00:38:31,400 But this year, food is in extremely short supply... 285 00:38:34,400 --> 00:38:37,520 ..and her family are struggling to find enough. 286 00:38:45,080 --> 00:38:49,120 Her youngest son is malnourished and weak... 287 00:38:54,320 --> 00:38:57,680 ..and is lagging behind his mother and brother. 288 00:39:06,560 --> 00:39:11,280 She leads them to a pocket of acacia forest that she knows well. 289 00:39:17,480 --> 00:39:20,800 But broken branches and fallen trees 290 00:39:20,800 --> 00:39:24,240 show that other elephants have already been here. 291 00:39:29,160 --> 00:39:31,840 There is very little left worth eating. 292 00:39:39,000 --> 00:39:43,720 Without food, his mother cannot produce enough milk. 293 00:39:48,040 --> 00:39:49,800 She has no choice... 294 00:39:52,800 --> 00:39:55,360 ..but to leave her weakening calf... 295 00:39:57,000 --> 00:40:00,080 ..and try to find food elsewhere. 296 00:40:06,400 --> 00:40:12,480 So, she and her elder son head off... 297 00:40:12,480 --> 00:40:14,760 ..without him. 298 00:40:38,080 --> 00:40:41,600 He can only hope for their return. 299 00:41:06,480 --> 00:41:09,160 FLIES BUZZ 300 00:41:15,560 --> 00:41:20,160 But for him, time has run out. 301 00:41:55,320 --> 00:41:58,600 There is nothing his mother, or anybody, 302 00:41:58,600 --> 00:42:01,360 could have done to save him. 303 00:42:01,360 --> 00:42:04,520 LOW RUMBLING 304 00:42:12,960 --> 00:42:16,120 With her elder son still alive, 305 00:42:16,120 --> 00:42:18,840 there is hope for her family. 306 00:42:20,680 --> 00:42:24,920 But only if they can adapt quickly. 307 00:42:35,280 --> 00:42:38,880 Some families are proving to be resilient. 308 00:42:44,000 --> 00:42:47,720 They're travelling long distances, away from Amboseli... 309 00:42:51,000 --> 00:42:55,240 ..and finding new sources of food and water. 310 00:43:12,680 --> 00:43:15,720 And despite the drought... 311 00:43:19,520 --> 00:43:22,520 ..calves are being born. 312 00:43:41,320 --> 00:43:44,360 But given the pace of change 313 00:43:44,360 --> 00:43:46,720 and the challenges they face... 314 00:43:48,120 --> 00:43:52,160 ..how many of Africa's elephants will be able to adapt? 315 00:43:53,280 --> 00:43:56,640 And can they do so fast enough? 316 00:44:11,040 --> 00:44:13,280 The Gobi Desert. 317 00:44:15,840 --> 00:44:19,960 A land of sand and snow. 318 00:44:24,480 --> 00:44:28,440 For at least a million years, this has been 319 00:44:28,440 --> 00:44:32,360 one of the most extreme habitats on Earth. 320 00:44:36,840 --> 00:44:39,680 Yet, footprints in the sand 321 00:44:39,680 --> 00:44:43,160 reveal that there is life here. 322 00:44:49,680 --> 00:44:51,600 A snow leopard. 323 00:44:51,600 --> 00:44:54,160 Seldom seen... 324 00:44:55,560 --> 00:45:00,560 ..especially in such hot, arid conditions. 325 00:45:04,680 --> 00:45:08,960 They live up to 2,000 metres above sea level, 326 00:45:08,960 --> 00:45:12,960 and patrol vast territories. 327 00:45:41,440 --> 00:45:44,400 They are rarely seen together. 328 00:45:52,200 --> 00:45:53,600 A mother... 329 00:45:56,840 --> 00:45:59,160 ..with cubs. 330 00:46:01,360 --> 00:46:04,240 Filmed here for the first time 331 00:46:04,240 --> 00:46:07,040 in such intimate detail. 332 00:46:09,840 --> 00:46:11,920 LEOPARDS SNARL 333 00:46:15,120 --> 00:46:18,440 And they give us a glimpse of their family life... 334 00:46:19,520 --> 00:46:21,480 ..such as it is. 335 00:46:34,080 --> 00:46:36,520 WATER LAPPING 336 00:46:38,080 --> 00:46:41,440 Snow leopard cubs stay with their mothers 337 00:46:41,440 --> 00:46:45,240 longer than those of almost any other big cat. 338 00:46:51,080 --> 00:46:55,040 They remain together for over two years... 339 00:46:58,040 --> 00:47:02,720 ..as they learn how to master the extremes. 340 00:47:10,200 --> 00:47:14,560 Which they do so well that they are able to thrive 341 00:47:14,560 --> 00:47:18,520 in some of the hottest, coldest... 342 00:47:18,520 --> 00:47:21,360 ..and highest places on Earth. 343 00:47:32,760 --> 00:47:36,680 Some animals, over millions of years, 344 00:47:36,680 --> 00:47:40,760 have evolved ways of surviving extremely hostile conditions. 345 00:47:43,800 --> 00:47:48,120 But as the extremes become more extreme... 346 00:47:48,120 --> 00:47:52,320 ..evolution is being outpaced. 347 00:48:00,280 --> 00:48:04,600 These extraordinary places are still inhabited 348 00:48:04,600 --> 00:48:07,040 by some of Earth's most resilient 349 00:48:07,040 --> 00:48:09,200 and adaptable species. 350 00:48:12,400 --> 00:48:14,680 But in our rapidly changing world... 351 00:48:16,240 --> 00:48:21,160 ..this is planet Earth as you may never see it again. 352 00:48:27,520 --> 00:48:29,360 LEOPARD YOWLS 353 00:48:44,840 --> 00:48:47,760 DAVID ATTENBOROUGH: The most extreme parts of our planet 354 00:48:47,760 --> 00:48:50,320 are not always found where you might expect. 355 00:48:53,320 --> 00:48:57,320 Hidden deep in the forests of Vietnam, a local man, 356 00:48:57,320 --> 00:49:00,600 Mr Ho Khanh, made an extraordinary discovery. 357 00:49:20,160 --> 00:49:22,240 He didn't know it at the time, 358 00:49:22,240 --> 00:49:26,600 but he had stumbled upon arguably the largest cave in the world. 359 00:49:38,800 --> 00:49:42,720 Inspired by Mr Ho Khanh's discovery, 360 00:49:42,720 --> 00:49:46,760 the Planet Earth team set out to film Hang Son Doong. 361 00:49:48,800 --> 00:49:50,960 But first, they have to get there. 362 00:49:52,360 --> 00:49:54,680 We've left the river valley, 363 00:49:54,680 --> 00:49:57,080 and we're walking uphill now to the entrance. 364 00:49:57,080 --> 00:49:59,080 It's just thick, thick forest. 365 00:50:00,880 --> 00:50:04,080 After trekking for two gruelling days, 366 00:50:04,080 --> 00:50:06,960 the team finally arrive. 367 00:50:06,960 --> 00:50:10,320 Down here is the entrance to Hang Son Doong, 368 00:50:10,320 --> 00:50:13,880 this tiny opening in this massive jungle. 369 00:50:13,880 --> 00:50:16,840 It's amazing that anyone ever found this place. 370 00:50:19,960 --> 00:50:22,640 We're about to go in for the next 18 days, 371 00:50:22,640 --> 00:50:25,200 and it's lights on from here on out. 372 00:50:26,960 --> 00:50:30,400 Local experts guide the team through the darkness. 373 00:50:31,600 --> 00:50:34,080 OK, I can go? Yes, yes. 374 00:50:35,440 --> 00:50:38,280 HEAVY BREATHING 375 00:50:42,200 --> 00:50:43,720 NERVOUS CHUCKLING 376 00:50:43,720 --> 00:50:45,240 I'm good. I'm OK. 377 00:50:45,240 --> 00:50:46,960 It's just really slippery. 378 00:50:48,960 --> 00:50:54,440 A complex system of ropes leads them down a 90-metre descent. 379 00:50:56,000 --> 00:50:58,000 RADIO CHATTER 380 00:50:59,640 --> 00:51:01,200 OK. 381 00:51:01,200 --> 00:51:02,440 Thanks. 382 00:51:06,440 --> 00:51:09,160 After four hours clambering over rocks... 383 00:51:09,160 --> 00:51:11,240 PAN SIZZLING 384 00:51:11,240 --> 00:51:14,080 ...they finally reach what will be their camp. 385 00:51:22,840 --> 00:51:26,040 The team wake to a new day - 386 00:51:26,040 --> 00:51:28,000 but in darkness. 387 00:51:28,000 --> 00:51:29,800 How did you sleep, Luke? 388 00:51:29,800 --> 00:51:31,920 Quite tired. 389 00:51:31,920 --> 00:51:34,160 These are all of the tents behind me. 390 00:51:34,160 --> 00:51:37,720 You can see they are pretty... 391 00:51:37,720 --> 00:51:39,240 ..pretty snug. 392 00:51:39,240 --> 00:51:42,400 This is where we have our breakfast every day, 393 00:51:42,400 --> 00:51:44,640 made by some amazing cooks. 394 00:51:46,120 --> 00:51:48,360 This is our charging station. 395 00:51:48,360 --> 00:51:50,760 Batteries for camera equipment, batteries for our lights, 396 00:51:50,760 --> 00:51:52,880 our head torches, which are so important. 397 00:51:53,880 --> 00:51:56,840 This is our water station. 398 00:51:56,840 --> 00:51:59,960 Everything that we have got has been brought in by someone, 399 00:51:59,960 --> 00:52:02,800 so you can't shower or wash. 400 00:52:02,800 --> 00:52:05,120 And it's not just water. 401 00:52:05,120 --> 00:52:09,720 Tonnes of equipment has had to be carried into the cave, 402 00:52:09,720 --> 00:52:12,080 every inch of the way. 403 00:52:15,240 --> 00:52:18,600 Now deep underground, the powerful lights 404 00:52:18,600 --> 00:52:20,960 reveal just how vast this cavern is. 405 00:52:22,360 --> 00:52:24,080 That is amazing. 406 00:52:24,080 --> 00:52:25,480 Look at that. It's... 407 00:52:25,480 --> 00:52:27,280 It's huge! 408 00:52:29,760 --> 00:52:32,080 As you turn on each of the lights, 409 00:52:32,080 --> 00:52:34,880 another part of this enormous chamber is revealed. 410 00:52:37,560 --> 00:52:40,600 It's magic. Absolutely magic. 411 00:52:43,760 --> 00:52:46,960 To try and capture the true scale, 412 00:52:46,960 --> 00:52:50,760 the team attempt a filming technique never tried before. 413 00:52:52,080 --> 00:52:55,400 We're trying to do a little bit of a dance with the drones, 414 00:52:55,400 --> 00:52:59,520 whereby we have one drone sort of slightly below the other, 415 00:52:59,520 --> 00:53:03,560 one filming the other one lighting a feature in the cave. 416 00:53:03,560 --> 00:53:05,920 It's a really tight space. 417 00:53:05,920 --> 00:53:07,960 You can't see anything. 418 00:53:07,960 --> 00:53:09,920 What could go wrong? 419 00:53:09,920 --> 00:53:11,760 DRONE BUZZES 420 00:53:13,600 --> 00:53:15,440 OK, slow down a bit. 421 00:53:17,080 --> 00:53:19,120 It's making everyone a little nervous. 422 00:53:20,160 --> 00:53:22,800 Yeah, don't go up any higher, Luke. 423 00:53:25,200 --> 00:53:27,480 OK, that's a nice height. 424 00:53:31,160 --> 00:53:34,400 With one drone lighting and one drone filming... 425 00:53:36,280 --> 00:53:41,280 ..they reveal the wonder of some of the world's largest stalagmites. 426 00:53:50,200 --> 00:53:53,360 As the days pass, the team settle into 427 00:53:53,360 --> 00:53:56,560 the everyday routine of life underground. 428 00:53:57,800 --> 00:54:01,960 This is the epitome of being off-grid. 429 00:54:01,960 --> 00:54:04,120 We call that one seesaw loo 430 00:54:04,120 --> 00:54:06,680 because it's kind of on a rock, 431 00:54:06,680 --> 00:54:09,320 and every time you sit on it and lean back, 432 00:54:09,320 --> 00:54:13,200 you're scared that there's going to be a disaster. 433 00:54:14,880 --> 00:54:18,240 The strangest thing is having no connection 434 00:54:18,240 --> 00:54:20,400 to the outside world whatsoever. 435 00:54:20,400 --> 00:54:23,800 Time is warped. Nothing makes sense underground. 436 00:54:28,480 --> 00:54:32,680 Without daylight to guide them, the team work round the clock 437 00:54:32,680 --> 00:54:35,560 to capture the cave's most unique features. 438 00:54:37,280 --> 00:54:39,600 Most special of all, 439 00:54:39,600 --> 00:54:43,200 an event which happens only a few weeks a year. 440 00:54:45,920 --> 00:54:48,400 To get there, they must navigate 441 00:54:48,400 --> 00:54:51,040 through boulder fields and tight squeezes. 442 00:54:52,200 --> 00:54:53,680 BLEEP 443 00:55:00,560 --> 00:55:03,160 Here, around half a million years ago, 444 00:55:03,160 --> 00:55:05,440 the cave roof collapsed, 445 00:55:05,440 --> 00:55:08,120 opening a window to the outside world. 446 00:55:12,560 --> 00:55:15,400 The team wait for just the right conditions. 447 00:55:16,760 --> 00:55:19,400 We've been sat here for hours. 448 00:55:19,400 --> 00:55:21,920 And finally, just on the right-hand side of the wall, 449 00:55:21,920 --> 00:55:24,400 we can see the first signs of light. 450 00:55:28,320 --> 00:55:30,720 After weeks of darkness 451 00:55:30,720 --> 00:55:32,600 200 metres underground... 452 00:55:33,920 --> 00:55:35,760 ..they see sunlight 453 00:55:35,760 --> 00:55:38,080 illuminate the cave floor. 454 00:55:43,200 --> 00:55:48,040 The way it just lights up this massive cave... 455 00:55:48,040 --> 00:55:51,440 ..when we've been trying to do it with about four or five lights 456 00:55:51,440 --> 00:55:54,840 and barely even lighting the smallest section. 457 00:55:54,840 --> 00:55:56,760 When the sun comes in, it just... 458 00:55:56,760 --> 00:55:58,160 HE PUFFS 459 00:56:00,400 --> 00:56:02,440 I think everyone's got goose bumps. 460 00:56:06,840 --> 00:56:11,600 I don't think I've ever appreciated sun so much in my life. 461 00:56:13,200 --> 00:56:15,200 It's incredible. 462 00:56:17,960 --> 00:56:21,000 Look at the backlit waterfalls. 463 00:56:21,000 --> 00:56:23,680 They're so slow coming down. 464 00:56:23,680 --> 00:56:26,400 It's literally one of the best things I've ever seen. 465 00:56:30,000 --> 00:56:33,840 For the crew, the sunbeam marks a profound end 466 00:56:33,840 --> 00:56:36,160 to their time in the cave. 467 00:56:39,440 --> 00:56:41,720 We're outside! 468 00:56:41,720 --> 00:56:44,840 It's so green, it's so bright. 469 00:56:44,840 --> 00:56:48,720 Being back above ground feels like where we belong. 470 00:56:50,720 --> 00:56:54,040 It's a wonder to imagine what other extremes 471 00:56:54,040 --> 00:56:58,360 still lie beneath the surface of planet Earth. 472 00:57:06,840 --> 00:57:09,160 Next time... 473 00:57:09,160 --> 00:57:11,400 ..the human world... 474 00:57:17,080 --> 00:57:21,920 ..where animals must find new ways to survive... 475 00:57:23,040 --> 00:57:25,040 ..amongst us. 476 00:57:31,760 --> 00:57:35,480 Habitat Explorer brings animals and their habitats to life. 477 00:57:35,480 --> 00:57:38,760 Explore this free interactive, and make origami animals. 478 00:57:38,760 --> 00:57:42,680 Go to bbc.co.uk/planetearth3 479 00:57:42,680 --> 00:57:45,240 and follow the links to the Open University. 480 00:57:45,240 --> 00:57:48,680 Or to order a free printed version, visit the website 481 00:57:48,680 --> 00:57:51,160 or call the numbers on the screen.