1 00:00:00,000 --> 00:00:02,400 DAVID ATTENBOROUGH: This is the entrance 2 00:00:02,520 --> 00:00:05,520 to one of the greatest of natural wonders. 3 00:00:22,760 --> 00:00:26,240 Hang So'n Đoong in Vietnam. 4 00:00:33,400 --> 00:00:37,120 It's thought to be the largest cave on Earth. 5 00:00:47,200 --> 00:00:50,560 Here, 200 metres underground, 6 00:00:50,680 --> 00:00:53,560 for just a few weeks a year, 7 00:00:53,680 --> 00:00:55,760 sunbeams cast light 8 00:00:55,880 --> 00:00:58,760 through a hole in the cave's roof. 9 00:01:16,400 --> 00:01:18,440 There is just enough light 10 00:01:18,560 --> 00:01:21,000 to enable plants to grow. 11 00:01:24,160 --> 00:01:25,520 But travel deeper 12 00:01:25,640 --> 00:01:27,640 and darkness returns. 13 00:01:31,040 --> 00:01:33,160 And you reach the limit 14 00:01:33,280 --> 00:01:35,240 of Where life can exist. 15 00:01:56,200 --> 00:01:59,320 These most extreme places on our planet 16 00:01:59,440 --> 00:02:03,240 are often hard for us to reach. 17 00:02:05,160 --> 00:02:09,000 Indeed, this cave was only recently discovered. 18 00:02:12,240 --> 00:02:16,240 And it contains many surprises. 19 00:02:23,240 --> 00:02:24,920 Cave pearls. 20 00:02:25,040 --> 00:02:27,120 Once tiny sand grains, 21 00:02:27,240 --> 00:02:30,560 now spheres the size of tennis balls. 22 00:02:40,720 --> 00:02:43,560 And giant stalagmites... 23 00:02:44,840 --> 00:02:46,360 ...formed over centuries 24 00:02:46,480 --> 00:02:49,760 by billions of individual drops of water. 25 00:02:58,400 --> 00:03:00,120 Underground rivers 26 00:03:00,240 --> 00:03:03,360 have carved this great passage through the limestone. 27 00:03:20,120 --> 00:03:22,640 Over five miles long 28 00:03:22,760 --> 00:03:24,720 and wide enough in places 29 00:03:24,840 --> 00:03:27,640 to accommodate a jumbo jet. 30 00:03:39,800 --> 00:03:41,160 And it is here, 31 00:03:41,280 --> 00:03:44,320 in the cave's furthest chamber, 32 00:03:44,440 --> 00:03:46,560 that life can be found. 33 00:03:53,880 --> 00:03:56,480 Translucent cave fish... 34 00:03:58,320 --> 00:03:59,720 ...and shrimps. 35 00:04:01,840 --> 00:04:04,120 Isolated for millennia, 36 00:04:04,240 --> 00:04:05,640 feeding on little more 37 00:04:05,760 --> 00:04:08,480 than the nutrients from the jungle above. 38 00:04:15,840 --> 00:04:18,360 The fish are completely blind 39 00:04:18,480 --> 00:04:23,000 and find their way about by touch alone. 40 00:04:30,880 --> 00:04:33,080 These underground worlds 41 00:04:33,200 --> 00:04:36,680 are at the very limit of Where life can exist. 42 00:04:45,760 --> 00:04:47,480 But above ground, 43 00:04:47,600 --> 00:04:49,840 our planet's most extreme conditions 44 00:04:49,960 --> 00:04:52,360 are found at the poles. 45 00:04:52,480 --> 00:04:54,200 (WIND WHISTLES) 46 00:04:58,240 --> 00:04:59,680 Here in the north, 47 00:04:59,800 --> 00:05:01,320 Winter temperatures can drop 48 00:05:01,440 --> 00:05:04,120 to -50 degrees Celsius. 49 00:05:07,440 --> 00:05:09,200 For four months of the year, 50 00:05:09,320 --> 00:05:12,240 there is barely a glimmer of sunlight. 51 00:05:18,520 --> 00:05:20,920 But here, too, 52 00:05:21,040 --> 00:05:23,280 life finds a way to survive. 53 00:05:27,840 --> 00:05:30,400 A pack of Arctic wolves. 54 00:05:36,480 --> 00:05:39,160 Their dense fur has protected them 55 00:05:39,280 --> 00:05:41,760 through months of bitter cold. 56 00:05:48,760 --> 00:05:51,680 But now the sun is returning. 57 00:06:05,160 --> 00:06:07,720 This is Ellesmere Island. 58 00:06:15,040 --> 00:06:16,080 The short summer 59 00:06:16,200 --> 00:06:17,920 gives the wolves a chance 60 00:06:18,040 --> 00:06:20,320 to rebuild their strength. 61 00:06:34,840 --> 00:06:36,280 But here in the Arctic... 62 00:06:37,920 --> 00:06:39,760 ...life is never easy. 63 00:06:52,240 --> 00:06:53,760 This pack is led 64 00:06:53,880 --> 00:06:56,120 by an experienced female. 65 00:07:05,240 --> 00:07:07,320 If she can't lead them to food, 66 00:07:07,440 --> 00:07:10,600 the coming Winter could be their last. 67 00:07:22,400 --> 00:07:25,120 Prey is spread over such a vast area 68 00:07:25,240 --> 00:07:28,360 that they must be constantly on the move... 69 00:07:32,920 --> 00:07:34,200 ...in search of it. 70 00:07:48,880 --> 00:07:50,560 But finding prey 71 00:07:50,680 --> 00:07:52,200 is only the beginning. 72 00:08:01,920 --> 00:08:03,160 Musk oxen. 73 00:08:04,240 --> 00:08:07,440 They are five times the size of a wolf. 74 00:08:22,880 --> 00:08:24,880 (SNORTING) 75 00:08:29,800 --> 00:08:32,440 The conflict between wolf and oxen 76 00:08:32,560 --> 00:08:34,560 is an ancient one, 77 00:08:34,680 --> 00:08:38,160 and has been waged since the Ice Age. 78 00:08:43,120 --> 00:08:45,240 (SNORTING AND GRUNTING) 79 00:08:48,280 --> 00:08:52,600 Each side knows the other's tactics. 80 00:08:54,920 --> 00:08:56,240 (SNARLING) 81 00:09:07,440 --> 00:09:09,760 The wolves try to isolate 82 00:09:09,880 --> 00:09:12,160 the weak or the young. 83 00:09:17,960 --> 00:09:19,240 A calf stumbles... 84 00:09:20,440 --> 00:09:21,760 ...and the pack leader 85 00:09:21,880 --> 00:09:23,520 immediately closes in. 86 00:09:33,680 --> 00:09:35,280 (GROWLING) 87 00:09:35,400 --> 00:09:38,120 But the herd regroups, 88 00:09:38,240 --> 00:09:39,960 horns at the ready. 89 00:09:49,520 --> 00:09:50,640 Stalemate. 90 00:09:52,960 --> 00:09:55,280 But it can only be brief. 91 00:10:19,960 --> 00:10:23,320 This time, the herd race to higher ground. 92 00:10:37,000 --> 00:10:39,120 As they reach the top of the ridge, 93 00:10:39,240 --> 00:10:40,800 the males turn 94 00:10:40,920 --> 00:10:42,560 to defend themselves. 95 00:10:42,680 --> 00:10:44,080 (GROWLING) 96 00:10:45,960 --> 00:10:47,080 Stalemate... 97 00:10:48,240 --> 00:10:49,240 ...again. 98 00:11:17,720 --> 00:11:19,920 Every unsuccessful hunt 99 00:11:20,040 --> 00:11:22,840 leaves the pack weakened. 100 00:11:38,640 --> 00:11:41,280 This could be an easier meal. 101 00:11:44,240 --> 00:11:45,560 (SNIFFING) 102 00:11:45,680 --> 00:11:47,600 But something is not right. 103 00:11:51,480 --> 00:11:55,040 This musk ox has died from a disease. 104 00:11:55,160 --> 00:11:57,120 (SNIFFING) 105 00:11:57,240 --> 00:11:58,600 Outbreaks like this 106 00:11:58,720 --> 00:12:00,880 are becoming increasingly common 107 00:12:01,000 --> 00:12:03,920 as the climate warms across the Arctic. 108 00:12:11,160 --> 00:12:12,600 Despite their hunger, 109 00:12:12,720 --> 00:12:15,360 the pack don't eat this carcass. 110 00:12:40,240 --> 00:12:41,680 Another herd. 111 00:12:42,720 --> 00:12:44,480 Another chance for the pack. 112 00:13:02,800 --> 00:13:03,840 This time, 113 00:13:03,960 --> 00:13:06,560 the wolves manage to split the herd. 114 00:13:08,320 --> 00:13:10,880 That may improve their chances. 115 00:13:29,080 --> 00:13:32,120 The lead female makes sure that the oxen 116 00:13:32,240 --> 00:13:34,240 do not reach the ridge again. 117 00:13:41,240 --> 00:13:43,080 With one less defender, 118 00:13:43,200 --> 00:13:45,240 the calf is more vulnerable. 119 00:14:02,920 --> 00:14:04,720 A much-needed meal. 120 00:14:18,000 --> 00:14:20,880 The wolves must continue to make the most 121 00:14:21,000 --> 00:14:23,600 of the remaining weeks of summer... 122 00:14:27,120 --> 00:14:29,760 ...so when Winter returns... 123 00:14:30,880 --> 00:14:33,360 ...they're ready to endure 124 00:14:33,480 --> 00:14:36,080 some of the harshest conditions 125 00:14:36,200 --> 00:14:39,240 that exist on Planet Earth. 126 00:15:01,560 --> 00:15:04,560 Such inhospitable extremes 127 00:15:04,680 --> 00:15:07,240 are not only found at the poles. 128 00:15:14,240 --> 00:15:16,200 As altitude increases... 129 00:15:17,320 --> 00:15:18,760 ...temperatures fall. 130 00:15:25,240 --> 00:15:26,160 Here, too, 131 00:15:26,280 --> 00:15:30,040 animals must make the most of the seasonal change. 132 00:15:38,240 --> 00:15:41,280 Particularly if you're ready to mate. 133 00:15:50,360 --> 00:15:52,640 A male common frog 134 00:15:52,760 --> 00:15:54,760 in the French Alps. 135 00:16:02,240 --> 00:16:04,400 He has spent all Winter 136 00:16:04,520 --> 00:16:06,760 hibernating under the ice. 137 00:16:11,200 --> 00:16:13,960 But now it's time to wake up. 138 00:16:15,760 --> 00:16:17,120 Being cold-blooded, 139 00:16:17,240 --> 00:16:20,240 he needs the sun to warm his body. 140 00:16:22,400 --> 00:16:25,560 But once it sets in a few hours' time, 141 00:16:25,680 --> 00:16:27,440 he could freeze to death. 142 00:16:29,920 --> 00:16:33,360 So finding a mate is urgent. 143 00:16:39,120 --> 00:16:40,560 The weather is good. 144 00:16:45,000 --> 00:16:47,040 And he starts to sledge. 145 00:16:50,120 --> 00:16:54,040 But this is not a one-frog race. 146 00:16:55,640 --> 00:16:58,760 Dozens are rushing to the breeding pool. 147 00:17:03,920 --> 00:17:06,240 This many frogs out on the slopes... 148 00:17:07,320 --> 00:17:09,080 ...doesn't go unnoticed. 149 00:17:13,520 --> 00:17:15,360 You can't afford any slips. 150 00:17:23,800 --> 00:17:25,880 It was all going so well. 151 00:17:40,520 --> 00:17:41,600 Made it. 152 00:17:41,720 --> 00:17:43,840 (CROAKING) 153 00:17:46,240 --> 00:17:49,520 But all the females have already got partners. 154 00:17:52,440 --> 00:17:54,880 There's only one thing for it. 155 00:17:59,280 --> 00:18:00,360 Wrestling. 156 00:18:17,080 --> 00:18:18,600 Not this time. 157 00:18:26,560 --> 00:18:27,920 Another latecomer. 158 00:18:30,000 --> 00:18:31,040 A female. 159 00:18:35,040 --> 00:18:36,560 Could she be the one? 160 00:18:47,320 --> 00:18:49,120 Sometimes being late 161 00:18:49,240 --> 00:18:50,600 can pay off. 162 00:19:14,480 --> 00:19:16,120 After all his efforts, 163 00:19:16,240 --> 00:19:19,120 the male is not going to let this chance 164 00:19:19,240 --> 00:19:21,000 slip through his fingers. 165 00:19:40,360 --> 00:19:42,720 This is the cedar forest 166 00:19:42,840 --> 00:19:45,760 of the Atlas Mountains in North Africa. 167 00:19:52,240 --> 00:19:53,880 These Barbary macaques 168 00:19:54,000 --> 00:19:56,080 enjoyed a summer of plenty, 169 00:19:56,200 --> 00:19:58,280 but now they're paying the price. 170 00:20:03,040 --> 00:20:05,800 Winter here can be brutal. 171 00:20:12,080 --> 00:20:15,680 In some years, over half the group perish. 172 00:20:21,000 --> 00:20:22,920 But in this macaque society, 173 00:20:23,040 --> 00:20:26,080 the more social the monkey, 174 00:20:26,200 --> 00:20:29,080 the more likely it is to survive. 175 00:20:31,720 --> 00:20:34,120 Keeping warm by huddling 176 00:20:34,240 --> 00:20:36,080 can make all the difference. 177 00:20:40,480 --> 00:20:42,520 But they can't huddle all day. 178 00:20:46,680 --> 00:20:50,240 They must, eventually, break apart to feed. 179 00:21:02,320 --> 00:21:05,120 They spread out, searching for food. 180 00:21:24,440 --> 00:21:25,480 But each night, 181 00:21:25,600 --> 00:21:29,720 the group return to huddle in the cedar trees. 182 00:21:43,720 --> 00:21:46,000 No-one wants to be left out. 183 00:21:48,480 --> 00:21:50,360 Especially the infants. 184 00:22:01,440 --> 00:22:04,560 This baby, somehow, has got separated. 185 00:22:20,720 --> 00:22:21,720 He must get back 186 00:22:21,840 --> 00:22:24,560 to the life-saving warmth of the huddle. 187 00:22:24,680 --> 00:22:26,040 And soon. 188 00:22:53,200 --> 00:22:55,320 But there is an obstacle. 189 00:22:59,600 --> 00:23:01,520 A grumpy male 190 00:23:01,640 --> 00:23:04,880 is giving the baby a rather cold shoulder. 191 00:23:08,520 --> 00:23:10,400 (GRUNTING) 192 00:23:12,000 --> 00:23:14,000 (CHITTERING) 193 00:23:16,120 --> 00:23:19,400 Maybe a more subtle approach. 194 00:23:28,040 --> 00:23:31,000 The male decides to leave. 195 00:23:46,720 --> 00:23:47,720 At last... 196 00:23:49,320 --> 00:23:51,520 ...an enthusiastic reception. 197 00:24:01,920 --> 00:24:03,880 The male has had less luck. 198 00:24:05,800 --> 00:24:07,960 It could be a long night. 199 00:24:13,960 --> 00:24:17,760 Huddling might seem a simple expression of affection, 200 00:24:17,880 --> 00:24:21,680 but among these macaques it's essential for survival. 201 00:24:25,400 --> 00:24:27,120 And not just here. 202 00:24:32,360 --> 00:24:36,000 5,000 miles away in the forests of Mexico, 203 00:24:36,120 --> 00:24:39,480 monarch butterflies, in millions, 204 00:24:39,600 --> 00:24:41,840 also huddle for survival. 205 00:24:47,320 --> 00:24:50,480 All of them have travelled across North America 206 00:24:50,600 --> 00:24:53,440 to escape the Winter that is coming there. 207 00:24:57,360 --> 00:24:58,520 This forest, 208 00:24:58,640 --> 00:25:01,160 at an altitude of 3,000 metres, 209 00:25:01,280 --> 00:25:04,120 might seem to offer little comfort... 210 00:25:06,320 --> 00:25:07,560 ...for temperatures here 211 00:25:07,680 --> 00:25:09,360 can drop below freezing... 212 00:25:10,840 --> 00:25:13,240 ...and that could kill the butterflies. 213 00:25:18,640 --> 00:25:21,560 But in fact, a thin layer of warm air 214 00:25:21,680 --> 00:25:25,160 trapped by the tree canopy protects them, 215 00:25:25,280 --> 00:25:28,040 and will do so throughout the Winter... 216 00:25:30,000 --> 00:25:33,800 ...provided that the forest remains undisturbed. 217 00:25:40,600 --> 00:25:43,200 Unpredictable extremes of weather, however, 218 00:25:43,320 --> 00:25:46,960 can disrupt this delicate balance. 219 00:25:50,880 --> 00:25:53,680 Strong winds can push deep into the forest, 220 00:25:53,800 --> 00:25:55,240 chilling the air. 221 00:25:56,760 --> 00:25:58,680 (WIND GUSTS) 222 00:26:01,480 --> 00:26:03,960 Hundreds of thousands of butterflies 223 00:26:04,080 --> 00:26:05,680 may be blown from the branches. 224 00:26:32,040 --> 00:26:36,080 In some years, storms kill 75% 225 00:26:36,200 --> 00:26:38,120 of hibernating monarchs. 226 00:26:55,280 --> 00:26:57,520 This one has survived... 227 00:26:57,640 --> 00:27:00,080 ...but on the ground. 228 00:27:03,600 --> 00:27:06,720 And night-time frosts are fatal. 229 00:27:08,200 --> 00:27:11,400 It can't stay here and live. 230 00:27:16,800 --> 00:27:18,800 It is too cold to fly, 231 00:27:18,920 --> 00:27:23,080 but it must get back into the tree branches somehow. 232 00:27:26,160 --> 00:27:28,240 So it starts to climb. 233 00:27:37,040 --> 00:27:39,840 It's the equivalent of a human 234 00:27:39,960 --> 00:27:42,120 climbing up a skyscraper. 235 00:28:13,440 --> 00:28:16,000 Safety at last. 236 00:28:21,640 --> 00:28:24,520 As spring returns, 237 00:28:24,640 --> 00:28:28,320 the monarchs wake from their long slumber. 238 00:28:44,840 --> 00:28:46,120 And now starts 239 00:28:46,240 --> 00:28:50,160 one of the Earth's greatest natural spectacles. 240 00:29:22,560 --> 00:29:25,920 But this mass awakening is under threat. 241 00:29:28,640 --> 00:29:30,200 The changing climate 242 00:29:30,320 --> 00:29:33,320 is creating more extreme weather, 243 00:29:33,440 --> 00:29:36,400 and storms are becoming more frequent. 244 00:29:38,800 --> 00:29:40,360 But, for now, 245 00:29:40,480 --> 00:29:43,640 this miracle of nature continues. 246 00:29:53,720 --> 00:29:54,720 Fire. 247 00:29:58,960 --> 00:30:02,160 Unpredictable and destructive. 248 00:30:09,760 --> 00:30:13,720 This is South Africa's fynbos. 249 00:30:13,840 --> 00:30:17,480 Botanically one of the most diverse habitats on Earth... 250 00:30:18,760 --> 00:30:21,840 ...with over 8,000 different plant species. 251 00:30:24,720 --> 00:30:26,360 Yet, every two decades, 252 00:30:26,480 --> 00:30:30,200 fire reduces the vegetation to ashes. 253 00:30:38,120 --> 00:30:39,960 This may seem catastrophic. 254 00:30:41,640 --> 00:30:45,240 But just days later, plants burst into life... 255 00:30:46,240 --> 00:30:48,160 ...and transform the landscape. 256 00:30:52,240 --> 00:30:54,040 Seeds and bulbs 257 00:30:54,160 --> 00:30:56,320 have been waiting for this event. 258 00:31:04,440 --> 00:31:08,520 90 million years of evolution alongside fire 259 00:31:08,640 --> 00:31:11,760 has made some plants so closely adapted 260 00:31:11,880 --> 00:31:15,880 that they can't germinate without the presence of smoke. 261 00:31:23,600 --> 00:31:26,160 Remarkably, around 50% 262 00:31:26,280 --> 00:31:28,560 of Planet Earth's terrestrial habitats 263 00:31:28,680 --> 00:31:32,000 depend on fire to remain healthy. 264 00:31:39,320 --> 00:31:42,040 In Northern Australia, these savannahs 265 00:31:42,160 --> 00:31:44,880 need seasonal fires to keep them open 266 00:31:45,000 --> 00:31:47,680 and prevent the spread of the nearby forest. 267 00:31:52,640 --> 00:31:54,840 And they are a vital habitat 268 00:31:54,960 --> 00:31:57,440 for this colourful resident. 269 00:32:03,760 --> 00:32:06,760 The rare golden-shouldered parrot. 270 00:32:09,760 --> 00:32:11,960 He and his lifelong partner... 271 00:32:13,440 --> 00:32:16,680 ...rely on grassland for both food... 272 00:32:18,080 --> 00:32:19,760 ...and a nest site. 273 00:32:21,960 --> 00:32:24,120 As usual, they've laid their eggs 274 00:32:24,240 --> 00:32:26,600 in a hole in a termite mound. 275 00:32:26,720 --> 00:32:28,320 (CHEEPING) 276 00:32:30,080 --> 00:32:32,840 And now there are four chicks. 277 00:32:36,040 --> 00:32:37,640 The thick mud walls 278 00:32:37,760 --> 00:32:40,360 don't only protect chicks from predators, 279 00:32:40,480 --> 00:32:41,840 but insulate them 280 00:32:41,960 --> 00:32:44,240 from the heat of the scorching sun. 281 00:32:51,480 --> 00:32:54,160 With fledging just days away, 282 00:32:54,280 --> 00:32:56,760 the parents are working overtime. 283 00:32:59,560 --> 00:33:03,120 They collect thousands of seeds every day. 284 00:33:25,680 --> 00:33:27,840 But they have a problem. 285 00:33:36,200 --> 00:33:39,600 The fire season is becoming more irregular. 286 00:33:58,560 --> 00:34:01,520 If they are not ready to fly, 287 00:34:01,640 --> 00:34:04,120 chicks have no escape. 288 00:34:28,560 --> 00:34:31,120 But choosing to nest in a termite mound 289 00:34:31,240 --> 00:34:33,040 has a surprise benefit. 290 00:34:35,280 --> 00:34:38,760 Their home is fireproof. 291 00:34:49,560 --> 00:34:51,320 Concrete-like walls 292 00:34:51,440 --> 00:34:55,280 keep chicks safe from the flames outside. 293 00:35:04,880 --> 00:35:07,000 The seeds of fire grass 294 00:35:07,120 --> 00:35:09,080 are now easier for the parrots to find 295 00:35:09,200 --> 00:35:10,720 on the blackened earth. 296 00:35:12,040 --> 00:35:15,200 And they are the parrots' favourite food. 297 00:35:23,600 --> 00:35:26,720 So for both adults and their fledgling chicks... 298 00:35:29,040 --> 00:35:32,280 ...fires are essential for survival. 299 00:35:40,000 --> 00:35:42,400 But their grassland home is shrinking, 300 00:35:42,520 --> 00:35:45,720 and only about 1,000 of these lovely parrots 301 00:35:45,840 --> 00:35:47,440 still survive. 302 00:35:49,560 --> 00:35:51,960 It's through the active management of fire 303 00:35:52,080 --> 00:35:56,920 by humans that their world now exists at all. 304 00:36:02,960 --> 00:36:06,760 Humans are affecting habitats across Planet Earth. 305 00:36:07,880 --> 00:36:10,160 For wildlife, all too often, 306 00:36:10,280 --> 00:36:13,480 the consequences have been disastrous. 307 00:36:22,360 --> 00:36:26,000 Elephants are among the most tolerant of animals. 308 00:36:26,120 --> 00:36:29,280 (GRUNTING) 309 00:36:29,400 --> 00:36:31,400 They are very long-lived, 310 00:36:31,520 --> 00:36:34,240 and knowledge passed between generations 311 00:36:34,360 --> 00:36:36,560 enables them to find food 312 00:36:36,680 --> 00:36:39,120 and little-used sources of water 313 00:36:39,240 --> 00:36:42,120 in times of great environmental stress. 314 00:36:44,400 --> 00:36:46,160 But even among elephants, 315 00:36:46,280 --> 00:36:49,640 survival strategies are now beginning to fail. 316 00:36:57,120 --> 00:37:01,400 In Amboseli, Kenya, years of extreme weather, 317 00:37:01,520 --> 00:37:04,440 combined with overgrazing by domestic cattle, 318 00:37:04,560 --> 00:37:08,680 are turning these grasslands to dust. 319 00:37:37,800 --> 00:37:40,960 This matriarch is over 40 years old... 320 00:37:42,920 --> 00:37:44,760 ...and she's already survived 321 00:37:44,880 --> 00:37:47,520 some of East Africa's worst droughts. 322 00:37:51,120 --> 00:37:55,240 But this year, food is in extremely short supply... 323 00:37:58,520 --> 00:38:01,320 ...and her family are struggling to find enough. 324 00:38:09,160 --> 00:38:13,240 Her youngest son is malnourished and weak... 325 00:38:18,320 --> 00:38:21,760 ...and is lagging behind his mother and brother. 326 00:38:30,640 --> 00:38:31,640 She leads them 327 00:38:31,760 --> 00:38:35,360 to a pocket of acacia forest that she knows well. 328 00:38:41,600 --> 00:38:44,640 But broken branches and fallen trees 329 00:38:44,760 --> 00:38:48,080 show that other elephants have already been here. 330 00:38:53,240 --> 00:38:55,960 There is very little left worth eating. 331 00:39:03,280 --> 00:39:04,280 Without food, 332 00:39:04,400 --> 00:39:07,360 his mother cannot produce enough milk. 333 00:39:12,080 --> 00:39:13,880 She has no choice... 334 00:39:16,520 --> 00:39:18,920 ...but to leave her weakening calf... 335 00:39:21,080 --> 00:39:23,920 ...and try to find food elsewhere. 336 00:39:30,240 --> 00:39:31,240 So she... 337 00:39:32,840 --> 00:39:36,880 ...and her elder son head off... 338 00:39:37,000 --> 00:39:38,840 ...without him. 339 00:40:02,240 --> 00:40:05,080 He can only hope for their return. 340 00:40:30,600 --> 00:40:32,600 (FLIES BUZZ) 341 00:40:39,480 --> 00:40:43,800 But for him, time has run out. 342 00:41:19,560 --> 00:41:22,640 There is nothing his mother, or anybody, 343 00:41:22,760 --> 00:41:25,160 could have done to save him. 344 00:41:25,280 --> 00:41:28,440 (LOW RUMBLING) 345 00:41:37,280 --> 00:41:39,640 With her elder son still alive, 346 00:41:39,760 --> 00:41:42,440 there is hope for her family. 347 00:41:44,840 --> 00:41:48,720 But only if they can adapt quickly. 348 00:41:59,760 --> 00:42:02,720 Some families are proving to be resilient. 349 00:42:08,200 --> 00:42:09,760 They're travelling long distances 350 00:42:09,880 --> 00:42:11,680 away from Amboseli... 351 00:42:15,080 --> 00:42:18,960 ...and finding new sources of food and water. 352 00:42:36,960 --> 00:42:39,160 And despite the drought... 353 00:42:43,840 --> 00:42:45,800 ...calves are being born. 354 00:43:05,440 --> 00:43:08,360 But given the pace of change 355 00:43:08,480 --> 00:43:10,560 and the challenges they face... 356 00:43:12,200 --> 00:43:14,040 ...how many of Africa's elephants 357 00:43:14,160 --> 00:43:15,640 will be able to adapt... 358 00:43:17,240 --> 00:43:20,040 ...and can they do so fast enough? 359 00:43:35,200 --> 00:43:36,800 The Gobi Desert. 360 00:43:39,960 --> 00:43:42,960 A land of sand and snow. 361 00:43:48,680 --> 00:43:52,440 For at least a million years, this has been 362 00:43:52,560 --> 00:43:55,680 one of the most extreme habitats on Earth. 363 00:44:01,240 --> 00:44:03,440 Yet footprints in the sand 364 00:44:03,560 --> 00:44:07,000 reveal that there is life here. 365 00:44:13,480 --> 00:44:15,600 A snow leopard. 366 00:44:15,720 --> 00:44:17,840 Seldom seen... 367 00:44:19,560 --> 00:44:23,640 ...especially in such hot, arid conditions. 368 00:44:28,800 --> 00:44:32,920 They live up to 2,000 metres above sea level, 369 00:44:33,040 --> 00:44:36,280 and patrol vast territories. 370 00:45:05,680 --> 00:45:08,000 They are rarely seen together. 371 00:45:16,320 --> 00:45:17,320 A mother... 372 00:45:21,080 --> 00:45:22,080 ...with cubs. 373 00:45:25,480 --> 00:45:27,960 Filmed here for the first time 374 00:45:28,080 --> 00:45:30,680 in such intimate detail. 375 00:45:39,240 --> 00:45:42,200 And they give us a glimpse of their family life... 376 00:45:43,720 --> 00:45:45,120 ...such as it is. 377 00:46:01,920 --> 00:46:05,040 Snow leopard cubs stay with their mothers 378 00:46:05,160 --> 00:46:08,560 longer than those of almost any other big cat. 379 00:46:15,240 --> 00:46:18,480 They remain together for over two years... 380 00:46:22,080 --> 00:46:26,000 ...as they learn how to master the extremes. 381 00:46:33,720 --> 00:46:38,840 Which they do so well that they are able to thrive 382 00:46:38,960 --> 00:46:41,800 in some of the hottest, coldest... 383 00:46:42,840 --> 00:46:45,240 ...and highest places on Earth. 384 00:46:56,960 --> 00:47:00,120 Some animals, over millions of years, 385 00:47:00,240 --> 00:47:04,280 have evolved ways of surviving extremely hostile conditions. 386 00:47:07,560 --> 00:47:11,480 But as the extremes become more extreme... 387 00:47:12,520 --> 00:47:15,280 ...evolution is being outpaced. 388 00:47:24,320 --> 00:47:28,400 These extraordinary places are still inhabited 389 00:47:28,520 --> 00:47:30,840 by some of Earth's most resilient 390 00:47:30,960 --> 00:47:32,920 and adaptable species. 391 00:47:36,520 --> 00:47:38,680 But in our rapidly changing world... 392 00:47:40,120 --> 00:47:42,000 ...this is Planet Earth 393 00:47:42,120 --> 00:47:44,760 as you may never see it again. 394 00:47:51,800 --> 00:47:53,240 (YOWLING) 395 00:48:08,680 --> 00:48:10,480 DAVID ATTENBOROUGH: The most extreme parts 396 00:48:10,600 --> 00:48:12,840 of our planet are not always found 397 00:48:12,960 --> 00:48:14,320 where you might expect. 398 00:48:17,280 --> 00:48:19,640 Hidden deep in the forests of Vietnam, 399 00:48:19,760 --> 00:48:22,120 a local man, Mr Ho Khanh, 400 00:48:22,240 --> 00:48:24,400 made an extraordinary discovery. 401 00:48:44,280 --> 00:48:46,080 He didn't know it at the time, 402 00:48:46,200 --> 00:48:47,480 but he had stumbled upon 403 00:48:47,600 --> 00:48:50,360 arguably the largest cave in the world. 404 00:49:02,960 --> 00:49:06,720 Inspired by Mr Ho Khanh's discovery, 405 00:49:06,840 --> 00:49:10,520 the Planet Earth team set out to film Hang Son Đoong. 406 00:49:12,320 --> 00:49:14,840 But first, they have to get there. 407 00:49:16,680 --> 00:49:18,800 We've left the river valley. 408 00:49:18,920 --> 00:49:21,000 We're walking uphill now to the entrance. 409 00:49:21,120 --> 00:49:22,920 It's just thick, thick forest. 410 00:49:24,800 --> 00:49:28,000 After trekking for two gruelling days, 411 00:49:28,120 --> 00:49:30,960 the team finally arrive. 412 00:49:31,080 --> 00:49:34,080 Down here is the entrance to Hang So'n Đoong, 413 00:49:34,200 --> 00:49:37,960 this tiny opening in this massive jungle. 414 00:49:38,080 --> 00:49:40,800 It's amazing that anyone ever found this place. 415 00:49:44,120 --> 00:49:46,680 We're about to go in for the next 18 days, 416 00:49:46,800 --> 00:49:49,320 and it's lights on from here on out. 417 00:49:50,840 --> 00:49:52,920 Local experts guide the team 418 00:49:53,040 --> 00:49:54,480 through the darkness. 419 00:49:55,720 --> 00:49:58,160 - OK, I can go? - Yes, please. 420 00:49:59,200 --> 00:50:02,400 (PANTS) 421 00:50:06,640 --> 00:50:07,640 (CHUCKLES) 422 00:50:07,760 --> 00:50:09,240 I'm good. I'm OK. 423 00:50:09,360 --> 00:50:10,880 It's just really slippery. 424 00:50:12,680 --> 00:50:14,680 A complex system of ropes 425 00:50:14,800 --> 00:50:18,080 leads them down a 90-metre descent. 426 00:50:20,200 --> 00:50:21,640 (RADIO CHATTER) 427 00:50:24,040 --> 00:50:26,160 OK. Thanks. 428 00:50:30,240 --> 00:50:32,920 After four hours clambering over rocks... 429 00:50:35,200 --> 00:50:38,240 ...they finally reach what will be their camp. 430 00:50:46,960 --> 00:50:50,040 The team wake to a new day. 431 00:50:50,160 --> 00:50:51,840 But in darkness. 432 00:50:51,960 --> 00:50:53,520 GEORGINA: How did you sleep, Luke? 433 00:50:53,640 --> 00:50:54,960 Quite tired. 434 00:50:56,080 --> 00:50:58,320 These are all of the tents behind me. 435 00:50:58,440 --> 00:51:01,640 You can see... they are pretty... 436 00:51:01,760 --> 00:51:03,520 ...pretty snug. 437 00:51:03,640 --> 00:51:05,160 This is Where we have our breakfast 438 00:51:05,280 --> 00:51:06,200 every day. 439 00:51:06,320 --> 00:51:08,640 Made by some amazing cooks. 440 00:51:10,240 --> 00:51:11,880 This is our charging station. 441 00:51:12,000 --> 00:51:14,680 Batteries for camera equipment, batteries for our lights, 442 00:51:14,800 --> 00:51:17,000 our head torches, which are so important. 443 00:51:18,080 --> 00:51:20,800 This is our water station. 444 00:51:20,920 --> 00:51:23,960 Everything that we have got has been brought in by someone, 445 00:51:24,080 --> 00:51:25,800 so you can't shower or wash. 446 00:51:26,960 --> 00:51:29,160 And it's not just water. 447 00:51:29,280 --> 00:51:30,680 Tonnes of equipment 448 00:51:30,800 --> 00:51:33,920 has had to be carried into the cave, 449 00:51:34,040 --> 00:51:36,240 every inch of the way. 450 00:51:39,320 --> 00:51:41,600 Now, deep underground, 451 00:51:41,720 --> 00:51:42,920 the powerful lights reveal 452 00:51:43,040 --> 00:51:45,160 just how vast this cavern is. 453 00:51:46,400 --> 00:51:47,960 That is amazing. 454 00:51:48,080 --> 00:51:50,920 - Look at that. - It's huge! 455 00:51:53,840 --> 00:51:55,840 As you turn on each of the lights, 456 00:51:55,960 --> 00:51:58,880 another part of this enormous chamber is revealed. 457 00:52:01,680 --> 00:52:04,200 It's magic. Absolutely magic. 458 00:52:08,440 --> 00:52:11,280 To try and capture the true scale, the team 459 00:52:11,400 --> 00:52:14,480 attempt a filming technique never tried before. 460 00:52:16,440 --> 00:52:17,440 We're trying to 461 00:52:17,560 --> 00:52:19,360 do a little bit of a dance with the drones, 462 00:52:19,480 --> 00:52:21,240 whereby we have one drone 463 00:52:21,360 --> 00:52:23,400 sort of slightly below the other, 464 00:52:23,520 --> 00:52:25,240 one filming the other one 465 00:52:25,360 --> 00:52:27,000 lighting a feature in the cave. 466 00:52:27,120 --> 00:52:30,080 It's a really tight space, 467 00:52:30,200 --> 00:52:31,800 you can't see anything. 468 00:52:31,920 --> 00:52:33,400 What could go wrong? 469 00:52:37,840 --> 00:52:39,280 OK, slow down a bit. 470 00:52:41,120 --> 00:52:43,320 It's making everyone a little nervous. 471 00:52:44,440 --> 00:52:46,920 Yeah, don't go up any higher, Luke. 472 00:52:49,520 --> 00:52:51,160 OK, that's a nice height. 473 00:52:55,240 --> 00:52:58,280 With one drone lighting and one drone filming... 474 00:53:00,440 --> 00:53:01,600 ...they reveal the wonder 475 00:53:01,720 --> 00:53:05,160 of some of the world's largest stalagmites. 476 00:53:14,520 --> 00:53:15,960 As the days pass, 477 00:53:16,080 --> 00:53:19,120 the team settle into the everyday routine 478 00:53:19,240 --> 00:53:20,480 of life underground. 479 00:53:21,760 --> 00:53:25,000 This is the epitome of being off-grid. 480 00:53:26,480 --> 00:53:27,960 We call that one seesaw loo 481 00:53:28,080 --> 00:53:30,720 because it's kind of on a rock 482 00:53:30,840 --> 00:53:33,440 and every time you sit on it and lean back, you're scared 483 00:53:33,560 --> 00:53:36,880 that there's going to be a disaster. 484 00:53:38,960 --> 00:53:42,120 The strangest thing is having no connection 485 00:53:42,240 --> 00:53:44,600 to the outside world whatsoever. 486 00:53:44,720 --> 00:53:45,840 Time is warped. 487 00:53:45,960 --> 00:53:47,920 Nothing makes sense underground. 488 00:53:52,560 --> 00:53:54,600 Without daylight to guide them, 489 00:53:54,720 --> 00:53:56,120 the team worked round the clock 490 00:53:56,240 --> 00:53:59,680 to capture the cave's most unique features. 491 00:54:01,360 --> 00:54:03,200 Most special of all, 492 00:54:03,320 --> 00:54:06,960 an event which happens only a few weeks a year. 493 00:54:09,640 --> 00:54:11,680 To get there, they must navigate 494 00:54:11,800 --> 00:54:15,080 through boulder fields and tight squeezes. 495 00:54:16,520 --> 00:54:17,600 (BLEEP) 496 00:54:24,440 --> 00:54:27,120 Here, around half a million years ago, 497 00:54:27,240 --> 00:54:29,200 the cave roof collapsed, 498 00:54:29,320 --> 00:54:31,920 opening a window to the outside world. 499 00:54:36,640 --> 00:54:39,480 The team wait for just the right conditions. 500 00:54:40,960 --> 00:54:42,760 We've been sat here for hours. 501 00:54:42,880 --> 00:54:45,680 And finally, just on the right-hand side of the wall, 502 00:54:45,800 --> 00:54:47,960 we can see the first signs of light. 503 00:54:52,440 --> 00:54:54,680 After weeks of darkness 504 00:54:54,800 --> 00:54:56,680 200 metres underground... 505 00:54:58,080 --> 00:54:59,360 ...they see sunlight 506 00:54:59,480 --> 00:55:01,720 illuminate the cave floor. 507 00:55:07,240 --> 00:55:10,720 The way it just lights up this massive cave. 508 00:55:12,400 --> 00:55:15,280 When we've been trying to do it with about four or five lights 509 00:55:15,400 --> 00:55:18,920 and barely even lighting the smallest section, 510 00:55:19,040 --> 00:55:21,680 when the sun comes in, it just... (PUFFS) 511 00:55:24,640 --> 00:55:26,520 I think everyone's got goose bumps. 512 00:55:30,920 --> 00:55:31,840 LUKE: I don't think 513 00:55:31,960 --> 00:55:35,240 I've ever appreciated sun so much in my life. 514 00:55:37,680 --> 00:55:39,360 It's incredible. 515 00:55:42,240 --> 00:55:45,040 Look at the backlit waterfalls. 516 00:55:45,160 --> 00:55:47,800 They're so slow coming down. 517 00:55:47,920 --> 00:55:49,880 It's literally one of the best things I've ever seen. 518 00:55:54,040 --> 00:55:55,520 For the crew, 519 00:55:55,640 --> 00:55:58,080 the sunbeam marks a profound end 520 00:55:58,200 --> 00:55:59,680 to their time in the cave. 521 00:56:03,840 --> 00:56:05,520 We're outside! 522 00:56:05,640 --> 00:56:08,640 It's so green. It's so bright. 523 00:56:08,760 --> 00:56:12,520 Being back above ground feels like Where we belong. 524 00:56:14,920 --> 00:56:17,800 It's a wonder to imagine What other extremes 525 00:56:17,920 --> 00:56:21,880 still lie beneath the surface of Planet Earth.