1 00:00:35,760 --> 00:00:38,479 This is the entrance 2 00:00:38,480 --> 00:00:41,880 to one of the greatest of natural wonders. 3 00:00:58,640 --> 00:01:02,640 Hang Son Doong, in Vietnam. 4 00:01:09,240 --> 00:01:12,960 It's thought to be the largest cave on Earth. 5 00:01:23,120 --> 00:01:26,439 Here, 200 metres underground, 6 00:01:26,440 --> 00:01:29,439 for just a few weeks a year, 7 00:01:29,440 --> 00:01:34,720 sunbeams cast light through a hole in the cave's roof. 8 00:01:52,160 --> 00:01:54,319 There is just enough light 9 00:01:54,320 --> 00:01:57,040 to enable plants to grow. 10 00:01:59,640 --> 00:02:01,639 But travel deeper, 11 00:02:01,640 --> 00:02:03,840 and darkness returns. 12 00:02:06,920 --> 00:02:09,319 And you reach the limit 13 00:02:09,320 --> 00:02:11,480 of where life can exist. 14 00:02:31,960 --> 00:02:35,199 These most extreme places on our planet 15 00:02:35,200 --> 00:02:39,040 are often hard for us to reach. 16 00:02:40,920 --> 00:02:45,080 Indeed, this cave was only recently discovered. 17 00:02:48,160 --> 00:02:52,440 And it contains many surprises. 18 00:02:58,960 --> 00:03:01,399 Cave pearls. 19 00:03:01,400 --> 00:03:03,559 Once tiny sand grains, 20 00:03:03,560 --> 00:03:06,720 now spheres the size of tennis balls. 21 00:03:16,480 --> 00:03:19,360 And giant stalagmites... 22 00:03:20,800 --> 00:03:22,679 ..formed over centuries 23 00:03:22,680 --> 00:03:26,000 by billions of individual drops of water. 24 00:03:33,960 --> 00:03:36,239 Underground rivers have carved 25 00:03:36,240 --> 00:03:39,480 this great passage through the limestone. 26 00:03:55,880 --> 00:03:58,519 Over five miles long 27 00:03:58,520 --> 00:04:00,879 and wide enough in places 28 00:04:00,880 --> 00:04:03,400 to accommodate a jumbo jet. 29 00:04:15,360 --> 00:04:17,319 And it is here, 30 00:04:17,320 --> 00:04:20,199 in the cave's furthest chamber, 31 00:04:20,200 --> 00:04:22,600 that life can be found. 32 00:04:29,640 --> 00:04:32,280 Translucent cave fish... 33 00:04:34,080 --> 00:04:35,720 ..and shrimps. 34 00:04:37,520 --> 00:04:39,999 Isolated for millennia, 35 00:04:40,000 --> 00:04:44,680 feeding on little more than the nutrients from the jungle above. 36 00:04:51,560 --> 00:04:54,839 The fish are completely blind 37 00:04:54,840 --> 00:04:58,800 and find their way about by touch alone. 38 00:05:06,480 --> 00:05:10,519 These underground worlds are at the very limit 39 00:05:10,520 --> 00:05:12,480 of where life can exist. 40 00:05:21,320 --> 00:05:23,519 But above ground, 41 00:05:23,520 --> 00:05:25,919 our planet's most extreme conditions 42 00:05:25,920 --> 00:05:28,359 are found at the poles. 43 00:05:33,680 --> 00:05:35,719 Here in the north, 44 00:05:35,720 --> 00:05:39,920 winter temperatures can drop to -50 degrees Celsius. 45 00:05:43,200 --> 00:05:45,479 For four months of the year, 46 00:05:45,480 --> 00:05:48,320 there is barely a glimmer of sunlight. 47 00:05:54,640 --> 00:05:57,039 But here, too, 48 00:05:57,040 --> 00:05:59,080 life finds a way to survive. 49 00:06:03,600 --> 00:06:06,880 A pack of Arctic wolves. 50 00:06:12,400 --> 00:06:15,359 Their dense fur has protected them 51 00:06:15,360 --> 00:06:18,080 through months of bitter cold. 52 00:06:24,440 --> 00:06:28,200 But now the sun is returning. 53 00:06:40,800 --> 00:06:44,080 This is Ellesmere Island. 54 00:06:50,920 --> 00:06:56,440 The short summer gives the wolves a chance to rebuild their strength. 55 00:07:10,440 --> 00:07:12,080 But here in the Arctic... 56 00:07:13,720 --> 00:07:16,360 ..life is never easy. 57 00:07:28,320 --> 00:07:32,280 This pack is led by an experienced female. 58 00:07:40,760 --> 00:07:43,519 If she can't lead them to food, 59 00:07:43,520 --> 00:07:46,960 the coming winter could be their last. 60 00:07:58,240 --> 00:08:01,199 Prey is spread over such a vast area 61 00:08:01,200 --> 00:08:04,400 that they must be constantly on the move... 62 00:08:08,680 --> 00:08:10,480 ..in search of it. 63 00:08:24,600 --> 00:08:28,160 But finding prey is only the beginning. 64 00:08:37,440 --> 00:08:39,999 Musk oxen. 65 00:08:40,000 --> 00:08:43,520 They are five times the size of a wolf. 66 00:09:05,560 --> 00:09:10,719 The conflict between wolf and oxen is an ancient one, 67 00:09:10,720 --> 00:09:13,960 and has been waged since the Ice Age. 68 00:09:24,440 --> 00:09:28,400 Each side knows the other's tactics. 69 00:09:43,040 --> 00:09:45,839 The wolves try to isolate 70 00:09:45,840 --> 00:09:48,200 the weak or the young. 71 00:09:53,600 --> 00:09:56,199 A calf stumbles... 72 00:09:56,200 --> 00:09:59,880 ..and the pack leader immediately closes in. 73 00:10:11,480 --> 00:10:13,999 But the herd regroups, 74 00:10:14,000 --> 00:10:16,160 horns at the ready. 75 00:10:25,280 --> 00:10:27,160 Stalemate. 76 00:10:28,720 --> 00:10:31,600 But it can only be brief. 77 00:10:55,760 --> 00:10:59,680 This time, the herd race to higher ground. 78 00:11:12,520 --> 00:11:15,399 As they reach the top of the ridge, 79 00:11:15,400 --> 00:11:18,319 the males turn to defend themselves. 80 00:11:21,640 --> 00:11:23,959 Stalemate... 81 00:11:23,960 --> 00:11:25,240 ..again. 82 00:11:53,520 --> 00:11:56,359 Every unsuccessful hunt 83 00:11:56,360 --> 00:11:59,160 leaves the pack weakened. 84 00:12:14,240 --> 00:12:17,320 This could be an easier meal. 85 00:12:21,360 --> 00:12:23,920 But something is not right. 86 00:12:27,280 --> 00:12:31,279 This musk ox has died from a disease. 87 00:12:33,000 --> 00:12:36,919 Outbreaks like this are becoming increasingly common 88 00:12:36,920 --> 00:12:40,160 as the climate warms across the Arctic. 89 00:12:46,640 --> 00:12:48,879 Despite their hunger, 90 00:12:48,880 --> 00:12:51,760 the pack don't eat this carcass. 91 00:13:15,680 --> 00:13:18,479 Another herd. 92 00:13:18,480 --> 00:13:20,920 Another chance for the pack. 93 00:13:38,960 --> 00:13:42,680 This time, the wolves manage to split the herd. 94 00:13:44,280 --> 00:13:46,960 That may improve their chances. 95 00:14:04,840 --> 00:14:07,839 The lead female makes sure that the oxen 96 00:14:07,840 --> 00:14:10,360 do not reach the ridge again. 97 00:14:17,000 --> 00:14:21,680 With one less defender, the calf is more vulnerable. 98 00:14:38,680 --> 00:14:41,480 A much-needed meal. 99 00:14:53,760 --> 00:14:56,759 The wolves must continue to make the most 100 00:14:56,760 --> 00:14:59,440 of the remaining weeks of summer... 101 00:15:02,680 --> 00:15:06,599 ..so when winter returns 102 00:15:06,600 --> 00:15:09,519 ..they're ready to endure 103 00:15:09,520 --> 00:15:11,919 some of the harshest conditions 104 00:15:11,920 --> 00:15:15,400 that exist on planet Earth. 105 00:15:37,320 --> 00:15:40,599 Such inhospitable extremes 106 00:15:40,600 --> 00:15:43,240 are not only found at the poles. 107 00:15:50,000 --> 00:15:52,879 As altitude increases... 108 00:15:52,880 --> 00:15:55,080 ..temperatures fall. 109 00:16:01,000 --> 00:16:06,160 Here, too, animals must make the most of the seasonal change. 110 00:16:13,880 --> 00:16:17,640 Particularly if you're ready to mate. 111 00:16:26,160 --> 00:16:28,839 A male common frog 112 00:16:28,840 --> 00:16:31,440 in the French Alps. 113 00:16:37,880 --> 00:16:43,360 He has spent all winter hibernating under the ice. 114 00:16:46,720 --> 00:16:49,720 But now it's time to wake up. 115 00:16:51,600 --> 00:16:56,040 Being cold-blooded, he needs the sun to warm his body. 116 00:16:58,160 --> 00:17:01,319 But once it sets in a few hours' time, 117 00:17:01,320 --> 00:17:03,840 he could freeze to death. 118 00:17:05,800 --> 00:17:09,720 So finding a mate is urgent. 119 00:17:14,600 --> 00:17:16,840 The weather is good. 120 00:17:20,440 --> 00:17:23,280 And he starts to sledge. 121 00:17:26,120 --> 00:17:30,320 But this is not a one-frog race. 122 00:17:31,760 --> 00:17:35,040 Dozens are rushing to the breeding pool. 123 00:17:39,800 --> 00:17:42,959 This many frogs out on the slopes... 124 00:17:42,960 --> 00:17:45,160 ..doesn't go unnoticed. 125 00:17:49,040 --> 00:17:51,360 You can't afford any slips. 126 00:17:59,760 --> 00:18:02,240 It was all going so well. 127 00:18:15,920 --> 00:18:17,799 Made it. 128 00:18:22,240 --> 00:18:25,320 But all the females have already got partners. 129 00:18:28,360 --> 00:18:31,440 There's only one thing for it. 130 00:18:34,760 --> 00:18:36,320 Wrestling. 131 00:18:52,720 --> 00:18:54,600 Not this time. 132 00:19:02,040 --> 00:19:03,960 Another latecomer. 133 00:19:05,480 --> 00:19:07,360 A female. 134 00:19:10,720 --> 00:19:12,800 Could she be the one? 135 00:19:22,960 --> 00:19:26,720 Sometimes being late can pay off. 136 00:19:50,400 --> 00:19:54,999 After all his efforts, the male is not going to let this chance 137 00:19:55,000 --> 00:19:57,360 slip through his fingers. 138 00:20:16,680 --> 00:20:21,960 This is the cedar forest of the Atlas Mountains, in North Africa. 139 00:20:27,880 --> 00:20:32,239 These Barbary macaques enjoyed a summer of plenty, 140 00:20:32,240 --> 00:20:34,240 but now they're paying the price. 141 00:20:38,480 --> 00:20:42,120 Winter here can be brutal. 142 00:20:48,280 --> 00:20:51,720 In some years, over half the group perish. 143 00:20:56,760 --> 00:20:58,799 But in this macaque society, 144 00:20:58,800 --> 00:21:02,039 the more social the monkey, 145 00:21:02,040 --> 00:21:04,880 the more likely it is to survive. 146 00:21:07,160 --> 00:21:11,880 Keeping warm by huddling can make all the difference. 147 00:21:16,120 --> 00:21:18,960 But they can't huddle all day. 148 00:21:22,680 --> 00:21:26,560 They must, eventually, break apart to feed. 149 00:21:38,000 --> 00:21:41,600 They spread out, searching for food. 150 00:22:00,200 --> 00:22:06,120 But each night, the group return to huddle in the cedar trees. 151 00:22:19,480 --> 00:22:22,000 No-one wants to be left out. 152 00:22:24,120 --> 00:22:27,000 Especially the infants. 153 00:22:37,120 --> 00:22:41,200 This baby, somehow, has got separated. 154 00:22:56,880 --> 00:23:00,199 He must get back to the life-saving warmth of the huddle, 155 00:23:00,200 --> 00:23:02,320 and soon. 156 00:23:28,640 --> 00:23:31,400 But there is an obstacle. 157 00:23:35,880 --> 00:23:40,880 A grumpy male is giving the baby a rather cold shoulder. 158 00:23:51,880 --> 00:23:55,960 Maybe a more subtle approach. 159 00:24:03,640 --> 00:24:07,680 The male decides to leave. 160 00:24:22,640 --> 00:24:25,159 At last... 161 00:24:25,160 --> 00:24:28,240 ..an enthusiastic reception. 162 00:24:37,440 --> 00:24:40,200 The male has had less luck. 163 00:24:41,440 --> 00:24:44,320 It could be a long night. 164 00:24:49,440 --> 00:24:53,879 Huddling might seem a simple expression of affection, 165 00:24:53,880 --> 00:24:57,480 but among these macaques it's essential for survival. 166 00:25:01,160 --> 00:25:03,360 And not just here. 167 00:25:08,120 --> 00:25:12,439 5,000 miles away, in the forests of Mexico, 168 00:25:12,440 --> 00:25:15,359 monarch butterflies, in millions, 169 00:25:15,360 --> 00:25:18,720 also huddle for survival. 170 00:25:23,320 --> 00:25:26,599 All of them have travelled across North America 171 00:25:26,600 --> 00:25:30,200 to escape the winter that is coming there. 172 00:25:33,120 --> 00:25:36,919 This forest, at an altitude of 3,000 metres, 173 00:25:36,920 --> 00:25:40,080 might seem to offer little comfort... 174 00:25:42,080 --> 00:25:46,159 ..for temperatures here can drop below freezing, 175 00:25:46,160 --> 00:25:49,680 and that could kill the butterflies. 176 00:25:54,560 --> 00:25:57,759 But in fact, a thin layer of warm air 177 00:25:57,760 --> 00:26:01,279 trapped by the tree canopy protects them, 178 00:26:01,280 --> 00:26:04,000 and will do so throughout the winter... 179 00:26:05,760 --> 00:26:10,160 ..provided that the forest remains undisturbed. 180 00:26:16,360 --> 00:26:19,439 Unpredictable extremes of weather, however, 181 00:26:19,440 --> 00:26:23,200 can disrupt this delicate balance. 182 00:26:26,600 --> 00:26:31,440 Strong winds can push deep into the forest, chilling the air. 183 00:26:37,240 --> 00:26:39,799 Hundreds of thousands of butterflies 184 00:26:39,800 --> 00:26:42,360 may be blown from the branches. 185 00:27:07,720 --> 00:27:14,480 In some years, storms kill 75% of hibernating monarchs. 186 00:27:31,000 --> 00:27:33,399 This one has survived... 187 00:27:33,400 --> 00:27:36,520 ..but on the ground. 188 00:27:39,800 --> 00:27:42,520 And night-time frosts are fatal. 189 00:27:43,960 --> 00:27:47,760 It can't stay here and live. 190 00:27:52,280 --> 00:27:55,399 It is too cold to fly, 191 00:27:55,400 --> 00:27:58,880 but it must get back into the tree branches somehow. 192 00:28:01,920 --> 00:28:04,440 So it starts to climb. 193 00:28:13,040 --> 00:28:18,000 It's the equivalent of a human climbing up a skyscraper. 194 00:28:49,040 --> 00:28:52,200 Safety, at last. 195 00:28:57,560 --> 00:29:00,639 As spring returns, 196 00:29:00,640 --> 00:29:04,400 the monarchs wake from their long slumber. 197 00:29:20,600 --> 00:29:26,240 And now starts one of the Earth's greatest natural spectacles. 198 00:29:58,240 --> 00:30:02,240 But this mass awakening is under threat. 199 00:30:04,400 --> 00:30:09,119 The changing climate is creating more extreme weather, 200 00:30:09,120 --> 00:30:12,200 and storms are becoming more frequent. 201 00:30:14,560 --> 00:30:19,440 But, for now, this miracle of nature continues. 202 00:30:29,480 --> 00:30:30,920 Fire. 203 00:30:34,800 --> 00:30:38,160 Unpredictable and destructive. 204 00:30:45,520 --> 00:30:49,519 This is South Africa's fynbos. 205 00:30:49,520 --> 00:30:53,200 Botanically one of the most diverse habitats on Earth... 206 00:30:54,440 --> 00:30:57,640 ..with over 8,000 different plant species. 207 00:31:00,640 --> 00:31:02,919 Yet, every two decades, 208 00:31:02,920 --> 00:31:06,160 fire reduces the vegetation to ashes. 209 00:31:13,880 --> 00:31:16,000 This may seem catastrophic. 210 00:31:17,200 --> 00:31:21,759 But just days later, plants burst into life... 211 00:31:21,760 --> 00:31:24,400 ..and transform the landscape. 212 00:31:28,200 --> 00:31:32,600 Seeds and bulbs have been waiting for this event. 213 00:31:40,200 --> 00:31:44,399 90 million years of evolution alongside fire 214 00:31:44,400 --> 00:31:47,879 has made some plants so closely adapted 215 00:31:47,880 --> 00:31:52,120 that they can't germinate without the presence of smoke. 216 00:31:59,360 --> 00:32:04,919 Remarkably, around 50% of planet Earth's terrestrial habitats 217 00:32:04,920 --> 00:32:08,240 depend on fire to remain healthy. 218 00:32:15,080 --> 00:32:19,759 In Northern Australia, these savannahs need seasonal fires 219 00:32:19,760 --> 00:32:24,040 to keep them open and prevent the spread of the nearby forest. 220 00:32:28,120 --> 00:32:31,199 And they are a vital habitat 221 00:32:31,200 --> 00:32:33,880 for this colourful resident. 222 00:32:39,520 --> 00:32:43,200 The rare golden-shouldered parrot. 223 00:32:45,520 --> 00:32:48,839 He and his lifelong partner.. 224 00:32:48,840 --> 00:32:53,639 rely on grassland for both food... 225 00:32:53,640 --> 00:32:56,000 ..and a nest site. 226 00:32:57,520 --> 00:33:02,199 As usual, they've laid their eggs in a hole in a termite mound. 227 00:33:05,800 --> 00:33:09,200 And now there are four chicks. 228 00:33:11,800 --> 00:33:16,399 The thick mud walls don't only protect chicks from predators, 229 00:33:16,400 --> 00:33:20,640 but insulate them from the heat of the scorching sun. 230 00:33:27,160 --> 00:33:30,319 With fledging just days away, 231 00:33:30,320 --> 00:33:32,880 the parents are working overtime. 232 00:33:35,680 --> 00:33:39,320 They collect thousands of seeds every day. 233 00:34:01,360 --> 00:34:04,080 But they have a problem. 234 00:34:12,040 --> 00:34:16,200 The fire season is becoming more irregular. 235 00:34:34,320 --> 00:34:37,479 If they are not ready to fly, 236 00:34:37,480 --> 00:34:40,400 chicks have no escape. 237 00:35:04,240 --> 00:35:08,960 But choosing to nest in a termite mound has a surprise benefit. 238 00:35:10,840 --> 00:35:14,600 Their home is fireproof. 239 00:35:25,320 --> 00:35:31,359 Concrete-like walls keep chicks safe from the flames outside. 240 00:35:40,360 --> 00:35:44,119 The seeds of fire grass are now easier for the parrots 241 00:35:44,120 --> 00:35:47,559 to find on the blackened earth... 242 00:35:47,560 --> 00:35:50,720 ..and they are the parrots' favourite food. 243 00:35:59,000 --> 00:36:03,760 So, for both adults and their fledgling chicks... 244 00:36:04,960 --> 00:36:08,240 ..fires are essential for survival. 245 00:36:15,680 --> 00:36:18,679 But their grassland home is shrinking, 246 00:36:18,680 --> 00:36:23,360 and only about 1,000 of these lovely parrots still survive. 247 00:36:25,280 --> 00:36:29,759 It's through the active management of fire by humans 248 00:36:29,760 --> 00:36:33,400 that their world now exists at all. 249 00:36:38,920 --> 00:36:43,639 Humans are affecting habitats across planet Earth. 250 00:36:43,640 --> 00:36:46,039 For wildlife, all too often, 251 00:36:46,040 --> 00:36:49,280 the consequences have been disastrous. 252 00:36:57,960 --> 00:37:01,879 Elephants are among the most tolerant of animals. 253 00:37:04,840 --> 00:37:07,479 They are very long-lived, 254 00:37:07,480 --> 00:37:12,079 and knowledge passed between generations enables them 255 00:37:12,080 --> 00:37:15,399 to find food and little-used sources of water in times 256 00:37:15,400 --> 00:37:18,040 of great environmental stress. 257 00:37:20,200 --> 00:37:22,519 But even among elephants, 258 00:37:22,520 --> 00:37:25,720 survival strategies are now beginning to fail. 259 00:37:32,800 --> 00:37:37,039 In Amboseli, Kenya, years of extreme weather, 260 00:37:37,040 --> 00:37:40,639 combined with overgrazing by domestic cattle, 261 00:37:40,640 --> 00:37:44,920 are turning these grasslands to dust. 262 00:38:13,480 --> 00:38:16,879 This matriarch is over 40 years old... 263 00:38:18,840 --> 00:38:20,719 ..and she's already survived 264 00:38:20,720 --> 00:38:23,720 some of East Africa's worst droughts. 265 00:38:26,880 --> 00:38:31,280 But this year, food is in extremely short supply... 266 00:38:34,200 --> 00:38:37,440 ..and her family are struggling to find enough. 267 00:38:44,920 --> 00:38:49,040 Her youngest son is malnourished and weak... 268 00:38:54,160 --> 00:38:57,560 ..and is lagging behind his mother and brother. 269 00:39:06,400 --> 00:39:11,160 She leads them to a pocket of acacia forest that she knows well. 270 00:39:17,360 --> 00:39:20,639 But broken branches and fallen trees 271 00:39:20,640 --> 00:39:24,160 show that other elephants have already been here. 272 00:39:29,000 --> 00:39:31,760 There is very little left worth eating. 273 00:39:38,840 --> 00:39:43,640 Without food, his mother cannot produce enough milk. 274 00:39:47,920 --> 00:39:49,680 She has no choice... 275 00:39:52,720 --> 00:39:55,280 ..but to leave her weakening calf... 276 00:39:56,920 --> 00:39:59,960 ..and try to find food elsewhere. 277 00:40:06,240 --> 00:40:12,399 So, she and her elder son head off... 278 00:40:12,400 --> 00:40:14,680 ..without him. 279 00:40:38,000 --> 00:40:41,480 He can only hope for their return. 280 00:41:15,480 --> 00:41:20,080 But for him, time has run out. 281 00:41:55,160 --> 00:41:58,479 There is nothing his mother, or anybody, 282 00:41:58,480 --> 00:42:01,279 could have done to save him. 283 00:42:12,880 --> 00:42:16,039 With her elder son still alive, 284 00:42:16,040 --> 00:42:18,720 there is hope for her family. 285 00:42:20,560 --> 00:42:24,800 But only if they can adapt quickly. 286 00:42:35,160 --> 00:42:38,800 Some families are proving to be resilient. 287 00:42:43,840 --> 00:42:47,640 They're travelling long distances, away from Amboseli... 288 00:42:50,840 --> 00:42:55,160 ..and finding new sources of food and water. 289 00:43:12,600 --> 00:43:15,600 And despite the drought... 290 00:43:19,440 --> 00:43:22,440 ..calves are being born. 291 00:43:41,240 --> 00:43:44,239 But given the pace of change 292 00:43:44,240 --> 00:43:46,600 and the challenges they face... 293 00:43:47,960 --> 00:43:52,080 ..how many of Africa's elephants will be able to adapt? 294 00:43:53,120 --> 00:43:56,560 And can they do so fast enough? 295 00:44:10,960 --> 00:44:13,200 The Gobi Desert. 296 00:44:15,760 --> 00:44:19,840 A land of sand and snow. 297 00:44:24,360 --> 00:44:28,279 For at least a million years, this has been 298 00:44:28,280 --> 00:44:32,240 one of the most extreme habitats on Earth. 299 00:44:36,720 --> 00:44:39,559 Yet, footprints in the sand 300 00:44:39,560 --> 00:44:43,040 reveal that there is life here. 301 00:44:49,560 --> 00:44:51,479 A snow leopard. 302 00:44:51,480 --> 00:44:54,040 Seldom seen... 303 00:44:55,360 --> 00:45:00,480 ..especially in such hot, arid conditions. 304 00:45:04,560 --> 00:45:08,839 They live up to 2,000 metres above sea level, 305 00:45:08,840 --> 00:45:12,880 and patrol vast territories. 306 00:45:41,320 --> 00:45:44,280 They are rarely seen together. 307 00:45:52,120 --> 00:45:53,520 A mother... 308 00:45:56,720 --> 00:45:59,080 ..with cubs. 309 00:46:01,280 --> 00:46:04,119 Filmed here for the first time 310 00:46:04,120 --> 00:46:06,920 in such intimate detail. 311 00:46:14,960 --> 00:46:18,360 And they give us a glimpse of their family life... 312 00:46:19,400 --> 00:46:21,360 ..such as it is. 313 00:46:37,920 --> 00:46:41,279 Snow leopard cubs stay with their mothers 314 00:46:41,280 --> 00:46:45,160 longer than those of almost any other big cat. 315 00:46:50,960 --> 00:46:54,960 They remain together for over two years... 316 00:46:57,840 --> 00:47:02,640 ..as they learn how to master the extremes. 317 00:47:10,040 --> 00:47:14,439 Which they do so well that they are able to thrive 318 00:47:14,440 --> 00:47:18,399 in some of the hottest, coldest... 319 00:47:18,400 --> 00:47:21,280 ..and highest places on Earth. 320 00:47:32,600 --> 00:47:36,479 Some animals, over millions of years, 321 00:47:36,480 --> 00:47:40,640 have evolved ways of surviving extremely hostile conditions. 322 00:47:43,680 --> 00:47:47,999 But as the extremes become more extreme... 323 00:47:48,000 --> 00:47:52,200 ..evolution is being outpaced. 324 00:48:00,120 --> 00:48:04,479 These extraordinary places are still inhabited 325 00:48:04,480 --> 00:48:06,919 by some of Earth's most resilient 326 00:48:06,920 --> 00:48:09,080 and adaptable species. 327 00:48:12,280 --> 00:48:14,600 But in our rapidly changing world... 328 00:48:16,080 --> 00:48:21,040 ..this is planet Earth as you may never see it again. 329 00:48:44,640 --> 00:48:47,559 The most extreme parts of our planet 330 00:48:47,560 --> 00:48:50,200 are not always found where you might expect. 331 00:48:53,160 --> 00:48:57,159 Hidden deep in the forests of Vietnam, a local man, 332 00:48:57,160 --> 00:49:00,480 Mr Ho Khanh, made an extraordinary discovery. 333 00:49:20,080 --> 00:49:22,079 He didn't know it at the time, 334 00:49:22,080 --> 00:49:26,480 but he had stumbled upon arguably the largest cave in the world. 335 00:49:38,720 --> 00:49:42,519 Inspired by Mr Ho Khanh's discovery, 336 00:49:42,520 --> 00:49:46,640 the Planet Earth team set out to film Hang Son Doong. 337 00:49:48,720 --> 00:49:50,840 But first, they have to get there. 338 00:49:52,240 --> 00:49:54,519 We've left the river valley, 339 00:49:54,520 --> 00:49:56,959 and we're walking uphill now to the entrance. 340 00:49:56,960 --> 00:49:59,000 It's just thick, thick forest. 341 00:50:00,720 --> 00:50:03,959 After trekking for two gruelling days, 342 00:50:03,960 --> 00:50:06,799 the team finally arrive. 343 00:50:06,800 --> 00:50:10,159 Down here is the entrance to Hang Son Doong, 344 00:50:10,160 --> 00:50:13,719 this tiny opening in this massive jungle. 345 00:50:13,720 --> 00:50:16,680 It's amazing that anyone ever found this place. 346 00:50:19,760 --> 00:50:22,519 We're about to go in for the next 18 days, 347 00:50:22,520 --> 00:50:25,120 and it's lights on from here on out. 348 00:50:26,800 --> 00:50:30,280 Local experts guide the team through the darkness. 349 00:50:31,520 --> 00:50:33,960 OK, I can go? Yes, yes. 350 00:50:43,600 --> 00:50:45,159 I'm good. I'm OK. 351 00:50:45,160 --> 00:50:46,840 It's just really slippery. 352 00:50:48,800 --> 00:50:54,320 A complex system of ropes leads them down a 90-metre descent. 353 00:50:59,520 --> 00:51:01,079 OK. 354 00:51:01,080 --> 00:51:02,360 Thanks. 355 00:51:06,280 --> 00:51:09,079 After four hours clambering over rocks... 356 00:51:11,080 --> 00:51:14,000 ...they finally reach what will be their camp. 357 00:51:22,760 --> 00:51:25,959 The team wake to a new day - 358 00:51:25,960 --> 00:51:27,919 but in darkness. 359 00:51:27,920 --> 00:51:29,679 How did you sleep, Luke? 360 00:51:29,680 --> 00:51:31,759 Quite tired. 361 00:51:31,760 --> 00:51:34,079 These are all of the tents behind me. 362 00:51:34,080 --> 00:51:37,639 You can see they are pretty... 363 00:51:37,640 --> 00:51:39,079 ..pretty snug. 364 00:51:39,080 --> 00:51:42,319 This is where we have our breakfast every day, 365 00:51:42,320 --> 00:51:44,600 made by some amazing cooks. 366 00:51:46,040 --> 00:51:48,199 This is our charging station. 367 00:51:48,200 --> 00:51:50,599 Batteries for camera equipment, batteries for our lights, 368 00:51:50,600 --> 00:51:52,760 our head torches, which are so important. 369 00:51:53,800 --> 00:51:56,639 This is our water station. 370 00:51:56,640 --> 00:51:59,879 Everything that we have got has been brought in by someone, 371 00:51:59,880 --> 00:52:02,679 so you can't shower or wash. 372 00:52:02,680 --> 00:52:04,959 And it's not just water. 373 00:52:04,960 --> 00:52:09,639 Tonnes of equipment has had to be carried into the cave, 374 00:52:09,640 --> 00:52:11,960 every inch of the way. 375 00:52:15,120 --> 00:52:18,519 Now deep underground, the powerful lights 376 00:52:18,520 --> 00:52:20,840 reveal just how vast this cavern is. 377 00:52:22,280 --> 00:52:23,959 That is amazing. 378 00:52:23,960 --> 00:52:25,399 Look at that. It's... 379 00:52:25,400 --> 00:52:27,200 It's huge! 380 00:52:29,640 --> 00:52:31,919 As you turn on each of the lights, 381 00:52:31,920 --> 00:52:34,760 another part of this enormous chamber is revealed. 382 00:52:37,480 --> 00:52:40,480 It's magic. Absolutely magic. 383 00:52:43,680 --> 00:52:46,799 To try and capture the true scale, 384 00:52:46,800 --> 00:52:50,680 the team attempt a filming technique never tried before. 385 00:52:51,920 --> 00:52:55,239 We're trying to do a little bit of a dance with the drones, 386 00:52:55,240 --> 00:52:59,359 whereby we have one drone sort of slightly below the other, 387 00:52:59,360 --> 00:53:03,479 one filming the other one lighting a feature in the cave. 388 00:53:03,480 --> 00:53:05,799 It's a really tight space. 389 00:53:05,800 --> 00:53:07,879 You can't see anything. 390 00:53:07,880 --> 00:53:09,799 What could go wrong? 391 00:53:13,520 --> 00:53:15,320 OK, slow down a bit. 392 00:53:16,920 --> 00:53:19,000 It's making everyone a little nervous. 393 00:53:20,120 --> 00:53:22,720 Yeah, don't go up any higher, Luke. 394 00:53:25,080 --> 00:53:27,400 OK, that's a nice height. 395 00:53:31,000 --> 00:53:34,280 With one drone lighting and one drone filming... 396 00:53:36,080 --> 00:53:41,160 ..they reveal the wonder of some of the world's largest stalagmites. 397 00:53:50,080 --> 00:53:53,199 As the days pass, the team settle into 398 00:53:53,200 --> 00:53:56,440 the everyday routine of life underground. 399 00:53:57,640 --> 00:54:01,879 This is the epitome of being off-grid. 400 00:54:01,880 --> 00:54:03,999 We call that one seesaw loo 401 00:54:04,000 --> 00:54:06,519 because it's kind of on a rock, 402 00:54:06,520 --> 00:54:09,159 and every time you sit on it and lean back, 403 00:54:09,160 --> 00:54:13,080 you're scared that there's going to be a disaster. 404 00:54:14,720 --> 00:54:18,159 The strangest thing is having no connection 405 00:54:18,160 --> 00:54:20,239 to the outside world whatsoever. 406 00:54:20,240 --> 00:54:23,720 Time is warped. Nothing makes sense underground. 407 00:54:28,320 --> 00:54:32,479 Without daylight to guide them, the team work round the clock 408 00:54:32,480 --> 00:54:35,480 to capture the cave's most unique features. 409 00:54:37,160 --> 00:54:39,439 Most special of all, 410 00:54:39,440 --> 00:54:43,080 an event which happens only a few weeks a year. 411 00:54:45,800 --> 00:54:48,239 To get there, they must navigate 412 00:54:48,240 --> 00:54:50,920 through boulder fields and tight squeezes. 413 00:55:00,400 --> 00:55:03,039 Here, around half a million years ago, 414 00:55:03,040 --> 00:55:05,279 the cave roof collapsed, 415 00:55:05,280 --> 00:55:08,000 opening a window to the outside world. 416 00:55:12,400 --> 00:55:15,280 The team wait for just the right conditions. 417 00:55:16,640 --> 00:55:19,239 We've been sat here for hours. 418 00:55:19,240 --> 00:55:21,799 And finally, just on the right-hand side of the wall, 419 00:55:21,800 --> 00:55:24,320 we can see the first signs of light. 420 00:55:28,240 --> 00:55:30,599 After weeks of darkness 421 00:55:30,600 --> 00:55:32,480 200 metres underground... 422 00:55:33,800 --> 00:55:35,639 ..they see sunlight 423 00:55:35,640 --> 00:55:38,000 illuminate the cave floor. 424 00:55:43,040 --> 00:55:47,879 The way it just lights up this massive cave... 425 00:55:47,880 --> 00:55:51,279 ..when we've been trying to do it with about four or five lights 426 00:55:51,280 --> 00:55:54,719 and barely even lighting the smallest section. 427 00:55:54,720 --> 00:55:56,639 When the sun comes in, it just... 428 00:56:00,320 --> 00:56:02,360 I think everyone's got goose bumps. 429 00:56:06,680 --> 00:56:11,480 I don't think I've ever appreciated sun so much in my life. 430 00:56:13,080 --> 00:56:15,120 It's incredible. 431 00:56:17,840 --> 00:56:20,879 Look at the backlit waterfalls. 432 00:56:20,880 --> 00:56:23,519 They're so slow coming down. 433 00:56:23,520 --> 00:56:26,320 It's literally one of the best things I've ever seen. 434 00:56:29,840 --> 00:56:33,759 For the crew, the sunbeam marks a profound end 435 00:56:33,760 --> 00:56:36,080 to their time in the cave. 436 00:56:39,360 --> 00:56:41,639 We're outside! 437 00:56:41,640 --> 00:56:44,639 It's so green, it's so bright. 438 00:56:44,640 --> 00:56:48,600 Being back above ground feels like where we belong. 439 00:56:50,560 --> 00:56:53,879 It's a wonder to imagine what other extremes 440 00:56:53,880 --> 00:56:58,280 still lie beneath the surface of planet Earth. 441 00:57:06,720 --> 00:57:09,039 Next time... 442 00:57:09,040 --> 00:57:11,280 ..the human world... 443 00:57:16,920 --> 00:57:21,840 ..where animals must find new ways to survive... 444 00:57:22,960 --> 00:57:24,920 ..amongst us. 445 00:57:31,600 --> 00:57:35,319 Habitat Explorer brings animals and their habitats to life. 446 00:57:35,320 --> 00:57:38,639 Explore this free interactive, and make origami animals. 447 00:57:38,640 --> 00:57:42,519 Go to bbc.co.uk/planetearth3 448 00:57:42,520 --> 00:57:45,079 and follow the links to the Open University. 449 00:57:45,080 --> 00:57:48,559 Or to order a free printed version, visit the website 450 00:57:48,560 --> 00:57:51,040 or call the numbers on the screen.