1 00:00:04,260 --> 00:00:06,060 In the heart of the Americas... 2 00:00:07,220 --> 00:00:08,900 ..where continents collide... 3 00:00:11,020 --> 00:00:13,900 ..there's a land full of natural riches... 4 00:00:19,700 --> 00:00:21,340 BIRDS HONK 5 00:00:25,420 --> 00:00:26,900 MONKEY GROWLS 6 00:00:33,900 --> 00:00:35,940 ..a land of towering giants... 7 00:00:39,020 --> 00:00:40,220 ..scorching sands... 8 00:00:43,780 --> 00:00:45,260 ..and secret rivers. 9 00:00:52,820 --> 00:00:55,100 Where great civilisations rose... 10 00:00:56,700 --> 00:00:57,980 ..and fell. 11 00:01:02,580 --> 00:01:05,420 To succeed here takes passion and spirit. 12 00:01:11,540 --> 00:01:14,540 This is a country rich in colour and culture... 13 00:01:19,620 --> 00:01:21,460 ..a festival of life. 14 00:01:34,940 --> 00:01:37,100 This is... 15 00:02:06,300 --> 00:02:10,820 In Mexico's far south there's a forest full of secrets. 16 00:02:16,060 --> 00:02:19,340 It's March, the peak of the dry season. 17 00:02:21,740 --> 00:02:24,020 This pool is fast disappearing... 18 00:02:25,380 --> 00:02:27,820 ..and it draws all the animals from the forest. 19 00:02:40,460 --> 00:02:42,220 Even the most elusive. 20 00:03:09,740 --> 00:03:14,220 For a young Morelet crocodile, the pool is also a hunting ground. 21 00:03:25,180 --> 00:03:27,300 There's plenty of potential prey. 22 00:03:29,460 --> 00:03:33,060 Trouble is, most are far too big. 23 00:03:47,940 --> 00:03:50,140 Finally, a realistic target. 24 00:04:10,900 --> 00:04:16,060 Even at his size, sneaking up in such shallow water isn't easy. 25 00:04:22,140 --> 00:04:24,020 He needs to change his technique. 26 00:04:29,620 --> 00:04:31,180 Perhaps an ambush will work. 27 00:04:51,500 --> 00:04:52,700 Almost. 28 00:05:21,540 --> 00:05:22,940 Success at last. 29 00:05:25,900 --> 00:05:28,460 But he's not the only crocodile here. 30 00:05:44,540 --> 00:05:47,660 And his taste of victory is short-lived. 31 00:05:54,820 --> 00:05:58,020 There's a reason water is so scarce in this corner of Mexico. 32 00:06:03,220 --> 00:06:08,260 This great forest, which stretches for almost 50,000 square miles, 33 00:06:08,260 --> 00:06:09,580 has no major rivers. 34 00:06:14,580 --> 00:06:17,100 And that's down to the region's unique geology. 35 00:06:22,380 --> 00:06:26,620 The forest stands upon a vast peninsula, the Yucatan. 36 00:06:33,300 --> 00:06:36,740 The entire peninsula is a gigantic slab of limestone. 37 00:06:40,540 --> 00:06:42,780 Limestone is incredibly porous. 38 00:06:45,380 --> 00:06:49,300 Any surface water swiftly drains away underground. 39 00:06:51,260 --> 00:06:53,980 And this shapes the lives of all who live here. 40 00:07:12,580 --> 00:07:14,700 For seven months of the year, 41 00:07:14,700 --> 00:07:17,580 virtually no rain falls, 42 00:07:17,580 --> 00:07:20,180 and by April, much of the forest is barren. 43 00:07:22,460 --> 00:07:25,540 For a mother spider monkey this can be challenging. 44 00:07:28,340 --> 00:07:31,420 She needs to produce enough milk to feed her baby. 45 00:07:39,100 --> 00:07:41,420 And that requires plenty of fresh fruit. 46 00:07:48,220 --> 00:07:52,500 Sometimes, that means travelling over two miles each day. 47 00:07:56,340 --> 00:08:00,860 But with long, slender fingers and no thumbs to get in the way 48 00:08:00,860 --> 00:08:03,180 this is child's play for an animal 49 00:08:03,180 --> 00:08:05,100 designed for life in the tree tops. 50 00:08:24,540 --> 00:08:27,140 She's taking her youngster to a special place... 51 00:08:28,860 --> 00:08:32,100 ..where she knows there's fruit all year round. 52 00:08:48,220 --> 00:08:51,540 The ancient city of Calakmul, 53 00:08:51,540 --> 00:08:55,260 once one of the most powerful in the Maya civilisation. 54 00:09:00,580 --> 00:09:04,660 In the seventh century, this city was home to 50,000 people. 55 00:09:12,900 --> 00:09:14,780 The people of Calakmul 56 00:09:14,780 --> 00:09:17,580 cultivated trees that would fruit throughout the year. 57 00:09:23,740 --> 00:09:28,340 Centuries later, these ancient gardens have left a lasting legacy. 58 00:09:40,820 --> 00:09:44,220 There's more fruit here than anywhere else in the forest... 59 00:09:45,540 --> 00:09:47,660 ..even at the height of the dry season. 60 00:09:51,500 --> 00:09:53,820 Over countless generations, 61 00:09:53,820 --> 00:09:56,900 the monkeys have passed this knowledge on to their youngsters. 62 00:10:03,260 --> 00:10:04,900 MONKEY CALLS 63 00:10:36,020 --> 00:10:38,380 At its peak in the eighth century, 64 00:10:38,380 --> 00:10:42,780 the Maya civilisation grew to almost 13 million people. 65 00:10:50,580 --> 00:10:57,020 Theirs was a highly sophisticated culture, advanced in mathematics, 66 00:10:57,020 --> 00:10:59,380 language and astronomy. 67 00:11:02,620 --> 00:11:06,580 The huge limestone temples, built to worship their gods, 68 00:11:06,580 --> 00:11:10,660 are a lasting testament to great feats of engineering. 69 00:11:16,380 --> 00:11:20,100 Almost every other ancient civilisation in history 70 00:11:20,100 --> 00:11:22,700 has been built beside a major river. 71 00:11:24,380 --> 00:11:27,420 So how did the Maya manage without one? 72 00:11:31,020 --> 00:11:34,340 There are almost 1.5 million direct descendants 73 00:11:34,340 --> 00:11:36,940 of the ancient Maya living in Mexico today. 74 00:11:39,740 --> 00:11:43,980 Don Roque and his wife Dona Su live on an isolated farm 75 00:11:43,980 --> 00:11:46,060 in the heart of the Yucatan's forest. 76 00:11:49,700 --> 00:11:52,180 GOAT BLEATS 77 00:12:05,860 --> 00:12:09,020 This farm has been in Don Roque's family for generations. 78 00:12:12,180 --> 00:12:15,140 With his children grown up and left home, 79 00:12:15,140 --> 00:12:17,740 nowadays, it's just him and Dona Su. 80 00:12:23,940 --> 00:12:26,660 They live off the land in a way 81 00:12:26,660 --> 00:12:29,860 little changed since the times of their ancestors. 82 00:12:39,540 --> 00:12:42,540 But there's more to their farm than meets the eye... 83 00:13:02,580 --> 00:13:08,060 ..a huge natural well, known in the Yucatan as a cenote. 84 00:13:18,780 --> 00:13:21,100 A cenote is a collapsed cave. 85 00:13:23,940 --> 00:13:27,060 Over thousands of years, rainwater has eroded 86 00:13:27,060 --> 00:13:30,500 the soft limestone on the surface. 87 00:13:30,500 --> 00:13:33,140 Eventually, the roof weakened and fell. 88 00:13:42,300 --> 00:13:45,900 This cenote provides Don Roque with water all year round. 89 00:13:55,060 --> 00:13:56,740 But it's far from unique. 90 00:14:03,220 --> 00:14:07,300 It's one of several thousand cenotes scattered across the peninsula. 91 00:14:12,380 --> 00:14:16,300 Cenote comes from the Maya word ts'onot, 92 00:14:16,300 --> 00:14:18,620 meaning holes with water. 93 00:14:25,220 --> 00:14:27,380 It was these mysterious pools 94 00:14:27,380 --> 00:14:30,140 that enabled the Maya civilisation 95 00:14:30,140 --> 00:14:32,260 to flourish without a major river. 96 00:14:39,180 --> 00:14:42,580 And the greatest ancient cities were founded beside them. 97 00:14:49,380 --> 00:14:51,580 Each cenote is an oasis. 98 00:14:56,540 --> 00:14:59,660 The surrounding forest remains lush, 99 00:14:59,660 --> 00:15:01,700 even at the height of the dry season. 100 00:15:07,500 --> 00:15:11,100 Filtered through the limestone, the water is full of minerals. 101 00:15:14,700 --> 00:15:18,140 It supports a unique community of plants and animals. 102 00:15:22,100 --> 00:15:26,700 Some fish in the Yucatan cenotes are found nowhere else on Earth. 103 00:15:34,620 --> 00:15:36,660 Within the dense forest, 104 00:15:36,660 --> 00:15:39,580 a cenote can remain hidden, even when close by. 105 00:15:43,580 --> 00:15:47,020 But the Maya had an almost sure-fire way of finding them... 106 00:15:49,300 --> 00:15:50,460 BIRD CALLS 107 00:15:52,300 --> 00:15:55,980 ..the distinctive call of the turquoise-browed motmot. 108 00:16:02,460 --> 00:16:05,300 Virtually every cenote has its own colony. 109 00:16:08,500 --> 00:16:11,580 Elsewhere, motmots prefer to nest alone, 110 00:16:11,580 --> 00:16:14,260 digging their burrows along river banks. 111 00:16:15,700 --> 00:16:17,180 MOTMOT CALLS 112 00:16:18,340 --> 00:16:21,660 Here in the Yucatan, they're forced together, 113 00:16:21,660 --> 00:16:25,740 sharing limited space on the cenote's soft limestone walls. 114 00:16:30,140 --> 00:16:33,340 Before a male can start looking for a mate, 115 00:16:33,340 --> 00:16:35,100 he needs to secure a nest site. 116 00:16:37,580 --> 00:16:39,820 But this cenote is already crowded. 117 00:16:39,820 --> 00:16:42,220 17 other pairs are nesting here. 118 00:16:48,980 --> 00:16:53,380 If he's to fit in, this new arrival must use the right body language. 119 00:16:56,140 --> 00:16:57,700 MOTMOT CALLS 120 00:17:06,300 --> 00:17:10,860 A raised turquoise brow is intended to intimidate rivals. 121 00:17:28,180 --> 00:17:30,820 The pendulous tick-tock of their tails 122 00:17:30,820 --> 00:17:34,100 has earned motmots the nickname clock birds. 123 00:17:41,020 --> 00:17:44,580 This male must persevere if he wants to breed this year. 124 00:17:52,700 --> 00:17:56,980 At last, he's earned his own little ledge of limestone. 125 00:18:03,100 --> 00:18:05,500 With a gift of food, 126 00:18:05,500 --> 00:18:08,540 he can get down to the business of attracting a mate. 127 00:18:19,100 --> 00:18:21,780 There are so many cenotes in the Yucatan 128 00:18:21,780 --> 00:18:23,820 because of a dramatic event 129 00:18:23,820 --> 00:18:26,060 that changed the history of life on Earth. 130 00:18:38,220 --> 00:18:40,900 Around 65 million years ago, 131 00:18:40,900 --> 00:18:45,740 a huge meteorite collided with what is now the Yucatan's north coast. 132 00:18:48,980 --> 00:18:51,220 Its effect was so catastrophic... 133 00:18:52,420 --> 00:18:54,900 ..it's believed to have wiped out the dinosaurs. 134 00:18:56,740 --> 00:19:01,220 In an instant, the impact fractured the limestone platform, 135 00:19:01,220 --> 00:19:04,820 creating what has become known as the ring of cenotes. 136 00:19:06,220 --> 00:19:10,540 Satellites have revealed over 200 cluster around its crater. 137 00:19:15,220 --> 00:19:19,260 Satellite technology is also helping modern-day explorers in the Yucatan. 138 00:19:22,140 --> 00:19:26,020 And new cenotes are still being discovered today. 139 00:19:26,020 --> 00:19:29,980 It should be about 50 metres that way. You see anything? 140 00:19:29,980 --> 00:19:34,500 Sam Meacham and Fred Devos are part of an international community 141 00:19:34,500 --> 00:19:37,940 of cave divers that has been exploring the Yucatan 142 00:19:37,940 --> 00:19:39,340 since the 1980s. 143 00:19:41,300 --> 00:19:45,700 I started out for the thrill of being able to explore something. 144 00:19:45,700 --> 00:19:48,740 And from a young age, I just was imbued 145 00:19:48,740 --> 00:19:50,980 with the desire to go out and explore this world. 146 00:19:52,340 --> 00:19:55,100 Each new cenote adds another piece to the puzzle. 147 00:19:58,460 --> 00:20:01,260 Oh, look at this, Sam. Wow. 148 00:20:01,260 --> 00:20:04,780 The moment of discovery, right? You can't beat it. 149 00:20:16,180 --> 00:20:20,220 In the last 30 years, cave divers in the Yucatan 150 00:20:20,220 --> 00:20:22,820 have revealed something extraordinary. 151 00:20:26,140 --> 00:20:29,660 The cenotes are not isolated wells. 152 00:20:34,260 --> 00:20:38,700 Each is connected by an intricate network of caves and passageways, 153 00:20:38,700 --> 00:20:42,580 spreading like a spider's web across the peninsula. 154 00:20:49,780 --> 00:20:55,260 Divers have discovered over 350 caves 155 00:20:55,260 --> 00:20:59,180 and nearly 1,000 miles of flooded tunnels. 156 00:21:00,980 --> 00:21:05,500 Including the two longest underwater cave systems on the planet. 157 00:21:36,820 --> 00:21:41,860 Many of these caverns have seen fewer visitors than the moon. 158 00:21:44,260 --> 00:21:47,140 The great majority remains uncharted. 159 00:22:09,300 --> 00:22:12,820 For the ancient Maya, the labyrinth of caves 160 00:22:12,820 --> 00:22:16,180 was out of reach but not beyond imagination. 161 00:22:18,180 --> 00:22:21,140 This secret underworld was held sacred. 162 00:22:22,180 --> 00:22:25,260 Home to the 12 gods of Xibalba, 163 00:22:25,260 --> 00:22:28,380 it was a place both feared and revered. 164 00:22:33,660 --> 00:22:37,940 But the ancient Maya recognised their link to this unknown world. 165 00:22:38,940 --> 00:22:40,540 Trees. 166 00:22:47,220 --> 00:22:50,220 Always plumbed into the water below, 167 00:22:50,220 --> 00:22:53,380 trees allow life above ground to flourish. 168 00:22:57,460 --> 00:23:02,100 No tree is better equipped to do this than the strangler fig. 169 00:23:03,340 --> 00:23:06,340 Its roots can reach 30 metres into the water below. 170 00:23:25,940 --> 00:23:30,100 Don Roque uses the fig's roots to enter the underworld... 171 00:23:31,780 --> 00:23:35,980 ..navigating his way down steps carved by his grandfather 172 00:23:35,980 --> 00:23:38,140 over half a century ago. 173 00:24:06,260 --> 00:24:10,340 At the bottom of the cenote, Don Roque has a secret garden. 174 00:24:16,980 --> 00:24:20,260 Here he cultivates a plant that wouldn't survive 175 00:24:20,260 --> 00:24:22,420 elsewhere on his farm. 176 00:24:22,420 --> 00:24:24,140 Coffee. 177 00:24:29,660 --> 00:24:33,900 Above ground it's too hot and dry for coffee to grow. 178 00:24:35,780 --> 00:24:39,820 Down here it's two degrees cooler and a lot more humid. 179 00:24:44,260 --> 00:24:48,260 Don Roque's cenote is also a haven for insects. 180 00:24:48,260 --> 00:24:51,500 Food for a colony of resident cave swallows. 181 00:25:10,940 --> 00:25:14,620 By May the swallows are intent on one thing. 182 00:25:16,180 --> 00:25:17,540 Breeding. 183 00:25:24,860 --> 00:25:27,740 The cenote provides everything they need. 184 00:25:34,260 --> 00:25:38,100 Down here they have a ready supply of soft mud. 185 00:25:42,740 --> 00:25:46,020 They use this to craft their nests 186 00:25:46,020 --> 00:25:50,140 together with fibres plucked from the hanging fig roots. 187 00:26:05,940 --> 00:26:09,180 All being well, in around a month's time, 188 00:26:09,180 --> 00:26:12,140 they'll each have a brood of up to five chicks. 189 00:26:19,100 --> 00:26:22,820 Don Roque's cenote has its own community of life 190 00:26:22,820 --> 00:26:25,460 thanks to the constant presence of water. 191 00:26:25,460 --> 00:26:29,100 But the rest of his farm requires something more. 192 00:26:30,100 --> 00:26:33,700 He plants his land with a mixture of corn, beans and squash 193 00:26:33,700 --> 00:26:36,380 in a tradition passed down the generations. 194 00:26:40,220 --> 00:26:43,780 A mix of crops helps maintain the fragile soil 195 00:26:43,780 --> 00:26:47,060 which lies in a thin layer on top of the limestone. 196 00:26:49,500 --> 00:26:52,820 But he can't draw enough water from his cenote 197 00:26:52,820 --> 00:26:55,580 to irrigate an entire field. 198 00:26:57,500 --> 00:27:01,180 For his crops to grow, Don Roque needs it to rain. 199 00:27:06,060 --> 00:27:08,860 By late May, change is in the air. 200 00:27:13,420 --> 00:27:16,220 The Yucatan relies on weather systems 201 00:27:16,220 --> 00:27:18,660 that develop thousands of miles away. 202 00:27:22,700 --> 00:27:26,180 Gathering ever more water as they cross the Atlantic, 203 00:27:26,180 --> 00:27:28,180 they bring the seasonal rains. 204 00:27:34,940 --> 00:27:39,140 The ancient Maya believed the rain was a gift from the gods. 205 00:27:43,660 --> 00:27:48,180 For Don Roque, the old gods are very much alive. 206 00:27:48,180 --> 00:27:50,260 RUMBLE OF THUNDER 207 00:27:57,660 --> 00:28:03,140 The most important of all is Chaac, the life-giving god of rain. 208 00:28:28,940 --> 00:28:32,660 It is Chaac who determines each thundercloud. 209 00:28:32,660 --> 00:28:34,620 CRACK OF LIGHTNING 210 00:28:39,260 --> 00:28:41,300 RUMBLE OF THUNDER 211 00:28:45,180 --> 00:28:48,100 Finally letting the rain fall. 212 00:28:56,980 --> 00:29:01,380 1,200 years ago, something happened that sent the Maya civilisation 213 00:29:01,380 --> 00:29:03,660 into a dramatic decline. 214 00:29:05,060 --> 00:29:07,820 Chaac withheld the seasonal rains. 215 00:29:10,700 --> 00:29:15,460 The Yucatan was gripped by a series of devastating droughts. 216 00:29:21,140 --> 00:29:23,780 Over 80% of the population vanished. 217 00:29:32,860 --> 00:29:36,660 One by one, the bustling cities were abandoned 218 00:29:36,660 --> 00:29:39,180 and reclaimed by the forest. 219 00:29:44,620 --> 00:29:46,700 MONKEYS HOWLING 220 00:29:46,700 --> 00:29:49,940 This year the gods have been kind. 221 00:29:49,940 --> 00:29:53,660 By July, it's raining almost every day. 222 00:29:54,860 --> 00:29:57,100 MONKEYS GRUNT AND HOWL 223 00:29:59,940 --> 00:30:03,380 This brings new growth to the forest. 224 00:30:03,380 --> 00:30:07,260 Good news for the Yucatan's noisiest residents. 225 00:30:08,540 --> 00:30:10,500 LOUD HOWL 226 00:30:16,540 --> 00:30:18,340 Howler monkeys. 227 00:30:22,140 --> 00:30:24,580 Their calls carry for over a mile. 228 00:30:25,580 --> 00:30:27,580 MONKEYS HOWL 229 00:30:29,220 --> 00:30:33,060 They howl to let neighbouring groups know their whereabouts. 230 00:30:43,220 --> 00:30:46,460 This patch of the forest is worth protecting. 231 00:30:48,980 --> 00:30:52,380 The troop has found a tree with young, succulent leaves - 232 00:30:52,380 --> 00:30:54,540 a particular favourite. 233 00:31:04,940 --> 00:31:08,100 But a diet of leaves takes time to digest. 234 00:31:13,940 --> 00:31:18,140 So howlers spend up to 80% of their time resting... 235 00:31:19,180 --> 00:31:22,860 ..making them amongst the most peaceful of all monkeys 236 00:31:22,860 --> 00:31:25,340 despite their fearsome calls. 237 00:31:29,740 --> 00:31:34,100 Baby howlers feed on their mother's milk until almost a year old. 238 00:31:35,180 --> 00:31:37,220 This is much easier to digest. 239 00:31:52,700 --> 00:31:56,460 With his mother trying to sleep off her lunch, 240 00:31:56,460 --> 00:32:00,100 nap time can be a bit dull for a baby. 241 00:32:15,220 --> 00:32:17,260 RUMBLE OF THUNDER 242 00:32:23,180 --> 00:32:26,980 All life in the Yucatan benefits from the rain. 243 00:32:37,180 --> 00:32:40,020 Trees burst into fruit. 244 00:32:46,100 --> 00:32:48,260 All the new plant growth 245 00:32:48,260 --> 00:32:51,140 provides food for an explosion of insect life. 246 00:32:52,220 --> 00:32:56,140 There are over 3,000 different species on the peninsula. 247 00:33:02,940 --> 00:33:05,180 The insects become food for others. 248 00:33:12,940 --> 00:33:17,380 Almost all the birds in the forest schedule their nesting to coincide 249 00:33:17,380 --> 00:33:19,380 with this seasonal bounty. 250 00:33:38,700 --> 00:33:42,140 The regular rain is also good news for Don Roque. 251 00:33:45,220 --> 00:33:48,820 In just six weeks his corn has shot up. 252 00:34:11,460 --> 00:34:14,660 But the explosion of plant-eating insects 253 00:34:14,660 --> 00:34:16,740 could cause Don Roque problems. 254 00:34:19,740 --> 00:34:25,020 For his crops to thrive, he needs the help of a natural ally. 255 00:34:38,180 --> 00:34:40,660 In the heart of the Yucatan, 256 00:34:40,660 --> 00:34:46,900 there's a cave the locals call El Volcan de los Murcielagos. 257 00:34:48,460 --> 00:34:50,380 The bat volcano. 258 00:35:00,500 --> 00:35:03,380 Every day, an hour before dusk, 259 00:35:03,380 --> 00:35:05,420 it erupts. 260 00:35:23,340 --> 00:35:27,140 More than three million bats exit the cave. 261 00:36:13,220 --> 00:36:16,900 The ancient Maya associated bats with death. 262 00:36:20,620 --> 00:36:24,660 This was a swarm straight from the underworld. 263 00:36:30,700 --> 00:36:34,020 But bats are vital to life in the Yucatan, 264 00:36:34,020 --> 00:36:36,620 keeping its insect population in check. 265 00:36:38,740 --> 00:36:42,620 Each can eat its own body weight in insects every night. 266 00:36:50,260 --> 00:36:53,500 While the bats are focused on the insects, 267 00:36:53,500 --> 00:36:55,620 others are watching them. 268 00:36:57,940 --> 00:37:00,180 Grey hawks... 269 00:37:02,020 --> 00:37:04,420 ..and bat falcons. 270 00:37:07,140 --> 00:37:10,220 These birds are skilled aerial predators, 271 00:37:10,220 --> 00:37:14,580 their talons perfectly designed to snatch the bats from the air. 272 00:37:34,940 --> 00:37:38,260 There are other more surprising hunters here. 273 00:37:41,860 --> 00:37:43,660 Brown jays. 274 00:37:50,780 --> 00:37:53,820 They normally eat insects and fruit. 275 00:38:05,020 --> 00:38:09,140 But the brown jays here have developed a taste for bats. 276 00:38:21,260 --> 00:38:24,380 The jays don't use talons to catch their prey. 277 00:38:27,740 --> 00:38:29,820 They use their beaks. 278 00:38:58,220 --> 00:39:02,900 The loss of ten or so bats barely makes a dent on the swarm. 279 00:39:13,820 --> 00:39:18,980 Tonight, the colony will consume around 30 tonnes of insects. 280 00:39:18,980 --> 00:39:23,780 A vital service that keeps the entire forest healthy. 281 00:39:50,260 --> 00:39:52,660 It's late September... 282 00:39:53,700 --> 00:39:58,300 ..and it has rained almost every day for the last three months. 283 00:39:59,340 --> 00:40:02,180 Don Roque's corn is now fully ripe. 284 00:40:50,980 --> 00:40:55,860 In a tradition dating back to the times of their ancient ancestors, 285 00:40:55,860 --> 00:40:59,100 the first corn of the harvest serves a special purpose. 286 00:41:05,180 --> 00:41:10,660 Dona Su crushes it to make a sacred drink called atole. 287 00:41:16,780 --> 00:41:21,100 This is offered to the gods in a ceremony of thanksgiving. 288 00:41:33,620 --> 00:41:36,140 SPEAKS IN NATIVE LANGUAGE 289 00:42:17,420 --> 00:42:22,460 From June to November, the Yucatan receives 100 centimetres of rain. 290 00:42:27,460 --> 00:42:32,580 Across the peninsula, that's over 36 trillion gallons. 291 00:42:41,820 --> 00:42:43,900 The cenotes are refilled. 292 00:42:50,420 --> 00:42:55,140 But most of this water seeps through the limestone into the underworld. 293 00:42:59,380 --> 00:43:04,020 The work of cave divers like Sam is furthering our understanding 294 00:43:04,020 --> 00:43:06,860 of the Yucatan's unique water system. 295 00:43:06,860 --> 00:43:08,860 'We're still exploring here. 296 00:43:08,860 --> 00:43:12,140 'And the more we explore, the more questions we ask. 297 00:43:13,300 --> 00:43:15,980 'Selfishly, I hope we finish in my lifetime. 298 00:43:15,980 --> 00:43:17,620 'I don't think we will.' 299 00:43:25,500 --> 00:43:30,180 Metre by metre, the divers are mapping the underground labyrinth. 300 00:43:32,980 --> 00:43:37,940 It's a painstaking process with every measurement taken by hand. 301 00:43:40,980 --> 00:43:44,380 But this has revealed something remarkable. 302 00:43:50,340 --> 00:43:54,500 The water in the caves isn't just locked within the limestone. 303 00:43:56,580 --> 00:43:58,180 It flows. 304 00:44:02,260 --> 00:44:05,900 The Yucatan's missing rivers are underground. 305 00:44:24,660 --> 00:44:28,700 Like all rivers, they are compelled towards the coast. 306 00:44:34,660 --> 00:44:40,460 Here, the fresh water meets salt water brought by the incoming tide. 307 00:44:48,620 --> 00:44:51,380 This border is called the halocline. 308 00:44:55,180 --> 00:44:57,980 Containing more dissolved particles, 309 00:44:57,980 --> 00:45:02,620 the salt water is heavier and lies below the fresh water... 310 00:45:04,220 --> 00:45:07,220 ..giving the illusion of a surface. 311 00:45:11,340 --> 00:45:13,620 But after months of heavy rain... 312 00:45:14,620 --> 00:45:17,220 ..the flow of fresh water prevails. 313 00:45:20,940 --> 00:45:24,340 At the coast, it leaves the underworld behind. 314 00:45:29,580 --> 00:45:33,900 Mangroves flourish in the mixture of salt and fresh water. 315 00:45:36,340 --> 00:45:40,780 The roots provide ideal nurseries for schools of fish. 316 00:45:44,940 --> 00:45:48,740 And gentle giants are drawn to the Yucatan's shores. 317 00:45:55,260 --> 00:46:00,260 Manatees come to drink at the outpouring of fresh water. 318 00:46:20,660 --> 00:46:24,220 As the fresh water flows through the mangroves, 319 00:46:24,220 --> 00:46:27,860 the trees release tannins, staining it brown. 320 00:46:48,460 --> 00:46:53,140 Eventually the fresh water reaches its final destination. 321 00:47:06,140 --> 00:47:11,380 It pours out into the Gulf of Mexico in huge volumes. 322 00:47:12,420 --> 00:47:15,460 DRAMATIC MUSIC 323 00:47:50,220 --> 00:47:54,220 Replenished by billions of gallons of fresh water, 324 00:47:54,220 --> 00:47:57,740 these vast coastal lagoons are the ideal home 325 00:47:57,740 --> 00:48:01,780 for a colony of the Yucatan's most colourful residents. 326 00:48:04,220 --> 00:48:06,140 Caribbean flamingos. 327 00:48:44,980 --> 00:48:47,500 It's been a good year for the colony, 328 00:48:47,500 --> 00:48:50,340 with nearly 15,000 new additions. 329 00:49:00,500 --> 00:49:04,340 Finding your chick in such a big crowd isn't easy. 330 00:49:04,340 --> 00:49:06,220 FLAMINGOS SQUAWK 331 00:49:06,220 --> 00:49:09,660 But flamingos have incredibly good hearing 332 00:49:09,660 --> 00:49:13,140 and her youngster has his own unique voice. 333 00:49:19,980 --> 00:49:21,820 Just six weeks ago, 334 00:49:21,820 --> 00:49:24,860 the chick was little bigger than his mother's beak. 335 00:49:43,220 --> 00:49:46,580 These lagoons are full of brine shrimps. 336 00:49:48,980 --> 00:49:52,220 The beta-carotene in these tiny crustaceans 337 00:49:52,220 --> 00:49:55,140 gives the flamingos their distinctive colour. 338 00:49:58,180 --> 00:50:02,940 But it will be another two years before he starts to turn pink. 339 00:50:48,100 --> 00:50:52,980 Here on the Yucatan's coast, as elsewhere on the peninsula, 340 00:50:52,980 --> 00:50:56,660 all owe their survival to the secret underworld. 341 00:50:57,700 --> 00:51:00,180 Without its great hidden rivers, 342 00:51:00,180 --> 00:51:04,660 this would be a hot, dry and hostile place. 343 00:51:06,820 --> 00:51:09,300 Born of a unique history, 344 00:51:09,300 --> 00:51:13,180 the underworld hidden beneath this vast forest 345 00:51:13,180 --> 00:51:16,580 gave rise to the ancient Maya civilisation... 346 00:51:18,980 --> 00:51:23,660 ..and it remains the lifeblood of the Yucatan. 347 00:51:37,260 --> 00:51:40,500 Capturing the beauty of the Yucatan's underworld 348 00:51:40,500 --> 00:51:44,180 meant sending a film crew into one of the most alien environments 349 00:51:44,180 --> 00:51:45,580 on the planet. 350 00:51:48,500 --> 00:51:51,140 To do this safely required expertise. 351 00:51:53,380 --> 00:51:57,180 Between them, this team has over 50 years' experience 352 00:51:57,180 --> 00:51:58,860 diving on the peninsula. 353 00:51:59,940 --> 00:52:02,780 I want to make sure we get that right, you know? 354 00:52:02,780 --> 00:52:05,740 Cameraman Mike Madden was one of the pioneers 355 00:52:05,740 --> 00:52:09,220 of cave exploration here back in the 1980s. 356 00:52:09,220 --> 00:52:12,580 And Sam Meacham is part of a team currently mapping 357 00:52:12,580 --> 00:52:15,260 one of the region's two longest caves. 358 00:52:16,220 --> 00:52:19,580 There's a common misconception that cave divers are these 359 00:52:19,580 --> 00:52:23,020 adrenaline junkie risk-takers cheating death on every dive. 360 00:52:24,100 --> 00:52:27,220 Were tarantulas in our risk management form? 361 00:52:27,220 --> 00:52:31,140 'When, in fact, we're probably some of the most cautious people 362 00:52:31,140 --> 00:52:32,820 'on the planet.' 363 00:52:37,020 --> 00:52:39,660 There's a very real need for caution. 364 00:52:41,500 --> 00:52:43,740 All diving carries risks. 365 00:52:43,740 --> 00:52:46,860 But cave diving heightens these risks. 366 00:52:53,700 --> 00:52:57,500 Surfacing isn't an option if you run out of air. 367 00:53:02,220 --> 00:53:05,700 This particular cave system, known as Taj Mahal, 368 00:53:05,700 --> 00:53:08,100 was first explored in 1995. 369 00:53:10,700 --> 00:53:14,180 It has over three miles of flooded passageways, 370 00:53:14,180 --> 00:53:16,820 some descending almost 30 metres. 371 00:53:18,660 --> 00:53:21,500 Filming here poses another challenge. 372 00:53:21,500 --> 00:53:24,140 Most of the cave is pitch black. 373 00:53:25,980 --> 00:53:28,700 So the first thing the divers need to do 374 00:53:28,700 --> 00:53:32,180 is set up a highly specialised underwater lighting rig. 375 00:53:36,580 --> 00:53:39,380 Four 1,200-watt lights. 376 00:53:41,380 --> 00:53:43,860 100 metres of cable. 377 00:53:46,820 --> 00:53:50,180 All of it has to be carried into the cave by hand. 378 00:53:56,660 --> 00:54:00,140 Lighting the caves on this scale is a new challenge for the team 379 00:54:00,140 --> 00:54:02,620 and draws on all their expertise. 380 00:54:05,820 --> 00:54:10,940 Since the 1980s, the dedicated work of divers like Sam and Mike 381 00:54:10,940 --> 00:54:15,340 has helped put nearly 1,000 miles of the Yucatan's underworld on the map. 382 00:54:16,380 --> 00:54:19,500 But the vast majority remains uncharted. 383 00:54:20,540 --> 00:54:25,180 It's pretty amazing that 35 years after the beginning 384 00:54:25,180 --> 00:54:28,340 of cave diving exploration in this area, 385 00:54:28,340 --> 00:54:31,980 I would say we're still really beginning to understand 386 00:54:31,980 --> 00:54:33,820 what's going on here. 387 00:54:34,980 --> 00:54:38,700 And now there's an added urgency to their work. 388 00:54:42,500 --> 00:54:47,180 Over the last 50 years the Yucatan's population has skyrocketed. 389 00:54:51,500 --> 00:54:56,180 The city of Cancun is one of the fastest-growing tourist destinations 390 00:54:56,180 --> 00:55:00,660 in the world, with over five million visitors every year. 391 00:55:06,260 --> 00:55:09,500 This creates jobs, but the demand for fresh water 392 00:55:09,500 --> 00:55:12,820 is putting pressure on the peninsula's underground rivers. 393 00:55:14,100 --> 00:55:17,500 The future of this area is dependent on tourism 394 00:55:17,500 --> 00:55:22,700 and so how do we sustainably manage these incredible natural resources 395 00:55:22,700 --> 00:55:26,660 so that Mexico can continue to receive 396 00:55:26,660 --> 00:55:30,180 10-12% of its gross domestic product through tourism 397 00:55:30,180 --> 00:55:32,340 on a 100-mile strip of beach? 398 00:55:34,700 --> 00:55:37,020 That's important to the future of Mexico, 399 00:55:37,020 --> 00:55:39,700 not only of this region but the country as a whole. 400 00:55:43,940 --> 00:55:47,340 Maps of the underground river systems are far from complete. 401 00:55:48,820 --> 00:55:52,220 New construction work may be taking place above caves 402 00:55:52,220 --> 00:55:54,420 that are yet to be discovered. 403 00:55:57,740 --> 00:56:04,220 The entire city of Cancun draws its drinking water from 142 cenotes, 404 00:56:04,220 --> 00:56:07,140 some of which are now at risk of pollution. 405 00:56:14,380 --> 00:56:17,380 It's a race against time to reveal the importance 406 00:56:17,380 --> 00:56:19,820 of the Yucatan's hidden underworld. 407 00:56:26,180 --> 00:56:29,420 Back at Taj Mahal, with the lights in place, 408 00:56:29,420 --> 00:56:31,860 the crew are ready to begin filming. 409 00:56:34,300 --> 00:56:37,340 All our focus is going to be this direction today. 410 00:56:37,340 --> 00:56:41,300 Every image they film must be meticulously planned. 411 00:56:45,460 --> 00:56:47,660 Get over. More light. 412 00:56:48,700 --> 00:56:50,700 Less light. 413 00:56:51,740 --> 00:56:53,500 Once the dive begins, 414 00:56:53,500 --> 00:56:56,180 all communication is through hand signals. 415 00:57:09,220 --> 00:57:13,660 At last the team can capture the splendour of the hidden underworld. 416 00:57:21,020 --> 00:57:23,980 For most of us, this flooded labyrinth 417 00:57:23,980 --> 00:57:28,100 remains just as inaccessible as it was to the ancient Maya. 418 00:57:30,260 --> 00:57:35,420 But for the cave divers devoted to exploring this world, 419 00:57:35,420 --> 00:57:40,180 the hope is that by opening our eyes to its beauty and importance, 420 00:57:40,180 --> 00:57:43,100 they will help safeguard its future. 421 00:58:01,260 --> 00:58:06,500 In the next episode, we head to Mexico's remotest region. 422 00:58:06,500 --> 00:58:08,340 The far north. 423 00:58:08,340 --> 00:58:12,380 A land of great deserts and rich prairies. 424 00:58:14,860 --> 00:58:17,460 Home to the toughest of characters... 425 00:58:18,460 --> 00:58:20,700 ..secret worlds... 426 00:58:21,820 --> 00:58:25,180 ..and some of Mexico's rarest wildlife.