1 00:00:22,280 --> 00:00:29,120 Hidden in this jungle are 3 ,000 years of human history, one of the world's 2 00:00:29,120 --> 00:00:30,600 greatest ancient civilizations. 3 00:00:32,800 --> 00:00:35,580 Here, on Mexico's Yucatan Peninsula, 4 00:00:36,460 --> 00:00:42,340 the Maya built cities, temples, and palaces, and yet we still don't know how 5 00:00:42,340 --> 00:00:43,340 they thrived. 6 00:00:46,460 --> 00:00:50,800 The forest has grown back, and nature has taken over again. 7 00:00:52,490 --> 00:00:54,610 leaving many riddles unsolved. 8 00:00:56,050 --> 00:00:58,750 Like the riddle of the missing river. 9 00:01:01,030 --> 00:01:06,530 Almost every other ancient civilization was founded beside a great river. 10 00:01:08,090 --> 00:01:09,430 But there are none here. 11 00:01:10,090 --> 00:01:11,570 Not even any streams. 12 00:01:12,710 --> 00:01:14,110 Where is the Nile? 13 00:01:14,490 --> 00:01:17,170 The Ganges or the Euphrates of the Maya? 14 00:01:22,960 --> 00:01:27,740 What they did have were thousands of these pretty little pools scattered 15 00:01:27,740 --> 00:01:28,740 the jungle. 16 00:01:29,480 --> 00:01:33,960 Called cenotes, they're the Yucatan's only source of fresh water. 17 00:01:34,540 --> 00:01:38,060 Could they by themselves have supported an entire civilization? 18 00:01:40,580 --> 00:01:45,480 The Maya believed that cenotes were entrances to another world. 19 00:01:47,860 --> 00:01:49,100 An underworld. 20 00:01:52,200 --> 00:01:57,940 but faith value they seem to be little more than beautiful jungle water holes 21 00:01:57,940 --> 00:02:04,720 was the underworld just a myth people today can do something the maya could 22 00:02:04,720 --> 00:02:11,720 have dreamt about breathe underwater these modern explorers have made some 23 00:02:11,720 --> 00:02:17,580 remarkable discoveries not only about the maya but about the forest and its 24 00:02:17,580 --> 00:02:20,960 animals too what they found in the underworld 25 00:02:22,030 --> 00:02:26,470 has changed our understanding of the Yucatan forever. 26 00:02:44,050 --> 00:02:49,530 The Yucatan, a peninsula the size of England, separating the Gulf of Mexico 27 00:02:49,530 --> 00:02:50,530 the Caribbean Sea. 28 00:03:00,140 --> 00:03:03,120 American -born Sam Meacham is a cave diver. 29 00:03:04,960 --> 00:03:08,300 He's been exploring the waters under the Yucatan for more than a decade. 30 00:03:10,860 --> 00:03:13,520 But he's still only seen a fraction of what's down there. 31 00:03:15,460 --> 00:03:21,400 His mission is to explore as many cenotes as he can, working with 32 00:03:21,400 --> 00:03:22,420 try to make sense of it all. 33 00:03:23,690 --> 00:03:29,030 The puzzle of the Yucatan Peninsula is extremely complex. I arrived here in 34 00:03:29,030 --> 00:03:33,990 with the intention of only being here for six months, and ten years later I 35 00:03:33,990 --> 00:03:37,770 myself still here, so interested and curious in what I've discovered. 36 00:03:40,650 --> 00:03:45,290 But Sam wasn't the first foreign explorer to be drawn to Mexico's jungles 37 00:03:45,290 --> 00:03:46,290 passion for adventure. 38 00:03:47,870 --> 00:03:49,630 Back in 1839, 39 00:03:50,400 --> 00:03:55,080 John Lloyd Stevens, an American diplomat and travel writer, set off into the 40 00:03:55,080 --> 00:03:58,840 Yucatan, inspired by rumors of a lost civilization. 41 00:04:00,620 --> 00:04:06,100 For a while, he found nothing, even though clues lay all around him. 42 00:04:10,160 --> 00:04:16,440 Finally, he stumbled upon the ruins of a great city, smothered by the jungle. 43 00:04:24,110 --> 00:04:30,110 The wild tales that Stevens told made his name as a famous Victorian explorer, 44 00:04:30,110 --> 00:04:35,390 hero of his time, and to some, the original Indiana Jones. 45 00:04:49,010 --> 00:04:53,290 Stephen's fantastic revelations have inspired a whole new generation of 46 00:04:53,290 --> 00:04:54,290 explorers. 47 00:04:55,130 --> 00:04:59,630 For me, one of the great motivating factors in what we do here is that I'm 48 00:04:59,630 --> 00:05:03,490 to explore in the 21st century, something I thought would never have 49 00:05:03,490 --> 00:05:04,590 possible in my lifetime. 50 00:05:06,550 --> 00:05:09,430 Just getting to the cenotes is an adventure in itself. 51 00:05:09,990 --> 00:05:14,310 Like the Maya ruins, they're scattered over thousands of square kilometres of 52 00:05:14,310 --> 00:05:15,310 trackless forest. 53 00:05:16,190 --> 00:05:17,650 But Sam's not alone. 54 00:05:19,270 --> 00:05:23,150 British -born Steve Bogarts shares Sam's passion for exploration. 55 00:05:23,830 --> 00:05:26,330 They've been cenote hunting together for years. 56 00:05:32,230 --> 00:05:36,610 With local help, they mount expeditions deep into the Yucatan's interior. 57 00:05:38,070 --> 00:05:41,030 It can take days to find a new cenote. 58 00:05:44,460 --> 00:05:48,460 As we travel through the jungle looking for cenotes, of course there's always 59 00:05:48,460 --> 00:05:53,580 the usual assembly of spiny trees and cactuses. We have crocodiles, we have 60 00:05:53,580 --> 00:05:56,500 snakes, scorpions, tarantulas, you name it, it's all there. 61 00:05:56,860 --> 00:06:00,740 But really, if you know what to look for and know where to go and where not to 62 00:06:00,740 --> 00:06:03,000 go, you can avoid a lot of these problems. 63 00:06:17,400 --> 00:06:20,320 Finally, a new, unexplored cenote. 64 00:06:21,260 --> 00:06:26,100 Never mind the jungle tracks, the real danger for Sam and Steve begins at the 65 00:06:26,100 --> 00:06:29,620 bottom of these enchanting little pools, considered sacred by the Maya. 66 00:06:30,620 --> 00:06:35,520 It's very easy to see how the ancient Maya would have perceived the cenotes as 67 00:06:35,520 --> 00:06:40,060 very sacred spaces. They're absolutely beautiful jewels out in the middle of 68 00:06:40,060 --> 00:06:41,060 this jungle. 69 00:06:42,830 --> 00:06:46,790 To walk up to the edge of a cenote and to look down into the crystal clear 70 00:06:46,790 --> 00:06:50,230 and see the fish swimming below in the natural daylight, casting these 71 00:06:50,230 --> 00:06:54,030 incredible shafts of light through the water, is very inspiring. 72 00:06:56,130 --> 00:07:01,130 As the sole sources of water in this jungle, these pools are also magnets for 73 00:07:01,130 --> 00:07:06,390 wildlife, and to cenote specialists like Greaves, their whole world. 74 00:07:13,000 --> 00:07:17,780 With thick forests on all sides, they seem as isolated as islands in an ocean. 75 00:07:33,040 --> 00:07:37,100 Peccaries, deer and other forest animals use cenotes as watering holes. 76 00:07:37,540 --> 00:07:39,500 But that doesn't mean they're easy to see. 77 00:07:39,960 --> 00:07:40,960 The jungle. 78 00:07:41,210 --> 00:07:42,590 dug its best to keep them hidden. 79 00:07:51,150 --> 00:07:53,810 But some animals you can't help but notice. 80 00:08:01,950 --> 00:08:02,950 Howler monkeys. 81 00:08:04,550 --> 00:08:08,250 Even if you don't see them at first you are sure to hear them. 82 00:08:09,000 --> 00:08:13,300 With calls that carry five kilometers, they're the loudest land animals in the 83 00:08:13,300 --> 00:08:14,300 world. 84 00:08:19,900 --> 00:08:21,920 Howlers are sloppy eaters. 85 00:08:22,440 --> 00:08:26,820 Coates following below can fill their stomach solely out of what they've 86 00:08:26,820 --> 00:08:27,820 dropped. 87 00:08:31,100 --> 00:08:32,100 Spider monkeys. 88 00:08:32,600 --> 00:08:36,480 They're infinitely quieter than howlers, but much more agile. 89 00:08:38,030 --> 00:08:41,929 With their hooking hands and long arms, these monkeys can live their entire 90 00:08:41,929 --> 00:08:43,530 lives in the dense forest canopy. 91 00:08:49,070 --> 00:08:54,870 For nine months of the year, there is no rainfall here, and much of the forest 92 00:08:54,870 --> 00:08:55,910 struggles to survive. 93 00:08:58,230 --> 00:09:00,950 But some trees seem immune to the drought. 94 00:09:01,910 --> 00:09:03,010 What's their secret? 95 00:09:04,910 --> 00:09:07,090 Such are the riddles of the Yucatan. 96 00:09:09,930 --> 00:09:11,710 The answers lie underground. 97 00:09:12,570 --> 00:09:17,350 But Sam and Steve won't get to go there until tomorrow. 98 00:09:29,450 --> 00:09:36,090 In the dark, the jungle seems even denser, and the sounds even stranger. 99 00:09:41,390 --> 00:09:44,110 This is when cenotes really come alive. 100 00:09:47,610 --> 00:09:51,150 Tapirs love water, for bathing as much as drinking. 101 00:09:51,570 --> 00:09:57,490 But visiting a cenote means coming into the open, which, for good reason, they 102 00:09:57,490 --> 00:09:58,830 only do after dark. 103 00:10:08,790 --> 00:10:14,090 Like watering holes anywhere, Cenotes are where predators, in this case 104 00:10:14,150 --> 00:10:15,150 come to hunt. 105 00:10:47,760 --> 00:10:51,640 But to the Maya, cenotes were more than just jungle watering holes. 106 00:10:53,460 --> 00:10:55,480 They were central to their world. 107 00:10:57,320 --> 00:11:00,520 Cities and temples were often built right next to them. 108 00:11:04,600 --> 00:11:10,240 These sacred wells were gateways to the underworld, a terrifying place of 109 00:11:10,240 --> 00:11:13,880 spirits and of fearsome gods who demanded respect. 110 00:11:20,040 --> 00:11:24,500 At the bottom of many cenotes lie offerings made to the underworld. 111 00:11:31,600 --> 00:11:36,660 For archaeologists, cenotes are time capsules that provide clues to how the 112 00:11:36,660 --> 00:11:39,480 ancient Maya lived and died. 113 00:11:44,340 --> 00:11:48,420 Sometimes even the people themselves were sacrificed to the gods they feared 114 00:11:48,420 --> 00:11:49,420 much. 115 00:11:50,060 --> 00:11:54,380 Every pot and skeleton has its own story to tell. 116 00:12:07,280 --> 00:12:13,680 The discoveries of underwater explorers are helping archaeologists rewrite 117 00:12:13,680 --> 00:12:15,900 the Yucatan's ancient history. 118 00:12:32,859 --> 00:12:37,420 Yucatan's explorers aren't just interested in the clues to Maya history 119 00:12:37,420 --> 00:12:39,060 they might find at the bottom of these pools. 120 00:12:39,900 --> 00:12:43,540 They want to know what might lie beyond them. 121 00:12:45,200 --> 00:12:48,160 Is there indeed an underworld? 122 00:12:52,040 --> 00:12:55,380 Could this cenote be a gateway to a whole new world? 123 00:12:55,860 --> 00:12:59,200 If it is, where does that world lead? 124 00:13:00,020 --> 00:13:01,440 Every new cenote... 125 00:13:02,440 --> 00:13:03,700 presents a new opportunity. 126 00:13:05,620 --> 00:13:10,640 Cenotes really present us with the truest form of exploration found today. 127 00:13:10,640 --> 00:13:15,400 we come up to the side of a cenote, we literally have no idea what we're going 128 00:13:15,400 --> 00:13:19,400 to find at the bottom of it until we actually get in and investigate. 129 00:13:19,760 --> 00:13:23,140 And for me, that's one of the greatest thrills about what we do. 130 00:13:24,460 --> 00:13:25,460 Ready? 131 00:13:25,720 --> 00:13:26,720 Almost. 132 00:13:27,180 --> 00:13:29,220 Cenotes aren't just simple pools. 133 00:13:29,500 --> 00:13:30,500 They're caves. 134 00:13:30,800 --> 00:13:32,680 Flooded caves whose roofs have collapsed. 135 00:13:33,460 --> 00:13:38,200 But Sam and Steve have yet to discover to what extent cenotes are connected to 136 00:13:38,200 --> 00:13:39,980 each other by flooded tunnels. 137 00:13:40,660 --> 00:13:44,640 If there is a network of tunnels down there, how far does it go? 138 00:13:51,320 --> 00:13:55,980 What they're doing is carefully charting an as yet uncharted part of the planet, 139 00:13:56,120 --> 00:13:59,300 somewhere no other human being has ever gone. 140 00:14:01,710 --> 00:14:04,450 It's one of the riskiest things an explorer can do. 141 00:14:24,190 --> 00:14:27,010 This type of diving isn't for everybody. 142 00:14:27,550 --> 00:14:31,610 and definitely you have to want to do it in order to be involved in it. 143 00:14:32,230 --> 00:14:36,650 The first cave dive that I ever did, actually, I was pretty nervous. 144 00:14:37,790 --> 00:14:41,930 I talked to an astronaut that sat on top of a rocket full of fuel and blasted 145 00:14:41,930 --> 00:14:42,930 off to the moon. 146 00:14:43,150 --> 00:14:46,270 Sure, I bet they were a little bit nervous, but look what we've gained 147 00:14:46,270 --> 00:14:47,270 space exploration. 148 00:14:47,710 --> 00:14:52,210 All those people were willing to take a risk to achieve an incredible goal. 149 00:14:56,880 --> 00:14:59,780 Sometimes there's hardly enough room to squeeze through. 150 00:15:00,820 --> 00:15:05,860 Getting stuck or damaging vital equipment now would be fatal. 151 00:15:07,280 --> 00:15:10,960 We're diving in an extremely hostile environment. It's underwater, it's dark, 152 00:15:11,000 --> 00:15:15,880 it's easy to get disoriented, and therefore it's easy to have panic 153 00:15:17,680 --> 00:15:23,880 There's two ways out of a panic situation, luck and death, and therefore 154 00:15:23,880 --> 00:15:25,280 is not an option for us. 155 00:15:25,640 --> 00:15:30,900 You really have to take three deep breaths, calm yourself and assure 156 00:15:30,900 --> 00:15:33,480 that you're able to get out of that situation. 157 00:15:55,939 --> 00:15:58,280 Exploration is rarely without risks. 158 00:15:59,760 --> 00:16:05,120 But one of the biggest rewards is seeing something that's never been seen 159 00:16:05,120 --> 00:16:06,120 before. 160 00:16:06,300 --> 00:16:10,260 What they've discovered down here is just staggering. 161 00:16:31,050 --> 00:16:32,470 The Maya did have an underworld. 162 00:16:34,010 --> 00:16:39,310 And it's as strange and as beautiful a place as any myth might describe. 163 00:16:47,410 --> 00:16:53,390 They've revealed a vast system of flooded caves, underpinning much of the 164 00:16:53,390 --> 00:16:54,349 peninsula. 165 00:16:54,350 --> 00:16:57,530 It's changed our view of the Yucatan forever. 166 00:17:07,240 --> 00:17:10,700 In a sense, this is like exploring outer space. 167 00:17:11,300 --> 00:17:17,380 The weightlessness, the utter strangeness, the thrill of the unknown. 168 00:17:20,440 --> 00:17:23,400 Cave divers call this inner space. 169 00:17:25,300 --> 00:17:28,180 Sam has got close to a long -held ambition. 170 00:17:29,200 --> 00:17:32,480 One of my childhood dreams was to become an astronaut. 171 00:17:33,070 --> 00:17:37,110 I'm not an astronaut now, but I feel that I'm as close as I can come to outer 172 00:17:37,110 --> 00:17:39,230 space exploration in the work that we do here. 173 00:17:40,430 --> 00:17:44,070 We're completely dependent on life support equipment. We travel into a 174 00:17:44,070 --> 00:17:47,630 completely alien and foreign environment that we don't know a whole lot about. 175 00:17:47,970 --> 00:17:52,310 And many of the cave systems that we dive in have seen fewer visitors than 176 00:17:52,310 --> 00:17:53,310 surface of the moon. 177 00:17:54,610 --> 00:17:58,670 It's amazing to think that a whole civilization once sat on top of all 178 00:17:58,790 --> 00:18:01,910 trying to imagine what was down here. 179 00:18:05,290 --> 00:18:09,770 The reality of this place can be as surreal as anything that Maya may have 180 00:18:09,770 --> 00:18:10,770 dreamt of. 181 00:18:13,150 --> 00:18:17,190 Sometimes, what seems to be air, isn't. 182 00:18:22,230 --> 00:18:23,810 It's just a different kind of water. 183 00:18:32,170 --> 00:18:35,270 Some caves contain layers of water that just don't mix. 184 00:18:36,090 --> 00:18:38,790 There's so much about this system that we don't yet understand. 185 00:18:39,670 --> 00:18:42,770 Sam and Steve's aim is to find out how it all connects. 186 00:18:44,010 --> 00:18:45,070 They're making maps. 187 00:18:54,730 --> 00:18:57,210 Light ahead reveals a new cenote. 188 00:19:00,910 --> 00:19:05,330 They'll record its position then swim back to where they started to dive and 189 00:19:05,330 --> 00:19:10,290 to return here overland The 190 00:19:10,290 --> 00:19:16,450 more they explore the more connections they find 191 00:19:16,450 --> 00:19:22,590 But they've got a long way to go There are still thousands of cenotes left to 192 00:19:22,590 --> 00:19:23,590 investigate 193 00:19:30,510 --> 00:19:33,510 The return journey is in many ways more difficult. 194 00:19:33,770 --> 00:19:36,810 GPS puts us right on target. 195 00:19:37,190 --> 00:19:39,790 Underground, they went where the tunnels led them. 196 00:19:40,270 --> 00:19:46,350 Up here, they're looking for one tiny pool among thousands, hidden somewhere 197 00:19:46,350 --> 00:19:47,329 a dense jungle. 198 00:19:47,330 --> 00:19:51,490 For this, they'll need satellite positioning and aerial photographs. 199 00:19:51,830 --> 00:19:54,250 Yeah, GPS puts us right on it, huh? 200 00:20:02,610 --> 00:20:04,130 Yeah, that's promising. 201 00:20:05,250 --> 00:20:10,770 State -of -the -art technology gets them close, but on the final stretch, they 202 00:20:10,770 --> 00:20:12,490 get a helping hand from birds. 203 00:20:15,870 --> 00:20:17,610 Turquoise -browed motmots. 204 00:20:18,010 --> 00:20:19,990 These are true cenote birds. 205 00:20:20,690 --> 00:20:25,670 They feed on the abundant insects near the water and often nest inside the 206 00:20:25,670 --> 00:20:26,670 caves. 207 00:20:35,530 --> 00:20:39,550 Their distinctive call almost always means there's a cenote nearby. 208 00:20:44,430 --> 00:20:47,750 It was the ancient Maya who first used them as guides to water. 209 00:20:48,150 --> 00:20:49,990 This works just as well today. 210 00:20:58,210 --> 00:21:04,430 Now they've located the new cenote, Sam and Steve need to find out if it has 211 00:21:04,430 --> 00:21:06,710 further connections with other parts of the system. 212 00:21:09,110 --> 00:21:13,230 In our corner of the Yucatan Peninsula, the collaborative efforts of cave diving 213 00:21:13,230 --> 00:21:19,810 explorers have mapped and explored over 550 kilometers of 214 00:21:19,810 --> 00:21:25,150 underground, underwater passageway in over 100 different cave systems. 215 00:21:25,650 --> 00:21:31,050 The promise of future exploration is high. There's so much left that we still 216 00:21:31,050 --> 00:21:32,530 have yet to explore. 217 00:21:40,910 --> 00:21:46,210 This may seem like nothing more than an elaborate game of join the dots, but 218 00:21:46,210 --> 00:21:50,790 each time Sam and Steve go back underground, they never lose sight of 219 00:21:50,790 --> 00:21:52,030 potential dangers of their work. 220 00:21:53,330 --> 00:21:59,770 One of the truisms of cave diving is that complacency breeds death, and every 221 00:21:59,770 --> 00:22:02,630 single dive we approach is as if it's the first dive we've done. 222 00:22:03,160 --> 00:22:08,400 And we have a ritual that we go through of matching our gear, checking for 223 00:22:08,400 --> 00:22:12,760 leaks, and making sure that everything is in optimal 100 % condition for 224 00:22:14,740 --> 00:22:17,480 Sam couldn't have a better dive buddy than Steve. 225 00:22:18,040 --> 00:22:22,120 He's one of the region's most experienced cave divers and a master 226 00:22:23,140 --> 00:22:25,160 He knows his equipment inside out. 227 00:22:25,380 --> 00:22:27,700 Is everything good here? Yeah, looks good. 228 00:22:27,900 --> 00:22:28,900 Okay. 229 00:22:31,560 --> 00:22:35,720 One of the first things you'll notice is that we're actually taking two tanks 230 00:22:35,720 --> 00:22:39,560 with us rather than one. That's because we're diving in an alien, potentially 231 00:22:39,560 --> 00:22:42,960 hostile environment, and we need redundancy in all our life support 232 00:22:43,040 --> 00:22:46,160 and gas supply is obviously very, very critical to us. 233 00:22:46,600 --> 00:22:50,600 We also use a gas management planning rule known as the rule of thirds. So 234 00:22:50,600 --> 00:22:54,360 use one third of our gas swimming into the cave, one third swimming back out 235 00:22:54,360 --> 00:22:57,580 again, so that when we surface, we have one third in reserve. And that's an 236 00:22:57,580 --> 00:23:02,060 emergency reserve should it take us longer to exit than we anticipated or if 237 00:23:02,060 --> 00:23:03,500 needed to share air with a buddy. 238 00:23:11,720 --> 00:23:15,020 A thin piece of white string, carefully laid. 239 00:23:15,630 --> 00:23:17,630 quite literally becomes their lifeline. 240 00:23:18,670 --> 00:23:23,990 It may be the only way that they can find their way back out of the 241 00:23:30,070 --> 00:23:30,670 They 242 00:23:30,670 --> 00:23:38,690 mark 243 00:23:38,690 --> 00:23:43,270 it with arrows that always point back towards the entrance and safety. 244 00:23:46,440 --> 00:23:47,940 It's also a measuring tape. 245 00:23:48,960 --> 00:23:52,120 Regularly spaced knots tell Sam and Steve how far they've gone. 246 00:23:57,920 --> 00:24:01,900 As we explore the cave systems, we try to be as smart as we can. 247 00:24:02,220 --> 00:24:06,680 And generally, we're trying to go in a particular direction. And we have 248 00:24:06,680 --> 00:24:07,900 compasses that work underwater. 249 00:24:08,160 --> 00:24:12,620 And using those compasses, we're able to determine which route to take. 250 00:24:17,950 --> 00:24:21,930 It's quite common to come up to a split in a passageway. 251 00:24:22,630 --> 00:24:25,550 We have to determine which is the best route to take. 252 00:24:25,750 --> 00:24:29,190 In some cases, that'll end up in a dead end, and we turn around and come back 253 00:24:29,190 --> 00:24:30,290 out and try the other way. 254 00:24:40,170 --> 00:24:44,450 Using spools of string, Yucatan's cave divers have measured the longest 255 00:24:44,450 --> 00:24:45,990 underwater cave in the world. 256 00:24:46,440 --> 00:24:49,000 over 133 kilometers long. 257 00:24:53,140 --> 00:24:57,420 Exploration wouldn't be exploration if everything always went to plan. 258 00:24:58,300 --> 00:25:03,600 This time, the divers have come to a passage too tight to squeeze through, 259 00:25:03,600 --> 00:25:04,600 they're forced to stop. 260 00:25:07,380 --> 00:25:11,420 They follow their safety line back and live to dive another day. 261 00:25:12,940 --> 00:25:16,620 But explorers wouldn't be explorers if they let such setbacks discourage them. 262 00:25:17,060 --> 00:25:18,900 There's always the thrill of the next dive. 263 00:25:22,140 --> 00:25:26,080 It's pretty much guaranteed that every time that we go into a cenote, it's 264 00:25:26,080 --> 00:25:29,560 to be a different experience. It's something new, it's something exciting, 265 00:25:29,560 --> 00:25:31,380 that's what really draws me in. 266 00:25:32,800 --> 00:25:36,420 One of the many interesting things of diving here is to watch all the wildlife 267 00:25:36,420 --> 00:25:38,500 that thrives in the crystal clear water. 268 00:25:40,100 --> 00:25:42,300 That includes sailfin mollies. 269 00:25:42,800 --> 00:25:47,160 small fish that stick to the bright sunlit zones in the open water pools of 270 00:25:47,160 --> 00:25:48,160 Cenotes. 271 00:25:48,700 --> 00:25:51,020 For a male, it's a hectic life. 272 00:25:51,600 --> 00:25:56,380 He has a three -dimensional territory to patrol and is constantly chasing other 273 00:25:56,380 --> 00:25:59,760 males out while trying to keep his harem of females in. 274 00:26:04,960 --> 00:26:09,420 In both cases, success depends on how effectively he displays his sail fin. 275 00:26:12,230 --> 00:26:14,410 It's a big job for a little fish. 276 00:26:31,790 --> 00:26:36,310 Some fish like these tetras have proved to be real opportunists. 277 00:26:37,170 --> 00:26:41,070 They've learned to follow divers' torches into the dark to feed right 278 00:26:41,070 --> 00:26:42,070 caves. 279 00:26:46,670 --> 00:26:51,870 Our divers take care not to bring any uninvited guests with them, because the 280 00:26:51,870 --> 00:26:57,550 underworld has its own unique creatures, an entire food chain of over 30 species 281 00:26:57,550 --> 00:27:00,790 that live out their lives in the pitch dark. 282 00:27:09,670 --> 00:27:15,070 Most cave animals are white, because in a world without light, colour is 283 00:27:15,070 --> 00:27:16,070 pointless. 284 00:27:16,520 --> 00:27:20,480 Even eyes are useless, and many creatures just don't have them. 285 00:27:23,140 --> 00:27:27,920 Down here, touch and smell are all that matter. 286 00:27:30,600 --> 00:27:36,780 Among the strangest and most ancient of cave beasts is the remipede, a sort of 287 00:27:36,780 --> 00:27:40,700 primitive centipede that's rarely seen, found only in waters exceptionally low 288 00:27:40,700 --> 00:27:41,700 in oxygen. 289 00:27:42,000 --> 00:27:44,640 Relics of one of the earliest chapters of life on Earth. 290 00:27:45,150 --> 00:27:49,550 They're among the cave's top predators, combing the water for shrimps and 291 00:27:49,550 --> 00:27:50,550 isopods. 292 00:27:53,630 --> 00:27:58,570 If the remipede doesn't seem to know which way is up, that's because in the 293 00:27:58,570 --> 00:28:02,370 water and in the dark, up and down aren't so relevant. 294 00:28:16,680 --> 00:28:22,640 In the underworld, even the fish are surreal, ghostly white, with blanks 295 00:28:22,640 --> 00:28:23,640 eyes should be. 296 00:28:28,040 --> 00:28:30,380 There are other signs of life down here. 297 00:28:34,100 --> 00:28:40,260 This is the perfectly preserved tooth of a Gomphotherium, a relative of the 298 00:28:40,260 --> 00:28:42,780 elephant that's been extinct for 10 ,000 years. 299 00:28:49,160 --> 00:28:54,860 Ancient animal remains, and these stalactites and stalagmites only ever 300 00:28:54,860 --> 00:28:59,740 in air, are hard evidence that these caves used to be dry. 301 00:29:00,780 --> 00:29:03,440 And Yucatan's history goes deeper still. 302 00:29:03,840 --> 00:29:10,020 The walls of these caves are made of soft limestone, telling us this was once 303 00:29:10,020 --> 00:29:11,200 huge coral reef. 304 00:29:26,830 --> 00:29:30,930 Some caves near the surface have air pockets and cracks in their ceilings 305 00:29:30,930 --> 00:29:32,270 allow bats to come and go. 306 00:29:33,190 --> 00:29:34,470 Cave swifts, too. 307 00:29:34,970 --> 00:29:37,930 It's the perfect sheltered place to roost and nest. 308 00:29:39,650 --> 00:29:42,430 No wonder the Maya thought bats were from the underworld. 309 00:29:43,050 --> 00:29:46,430 They would have seen them flying straight out of the ground as night 310 00:29:50,570 --> 00:29:52,630 By exploring underground... 311 00:29:52,940 --> 00:29:57,340 Yucatan's divers are peeling back the many layers of the peninsula and are 312 00:29:57,340 --> 00:30:01,580 slowly revealing the incredible relationship between its flooded caves 313 00:30:01,580 --> 00:30:03,580 everything they affect at the surface. 314 00:30:07,240 --> 00:30:10,160 There are many ways in which these two worlds connect. 315 00:30:12,820 --> 00:30:13,820 Tree roots. 316 00:30:16,020 --> 00:30:22,100 This is the jungle's secret. How, with hardly any surface water, it can still 317 00:30:22,100 --> 00:30:23,280 grow so dense. 318 00:30:25,320 --> 00:30:29,920 Some trees and vines push their roots through gaps in the limestone to the 319 00:30:29,920 --> 00:30:31,260 permanent water supply below. 320 00:30:33,780 --> 00:30:38,600 It doesn't matter how dry it gets on the surface, they rely on the underworld. 321 00:30:40,180 --> 00:30:44,660 These deep -rooted trees provide animals with a year -round supply of leaves, 322 00:30:44,900 --> 00:30:47,000 flowers and fruit. 323 00:30:55,850 --> 00:31:00,490 This vital connection between the forest and the ground beneath it must have 324 00:31:00,490 --> 00:31:01,490 intrigued the Maya. 325 00:31:01,870 --> 00:31:05,610 It could only have reinforced their belief in the power of the underworld. 326 00:31:07,790 --> 00:31:10,150 They too relied on its gift of water. 327 00:31:11,450 --> 00:31:16,990 A few cenotes could help a whole city survive even the harshest of dry 328 00:31:27,020 --> 00:31:30,440 But Sam doesn't just look to archaeology for his understanding of the Maya. 329 00:31:31,040 --> 00:31:32,380 He can talk to them. 330 00:31:34,060 --> 00:31:37,360 Direct descendants of the ancient Maya still live here. 331 00:31:38,340 --> 00:31:45,080 One of them is Don Femine Zip, a good friend of Sam's. 332 00:31:48,580 --> 00:31:55,160 The Maya still practice flash and burn farming. 333 00:31:55,740 --> 00:31:58,960 growing crops than letting the forest grow back to replenish the soil. 334 00:32:00,180 --> 00:32:03,580 In fact, the ancient Maya did this on a grand scale. 335 00:32:04,500 --> 00:32:08,900 Incredibly, most of the jungle here, previously thought to be pristine, has 336 00:32:08,900 --> 00:32:12,760 actually been cut down and regrown many times over the last 2 ,000 years. 337 00:32:17,080 --> 00:32:21,160 The Maya may have stopped building large cities and temples, but they live on 338 00:32:21,160 --> 00:32:23,360 today as skillful farmers, thriving. 339 00:32:23,740 --> 00:32:30,440 despite the thin soils and harsh seasons of the Yucatan Maya 340 00:32:30,440 --> 00:32:43,840 communities 341 00:32:43,840 --> 00:32:48,140 are close -knit and the Mayan language is still spoken 342 00:33:04,960 --> 00:33:08,140 Sunote is derived from the Maya word for well. 343 00:33:08,820 --> 00:33:10,940 Almost every village is built around one. 344 00:33:11,600 --> 00:33:14,980 Other sunotes mark boundaries between the communities. 345 00:33:23,310 --> 00:33:27,690 The notice were and are quite literally central to their world. 346 00:33:36,090 --> 00:33:40,410 As well as a distinct language, the Maya have a distinct set of beliefs. 347 00:33:40,970 --> 00:33:46,390 Their stories and fables pass down the generations, describe everything around 348 00:33:46,390 --> 00:33:51,630 them. The cenotes, the jungle, the animals. 349 00:33:56,230 --> 00:34:01,330 One Maya belief is that the powerful forces of the underworld determine their 350 00:34:01,330 --> 00:34:03,730 prosperity and their destiny. 351 00:34:10,130 --> 00:34:13,190 Don Fermin still practices the Maya religion. 352 00:34:13,730 --> 00:34:20,170 He prays to the gods of his ancestors and regards cenotes as windows into 353 00:34:20,170 --> 00:34:21,170 world. 354 00:34:25,670 --> 00:34:31,350 In advance of Tham's more difficult dive, Don Thamin sometimes makes 355 00:34:31,350 --> 00:34:34,409 to the underworld, asking for a safe passage. 356 00:34:35,570 --> 00:34:38,310 And this dive will be difficult. 357 00:34:42,409 --> 00:34:46,710 But it will reveal yet another twist in the Yucatan's many -layered history. 358 00:34:47,350 --> 00:34:48,830 A cosmic event. 359 00:34:49,340 --> 00:34:54,420 It affected not only the world of the ancient Maya, but possibly the rest of 360 00:34:54,420 --> 00:34:55,420 world as well. 361 00:35:00,140 --> 00:35:05,160 Some cenotes near the northwestern tip of the Yucatan aren't at all like the 362 00:35:05,160 --> 00:35:06,820 Sam and Steve are used to exploring. 363 00:35:08,520 --> 00:35:14,440 These are much deeper, sheer, vertical sinkholes, known as pit cenotes. 364 00:35:23,020 --> 00:35:27,060 This cenote is definitely a lot deeper than ones we normally would encounter. 365 00:35:27,460 --> 00:35:31,780 Today we got to about 45 meters of depth and still we couldn't see the bottom. 366 00:35:39,260 --> 00:35:41,240 This appears to be the bottom. 367 00:35:42,040 --> 00:35:43,080 But it isn't. 368 00:35:43,960 --> 00:35:47,840 It's a cloud of hydrogen sulfide made from rotting vegetation. 369 00:35:51,200 --> 00:35:52,200 It's toxic. 370 00:35:52,590 --> 00:35:55,610 and corrosive not somewhere you'd want to hang around 371 00:35:55,610 --> 00:36:05,410 the 372 00:36:05,410 --> 00:36:09,730 hydrogen sulfide layer is actually pretty intense as you're descending down 373 00:36:09,730 --> 00:36:13,830 the cenote it gives the appearance that you're coming up on the floor and all of 374 00:36:13,830 --> 00:36:18,010 a sudden you realize it's not the floor it's a cloud it's made up of sulfur 375 00:36:18,010 --> 00:36:22,800 primarily so it's got a rotten egg smell to it In extreme cases where it's very 376 00:36:22,800 --> 00:36:25,920 strong, you can feel it burning any exposed skin that you have. 377 00:36:47,280 --> 00:36:49,600 Why are these cenotes so different? 378 00:36:50,860 --> 00:36:54,460 They're evidence of a critical turning point in the Yucatan's distant history. 379 00:36:55,220 --> 00:37:00,300 Something that was only noticed 20 years ago, when satellites gave us a new 380 00:37:00,300 --> 00:37:01,760 perspective on life on Earth. 381 00:37:06,760 --> 00:37:12,820 If you look at normal cenotes from space, their pattern is scattered and 382 00:37:14,860 --> 00:37:18,500 But the pit cenotes form a distinct semicircle. 383 00:37:19,420 --> 00:37:21,020 165 kilometers across. 384 00:37:24,860 --> 00:37:28,800 Seismic studies have shown that the circle is completed under the sea. 385 00:37:29,540 --> 00:37:32,240 So what does this huge circle represent? 386 00:37:33,280 --> 00:37:39,780 The answer lies at least 65 million years ago, when the Yucatan was a 387 00:37:39,780 --> 00:37:40,780 tropical sea. 388 00:37:41,640 --> 00:37:47,660 The disastrous event that caused the circle was so massive that some think It 389 00:37:47,660 --> 00:37:49,500 could have led to the demise of the dinosaurs. 390 00:37:52,220 --> 00:37:59,120 An enormous meteor, heading for what is now the very tip of the Yucatan 391 00:37:59,120 --> 00:38:00,120 Peninsula. 392 00:38:16,340 --> 00:38:22,760 Imagine at the moment that this meteor slammed into our planet, it was so huge 393 00:38:22,760 --> 00:38:27,620 that if one edge of it was touching our planet, the outer edge of it would be at 394 00:38:27,620 --> 00:38:30,720 the same altitude as a commercial jetliner flies today. 395 00:38:30,940 --> 00:38:36,240 The immense impact crater was gradually buried under limestone, built up by 396 00:38:36,240 --> 00:38:38,120 coral reefs over millions of years. 397 00:38:38,860 --> 00:38:40,520 But the crater's shape... 398 00:38:40,780 --> 00:38:44,580 was echoed in the way this limestone then eroded to form the distinctive 399 00:38:44,580 --> 00:38:46,240 semicircle of pits and otus. 400 00:38:48,980 --> 00:38:53,720 When the Maya arrived, they built great cities and temples around these sacred 401 00:38:53,720 --> 00:38:58,140 wells, unwittingly outlining the footprint of this global catastrophe. 402 00:39:01,880 --> 00:39:06,220 Once again, the Yucatan's history can be read by looking deep into its 403 00:39:06,220 --> 00:39:07,220 landscape. 404 00:39:11,340 --> 00:39:16,460 But it has one more secret to reveal One last riddle to be solved 405 00:39:16,460 --> 00:39:28,900 When 406 00:39:28,900 --> 00:39:32,700 it does rain here, it rains hard 407 00:39:32,700 --> 00:39:39,060 But this huge amount of water 408 00:39:39,790 --> 00:39:41,210 doesn't settle on the ground. 409 00:39:41,930 --> 00:39:43,170 It vanishes. 410 00:39:44,890 --> 00:39:49,010 It seeps through the limestone into the underworld. 411 00:39:51,450 --> 00:39:55,190 But this freshwater is only the top layer. 412 00:39:55,610 --> 00:40:00,050 It floats above an enormous body of much heavier saltwater. 413 00:40:01,370 --> 00:40:05,310 This is the halocline, the interface between the two. 414 00:40:06,920 --> 00:40:11,740 It's this contrast between the gin -clear freshwater and the hazier 415 00:40:11,740 --> 00:40:14,440 that can make diving here so surreal. 416 00:40:21,580 --> 00:40:27,480 Divers have discovered that the freshwater here does more than just 417 00:40:31,320 --> 00:40:35,360 It flows in huge underground rivers. 418 00:40:36,110 --> 00:40:39,970 Probably the largest underground river system in the world. 419 00:40:46,370 --> 00:40:52,770 Nearly two centuries ago, John Lloyd Stevens rediscovered the Maya 420 00:40:54,210 --> 00:40:58,150 People have long wondered how they thrived without a great river. 421 00:40:59,910 --> 00:41:03,570 Now, we appear to have found their Nile. 422 00:41:08,970 --> 00:41:12,490 These great rivers must flow out to sea, but where? 423 00:41:13,030 --> 00:41:14,930 Sam needs to find out. 424 00:41:16,710 --> 00:41:20,670 He comes across the skeleton of a manatee, a sea mammal. 425 00:41:21,410 --> 00:41:22,950 He must be getting close. 426 00:41:33,550 --> 00:41:36,930 Meter by meter, cenote to cenote. 427 00:41:38,090 --> 00:41:44,230 Cave divers are mapping the rivers from source to sea But 428 00:41:44,230 --> 00:41:48,430 while doing so they've made an alarming discovery 429 00:41:48,430 --> 00:41:52,570 The 430 00:41:52,570 --> 00:41:57,130 modern world is taking over 431 00:41:57,130 --> 00:42:04,000 I'm amazed at the changes 432 00:42:04,000 --> 00:42:08,200 that have taken place in such a short time in this area. It seems that every 433 00:42:08,200 --> 00:42:11,260 time I go out my door, there's a new building that's been built. 434 00:42:11,840 --> 00:42:16,580 The coastal strip of Cancun and the Riviera Maya is one of the fastest 435 00:42:16,580 --> 00:42:18,000 tourist areas in the world. 436 00:42:18,420 --> 00:42:22,320 There's one specific occasion where we're actually diving beneath a major 437 00:42:22,320 --> 00:42:23,420 construction project. 438 00:42:23,800 --> 00:42:28,860 And as we were diving along, the entire cave was literally shaking as we were 439 00:42:28,860 --> 00:42:29,860 diving through it. 440 00:42:30,040 --> 00:42:32,780 And it wasn't until the next day that we came back. 441 00:42:33,290 --> 00:42:37,790 that we realized that they had been perforating through the ceiling of the 442 00:42:37,950 --> 00:42:41,850 and along one of the lines that Steve had laid the previous day, there was 443 00:42:41,850 --> 00:42:45,790 actually a cement piling going right down through the cave system. 444 00:42:46,230 --> 00:42:51,650 New construction could inadvertently block or pollute the great underground 445 00:42:51,650 --> 00:42:56,110 rivers of the Yucatan, with far -reaching effects still too complex for 446 00:42:56,110 --> 00:43:01,450 understand. The Maya underworld faces a new chapter in its long and varied 447 00:43:01,450 --> 00:43:02,450 history. 448 00:43:06,960 --> 00:43:11,040 The decline of the ancient Maya could teach us a thing or two. 449 00:43:13,820 --> 00:43:16,960 Some say they developed too far too fast. 450 00:43:17,960 --> 00:43:21,980 Others, that a succession of droughts left them without water. 451 00:43:26,880 --> 00:43:30,400 Everyone here still relies on the underworld. 452 00:43:30,940 --> 00:43:34,380 It is and always was the lifeblood of the peninsula. 453 00:43:34,980 --> 00:43:41,220 Without it, the Yucatan would be a hot, dry and hostile place. 454 00:43:47,520 --> 00:43:52,600 By mapping the course of every river to the sea, Sam and other divers are hoping 455 00:43:52,600 --> 00:43:56,120 to draw attention to them, so further damage can be avoided. 456 00:44:03,850 --> 00:44:08,610 Their work has not only helped us understand the Yucatan's past, but it 457 00:44:08,610 --> 00:44:09,850 to safeguard its future. 458 00:44:16,410 --> 00:44:19,750 Sam's journey down this river is nearly over. 459 00:44:20,530 --> 00:44:22,810 There's more light and more air. 460 00:44:23,370 --> 00:44:26,230 And the roots are roots of mangroves. 461 00:44:28,830 --> 00:44:30,190 And there are manatees. 462 00:44:34,060 --> 00:44:39,620 These gentle herbivores come to the underworld outflow to drink fresh water 463 00:44:39,620 --> 00:44:40,620 to cool off. 464 00:44:40,720 --> 00:44:44,160 What they mean to Sam is that he's made it. 465 00:44:57,000 --> 00:44:59,100 One last tunnel. 466 00:45:02,700 --> 00:45:07,660 And a journey that began in a jungle pool ends up off a Caribbean beach. 467 00:45:10,300 --> 00:45:15,220 Tomorrow he'll be back in the forest, looking for a new cenote and the next 468 00:45:15,220 --> 00:45:19,060 river. And when all the cenotes are explored and all the maps are finished, 469 00:45:19,260 --> 00:45:21,760 maybe the Yucatan will be better understood. 470 00:45:28,820 --> 00:45:34,310 In a more mystical way, The ancient Maya understood it. 471 00:45:35,070 --> 00:45:37,530 They knew they were at the mercy of the underworld. 472 00:45:41,810 --> 00:45:46,390 At the ruins, archaeologists are revealing ever more about this great 473 00:45:46,390 --> 00:45:49,990 civilization, how they lived and what they believed. 474 00:45:54,030 --> 00:45:58,490 But now, a whole new frontier has opened up underground. 475 00:46:06,380 --> 00:46:10,780 Sam and Steve are not the first explorers to have been enchanted by the 476 00:46:10,780 --> 00:46:16,160 of the Yucatan, but they have quite literally taken exploration to a whole 477 00:46:16,160 --> 00:46:17,160 level. 478 00:46:20,280 --> 00:46:24,500 To this day, it's only thought that we've charted a fraction of the Maya 479 00:46:24,500 --> 00:46:29,160 underworld, and many of these areas still remain untouched and uncharted. 480 00:46:34,940 --> 00:46:36,760 Sam continues with his passion. 481 00:46:37,400 --> 00:46:40,160 He certainly has his work cut out for him in the coming years. 482 00:46:43,760 --> 00:46:48,440 My feelings about exploration can be very easily summarized in a poem that I 483 00:46:48,440 --> 00:46:53,040 read about the Yukon Gold Rush. And in that, the author says, it's not the 484 00:46:53,220 --> 00:46:54,660 it's finding the gold. 485 00:46:55,900 --> 00:47:00,160 It's finding the cenote and diving down into it and seeing what's there that 486 00:47:00,160 --> 00:47:01,460 really is the thrill for me. 487 00:47:05,230 --> 00:47:09,270 Really, for all of us, it's a motivation to think that you can live in the 21st 488 00:47:09,270 --> 00:47:11,130 century and still be able to explore. 489 00:47:12,570 --> 00:47:17,870 We're only just scratching the surface of what exists here. I have absolutely 490 00:47:17,870 --> 00:47:22,710 doubt that this place will continue to provide incredible scientific 491 00:47:22,710 --> 00:47:23,850 for years to come. 492 00:47:27,330 --> 00:47:33,370 Sam and his explorer colleagues have risky yet fascinating days ahead of 493 00:47:33,960 --> 00:47:37,900 unveiling the many secrets of the Maya underworld.