1 00:00:15,560 --> 00:00:17,830 Of all our planet's forces, 2 00:00:17,880 --> 00:00:22,470 perhaps none has greater power over us than water. 3 00:00:25,760 --> 00:00:30,270 For me, water's the most magical force on Earth. 4 00:00:31,720 --> 00:00:35,150 The presence of water shapes, renews and nourishes our planet. 5 00:00:35,200 --> 00:00:37,870 Oh, my gosh! You're getting all wet there! 6 00:00:42,600 --> 00:00:44,590 It's our planet's lifeblood. 7 00:00:44,640 --> 00:00:46,830 It pumps through it continuously, 8 00:00:46,880 --> 00:00:48,790 delivering vital ingredients for life. 9 00:00:48,840 --> 00:00:50,790 Ah, it's glorious. 10 00:01:01,200 --> 00:01:04,310 Water makes Earth alive. 11 00:01:07,440 --> 00:01:09,950 Yet water is just one of the ways 12 00:01:10,000 --> 00:01:13,870 that the power of the planet has shaped our lives. 13 00:01:15,880 --> 00:01:18,270 The Earth has immense power... 14 00:01:22,120 --> 00:01:26,070 ...and yet that's rarely mentioned in our history books. 15 00:01:27,240 --> 00:01:29,670 I'm here to change that. 16 00:01:31,920 --> 00:01:36,830 In this series, I'm exploring four great planetary forces 17 00:01:36,880 --> 00:01:39,390 that have influenced our history. 18 00:01:44,240 --> 00:01:46,110 The power of the deep Earth... 19 00:01:49,440 --> 00:01:51,390 ...that fuelled technological innovation. 20 00:01:53,480 --> 00:01:54,990 Wind. 21 00:01:55,040 --> 00:01:58,190 It has shaped the fate of entire continents. 22 00:01:58,240 --> 00:02:00,550 And fire... 23 00:02:03,000 --> 00:02:06,510 ...which gave us the power to conquer the planet. 24 00:02:07,880 --> 00:02:10,870 But I'm going to start with water. 25 00:02:11,920 --> 00:02:16,070 The magic of water is that it's constantly transforming itself, 26 00:02:16,120 --> 00:02:20,990 shifting between guises and from place to place. 27 00:02:21,040 --> 00:02:26,270 Our struggle to control it has been behind the rise and fall 28 00:02:26,320 --> 00:02:29,390 of some of the greatest civilisations on Earth. 29 00:02:55,200 --> 00:02:58,710 The centre of the Sahara Desert in North Africa. 30 00:03:00,680 --> 00:03:03,270 One of the driest places on Earth. 31 00:03:08,600 --> 00:03:12,190 I'm over six hours' drive from civilisation. 32 00:03:14,760 --> 00:03:19,230 Temperatures here regularly reach 40 degrees Celsius, 33 00:03:19,280 --> 00:03:22,590 and there's less than a centimetre of rainfall each year. 34 00:03:24,280 --> 00:03:25,750 Ah.... 35 00:03:28,880 --> 00:03:30,350 The whole thing's moving. 36 00:03:33,760 --> 00:03:36,190 It's like walking on water. 37 00:03:37,520 --> 00:03:40,630 Yet hidden amongst these dry dunes are clues 38 00:03:40,680 --> 00:03:46,110 that point to the dramatic influence the planet has had on human lives. 39 00:03:53,360 --> 00:03:56,550 I've come here because although you'd never know it, 40 00:03:56,600 --> 00:04:00,270 the story of this place is all about water. 41 00:04:00,320 --> 00:04:04,750 The clues are etched into that rock face there. 42 00:04:04,800 --> 00:04:09,030 Prehistoric rock art dating back 6,000 years, 43 00:04:09,080 --> 00:04:12,830 and depicting the most unlikely cast of characters you've ever seen. 44 00:04:16,040 --> 00:04:18,710 Wow, what is that? 45 00:04:18,760 --> 00:04:20,590 It's a giraffe... 46 00:04:20,640 --> 00:04:23,390 It's a giraffe, look at it, there's the neck. 47 00:04:23,440 --> 00:04:25,590 There's its ears, that's an eye, 48 00:04:25,640 --> 00:04:27,750 and its mouth. 49 00:04:27,800 --> 00:04:29,350 T hat's really natural, isn't it? 50 00:04:29,400 --> 00:04:31,950 And this looks like the giraffe dipping its head down, 51 00:04:32,000 --> 00:04:35,190 drinking some water - we've got a herd of giraffes here! 52 00:04:43,800 --> 00:04:46,190 There's two cats. 53 00:04:46,240 --> 00:04:48,310 They're fighting. 54 00:04:52,440 --> 00:04:54,750 This... What is this? 55 00:04:54,800 --> 00:04:58,830 It looks like the figure of a man, but he's wearing a bikini. 56 00:05:04,520 --> 00:05:08,350 And this is clearly a crocodile, which is especially odd here. 57 00:05:08,400 --> 00:05:11,510 This is an aquatic animal, it doesn't just paddle around. 58 00:05:11,560 --> 00:05:14,550 It needs a lot of water to live in. In fact, 59 00:05:14,600 --> 00:05:17,990 all the creatures that are depicted on these rocks are not desert animals - 60 00:05:18,040 --> 00:05:20,430 they need wet conditions. 61 00:05:24,600 --> 00:05:28,270 In such a parched wilderness, how can this be? 62 00:05:31,280 --> 00:05:36,710 The only explanation is that 6,000 years ago, this place was wet. 63 00:05:43,440 --> 00:05:48,070 Once you know what to look for, the evidence is all around. 64 00:05:50,560 --> 00:05:54,150 Up there is a river valley that's been carved out into the rock, 65 00:05:54,200 --> 00:05:56,470 and it's been carved by running water 66 00:05:56,520 --> 00:06:00,310 which has flowed down here, smoothing off this rock bed, 67 00:06:00,360 --> 00:06:03,230 and then cascaded down into the valley and off there. 68 00:06:03,280 --> 00:06:07,070 6,000 years ago, that was a big river. 69 00:06:22,200 --> 00:06:26,310 Satellite images reveal that the river bed I'm standing in 70 00:06:26,360 --> 00:06:29,710 is just one of a network of past river valleys 71 00:06:29,760 --> 00:06:32,030 that crisscross the Sahara Desert. 72 00:06:37,680 --> 00:06:43,550 10,000 years ago, this dry, empty place was entirely different. 73 00:06:46,760 --> 00:06:50,150 Little is known about the early Saharans who lived here then, 74 00:06:50,200 --> 00:06:54,470 but we do know that they depended entirely on water. 75 00:06:57,640 --> 00:07:00,430 Water formed the lakes in which they swam. 76 00:07:01,640 --> 00:07:05,350 Water nourished the plants which fed the animals they hunted. 77 00:07:08,760 --> 00:07:12,230 Water filled the clay pots from which they drank. 78 00:07:17,720 --> 00:07:20,510 But then the climate changed. 79 00:07:20,560 --> 00:07:25,950 About 5, 500 years ago, the Sahara began to dry. 80 00:07:26,000 --> 00:07:30,670 The rains failed, the rivers shrank, and the lakes dried out. 81 00:07:31,840 --> 00:07:34,830 For the early Saharan people there was only one option - 82 00:07:34,880 --> 00:07:39,270 to follow the rains and abandon the desert. 83 00:07:43,600 --> 00:07:46,870 The fortunes of the early Saharan people 84 00:07:46,920 --> 00:07:49,070 reveal a universal, timeless truth - 85 00:07:49,120 --> 00:07:52,630 our fate is inextricably linked to water. 86 00:07:52,680 --> 00:07:56,470 The problem is, water never stands still. 87 00:07:56,520 --> 00:07:59,590 It's always on the move across the planet. 88 00:08:04,200 --> 00:08:06,590 We think of this as a blue planet. 89 00:08:06,640 --> 00:08:11,030 But while water is abundant, most of it is no use. 90 00:08:12,880 --> 00:08:18,270 More than 97% of the Earth's water is salty ocean, which we can't drink 91 00:08:18,320 --> 00:08:20,470 or use to grow crops. 92 00:08:23,960 --> 00:08:29,550 Less than 3% is fresh water, on which all human life hangs. 93 00:08:30,640 --> 00:08:34,830 What's more, that tiny fraction is often hard to pin down, 94 00:08:34,880 --> 00:08:39,390 because fresh water has a life cycle all of its own. 95 00:08:46,600 --> 00:08:52,750 I'm about to explore that cycle, in all its elusive glory. 96 00:08:57,120 --> 00:08:59,950 You know, water seems so familiar, doesn't it? 97 00:09:00,000 --> 00:09:02,470 But to see its remarkable qualities 98 00:09:02,520 --> 00:09:04,670 you have to go to some extreme lengths. 99 00:09:15,800 --> 00:09:17,710 Here we go... 100 00:09:17,760 --> 00:09:20,310 Ho-ho! Feel that! 101 00:09:22,240 --> 00:09:24,270 Here we go! 102 00:09:24,320 --> 00:09:27,790 Oh... Hey-hey! Oh, we're off! 103 00:09:27,840 --> 00:09:29,750 Oh, my God! 104 00:09:29,800 --> 00:09:31,630 It's a bit bouncy! 105 00:09:33,760 --> 00:09:36,630 I shouldn't have had that bacon and eggs this morning. 106 00:09:42,160 --> 00:09:44,910 O-o-o-h! 107 00:09:47,480 --> 00:09:52,710 The fresh water that we depend on begins its life in the oceans. 108 00:09:55,400 --> 00:09:59,750 As the sun's rays beat down on the surface of the sea, 109 00:09:59,800 --> 00:10:03,310 they heat the water molecules until some evaporate. 110 00:10:04,440 --> 00:10:08,550 It's the start of an extraordinary journey. 111 00:10:08,600 --> 00:10:11,830 You know, when water evaporates, 112 00:10:11,880 --> 00:10:14,710 it feels as if it vanishes into thin air. 113 00:10:14,760 --> 00:10:16,870 But although we barely notice it, 114 00:10:16,920 --> 00:10:20,950 water molecules are suspended around us all the time. 115 00:10:23,200 --> 00:10:26,110 It's just that we're only aware of it when they clump together 116 00:10:26,160 --> 00:10:28,310 as cloud. 117 00:10:33,160 --> 00:10:39,270 At any one time, less than a thousandth of the world's fresh water is up here 118 00:10:39,320 --> 00:10:41,670 in the atmosphere. 119 00:10:41,720 --> 00:10:46,870 It may not seem much, but this is what spreads water from the seas to the land. 120 00:10:49,360 --> 00:10:53,750 A water molecule doesn't hang around up here for very long. 121 00:10:53,800 --> 00:10:56,790 In fact, it spends less time up here in the atmosphere 122 00:10:56,840 --> 00:10:58,870 than at any other time on its journey - 123 00:10:58,920 --> 00:11:00,710 a mere nine days 124 00:11:00,760 --> 00:11:05,830 until the typical water molecule crashes to Earth as rain. 125 00:11:15,120 --> 00:11:17,310 For most of us, rain is perhaps 126 00:11:17,360 --> 00:11:19,990 the most familiar stage of the water cycle, 127 00:11:20,040 --> 00:11:23,230 but notoriously the least reliable. 128 00:11:24,520 --> 00:11:27,910 As the water falls as rain, it joins a bigger system, 129 00:11:27,960 --> 00:11:33,910 cascading and carving its way across the land surface as streams and rivers. 130 00:11:38,640 --> 00:11:40,310 Look at that! 131 00:11:40,360 --> 00:11:42,430 Water absolutely everywhere! 132 00:11:44,520 --> 00:11:48,190 Rivers and rain are the part of the water cycle that we depend on. 133 00:11:49,760 --> 00:11:52,590 Whoo-hoo! 134 00:11:52,640 --> 00:11:57,110 And yet they're only a tiny proportion of the world's fresh water... 135 00:11:58,680 --> 00:12:03,390 ...a measly 2% of all fresh water on the planet. 136 00:12:03,440 --> 00:12:06,030 The rest of the Earth's fresh water 137 00:12:06,080 --> 00:12:09,270 is locked away down there, on the ground. 138 00:12:09,320 --> 00:12:11,830 Oh... 139 00:12:11,880 --> 00:12:13,390 Oh! 140 00:12:13,440 --> 00:12:14,750 What a landing! 141 00:12:15,880 --> 00:12:19,310 The vast majority of it is stored as ice. 142 00:12:22,560 --> 00:12:25,350 Most of the rest seeps deep into the Earth, 143 00:12:25,400 --> 00:12:28,350 where it's known as groundwater. 144 00:12:29,920 --> 00:12:32,790 Hidden away down here 145 00:12:32,840 --> 00:12:35,950 is the planet's second-largest store of fresh water. 146 00:12:41,840 --> 00:12:45,870 But in the end, all water arrives back in the oceans, 147 00:12:45,920 --> 00:12:47,750 and the cycle begins again. 148 00:12:58,640 --> 00:13:00,710 What that circulation means for us humans 149 00:13:00,760 --> 00:13:04,310 is that water is a moving target. 150 00:13:04,360 --> 00:13:09,110 We constantly have to seek it out on its endless cycle and intercept it 151 00:13:09,160 --> 00:13:11,990 wherever and whenever we can. 152 00:13:12,040 --> 00:13:13,550 This quest to... 153 00:13:13,600 --> 00:13:19,350 to pin down water has played a defining role in human history. 154 00:13:22,840 --> 00:13:25,670 You can trace the impact of our quest for water 155 00:13:25,720 --> 00:13:29,190 right back to the dawn of civilisation, 156 00:13:29,240 --> 00:13:31,310 about 12,000 years ago. 157 00:13:31,360 --> 00:13:34,110 It all began with a big block of ice. 158 00:13:36,800 --> 00:13:38,430 12,000 years ago, 159 00:13:38,480 --> 00:13:43,510 much of the northern hemisphere was covered in a single, huge ice sheet. 160 00:13:49,000 --> 00:13:51,630 And even today you can see its legacy... 161 00:13:53,280 --> 00:13:55,870 ...here in Iceland. 162 00:14:00,360 --> 00:14:06,030 This glacier is a tiny remnant of that once enormous expanse of ice. 163 00:14:18,240 --> 00:14:23,750 Ice is like a storage cupboard in the circulation of water around the planet, 164 00:14:23,800 --> 00:14:28,070 a store into which water can be deposited or withdrawn. 165 00:14:31,000 --> 00:14:34,430 And it was a shift in the amount of water locked up here 166 00:14:34,480 --> 00:14:39,790 that was to drive one of the greatest ever transformations of human society. 167 00:14:42,640 --> 00:14:46,150 Today, the ice sheet here is melting and retreating, 168 00:14:46,200 --> 00:14:48,870 and releasing this great armada of icebergs. 169 00:14:48,920 --> 00:14:51,630 But if you go back 12,500 years ago, 170 00:14:51,680 --> 00:14:54,630 it's a very different story. 171 00:14:54,680 --> 00:14:57,830 Then the ice was expanding, 172 00:14:57,880 --> 00:15:01,510 sucking moisture out of the atmosphere in vast quantities 173 00:15:01,560 --> 00:15:03,470 and locking it away in the ice. 174 00:15:03,520 --> 00:15:06,910 And the effects of that were felt right across the planet. 175 00:15:21,600 --> 00:15:24,110 Thousands of kilometres away in the Middle East... 176 00:15:26,760 --> 00:15:30,310 ...it led to a drought which lasted for centuries. 177 00:15:32,440 --> 00:15:36,150 It had its most profound impact in what would become known 178 00:15:36,200 --> 00:15:42,670 as the Fertile Crescent, an area famed for its exceptionally rich soil. 179 00:15:48,520 --> 00:15:52,350 This drought would trigger the start of the defining characteristic 180 00:15:52,400 --> 00:15:54,030 of human civilisation. 181 00:15:57,720 --> 00:16:01,470 Back then, every human on the planet was a hunter-gatherer. 182 00:16:01,520 --> 00:16:06,670 Those living in the Fertile Crescent, the Natufians, thrived on rich pickings 183 00:16:06,720 --> 00:16:11,110 of fruit and berries, with plenty of deer and ibex to hunt. 184 00:16:12,880 --> 00:16:17,750 But as the drought took hold, to survive they would have to adapt. 185 00:16:19,560 --> 00:16:23,550 They came up with two distinct strategies. 186 00:16:23,600 --> 00:16:27,790 One group developed this, the Harif point, 187 00:16:27,840 --> 00:16:30,110 a new, state-of-the-art arrowhead 188 00:16:30,160 --> 00:16:34,110 that allowed them to tackle a drought by hunting more efficiently. 189 00:16:34,160 --> 00:16:38,190 But a second group came up with something a little bit more subtle. 190 00:16:38,240 --> 00:16:42,150 Although you wouldn't know it, this is a sickle, 191 00:16:42,200 --> 00:16:46,550 and it offered a completely new approach to gathering food. 192 00:16:46,600 --> 00:16:50,270 This small, stone blade represented 193 00:16:50,320 --> 00:16:55,070 a decision not to chase food, but to stay put and grow it. 194 00:16:58,400 --> 00:17:01,310 The Harif point did a good job for the hunters. 195 00:17:03,600 --> 00:17:07,310 But it was the sickle that really changed history. 196 00:17:08,600 --> 00:17:12,470 In a drought, it's safer to stay close to water, 197 00:17:12,520 --> 00:17:18,030 but that decision to remain in one place meant planting crops was essential. 198 00:17:18,080 --> 00:17:20,830 If you go foraging in the forest, 199 00:17:20,880 --> 00:17:23,710 you can only collect so much food with your bare hands, 200 00:17:23,760 --> 00:17:27,470 but if you've got one of these, you can harvest fast and furious, 201 00:17:27,520 --> 00:17:31,070 and for the same amount of effort, you can collect far more food. 202 00:17:31,120 --> 00:17:33,510 With this simple tool, 203 00:17:33,560 --> 00:17:37,110 these people had begun the agricultural revolution. 204 00:17:37,160 --> 00:17:40,870 And the rest, as they say, is history. 205 00:17:44,360 --> 00:17:48,630 A lack of water and a simple but ingenious response 206 00:17:48,680 --> 00:17:51,190 led to the birth of civilisation. 207 00:17:54,880 --> 00:17:58,310 But once farming took hold, it had a profound impact 208 00:17:58,360 --> 00:18:00,270 on our relationship with water. 209 00:18:02,040 --> 00:18:05,910 No longer could we simply follow the rains. 210 00:18:05,960 --> 00:18:10,030 Now people needed regular, reliable sources of water 211 00:18:10,080 --> 00:18:12,430 to make sure their crops grew. 212 00:18:14,560 --> 00:18:18,190 So the need for water began to define 213 00:18:18,240 --> 00:18:20,910 where the first civilisations could flourish. 214 00:18:22,960 --> 00:18:26,270 That led people to the one stage of the water cycle 215 00:18:26,320 --> 00:18:29,190 that offers reliable fresh water - 216 00:18:29,240 --> 00:18:31,270 rivers. 217 00:18:33,000 --> 00:18:34,350 Across the planet, 218 00:18:34,400 --> 00:18:37,310 rivers cover a tiny proportion of the Earth's surface, 219 00:18:37,360 --> 00:18:40,990 but for the first farmers, they became magnets. 220 00:18:46,480 --> 00:18:49,470 But rivers did more than supply a steady source of water. 221 00:18:49,520 --> 00:18:53,230 They changed the very character of the civilisations 222 00:18:53,280 --> 00:18:55,590 that grew up along them, 223 00:18:55,640 --> 00:19:00,830 influencing everything from politics to social organisation. 224 00:19:03,760 --> 00:19:08,470 The power of rivers to shape history is graphically illustrated 225 00:19:08,520 --> 00:19:12,190 by perhaps the greatest of all early civilisations... 226 00:19:18,520 --> 00:19:20,790 ...Ancient Egypt. 227 00:19:22,160 --> 00:19:25,030 You might think you know the story - 228 00:19:25,080 --> 00:19:27,990 a mighty civilisation that built the pyramids 229 00:19:28,040 --> 00:19:32,110 under the autocratic rule of ruthless Pharaohs. 230 00:19:34,160 --> 00:19:37,750 But if you want to understand what really made Egypt great, 231 00:19:37,800 --> 00:19:42,150 you have to leave the pyramids and the temples behind... 232 00:19:45,040 --> 00:19:50,830 ...and come here, to a small place that hardly anyone visits. 233 00:19:50,880 --> 00:19:54,790 You know, at first glance these look like your average, everyday, 234 00:19:54,840 --> 00:19:57,350 2,000-year-old steps. 235 00:19:59,520 --> 00:20:03,990 But this staircase is what made Ancient Egypt tick. 236 00:20:04,040 --> 00:20:08,350 You get an idea of its true purpose by the markings on the side wall - 237 00:20:08,400 --> 00:20:11,990 these grooves were carefully carved into the marble - 238 00:20:12,040 --> 00:20:16,350 because this was a beautifully simple measuring device. 239 00:20:16,400 --> 00:20:20,230 And to see what it was measuring, you have to pop round the corner. 240 00:20:22,920 --> 00:20:24,470 Oh! 241 00:20:24,520 --> 00:20:25,910 It's all wet! 242 00:20:28,080 --> 00:20:30,670 And this is it - 243 00:20:30,720 --> 00:20:32,310 the Nile river. 244 00:20:32,360 --> 00:20:36,830 That set of steps and markings is a Nilometer. 245 00:20:36,880 --> 00:20:40,630 It measured the changing level of the river. 246 00:20:40,680 --> 00:20:44,710 Each year when it flooded, the maximum height that the waters came to 247 00:20:44,760 --> 00:20:47,630 would directly predict the yield of the crops 248 00:20:47,680 --> 00:20:50,630 and, with that, the profits that the farmers made. 249 00:20:53,880 --> 00:20:55,910 It worked because the water of the river 250 00:20:55,960 --> 00:20:58,990 carried something special within it - 251 00:20:59,040 --> 00:21:00,870 an almost invisible treasure 252 00:21:00,920 --> 00:21:05,390 that was the secret of Egypt's economic might. 253 00:21:06,960 --> 00:21:10,390 What made Egypt great is this stuff - 254 00:21:10,440 --> 00:21:12,030 silt. 255 00:21:12,080 --> 00:21:16,230 It's a rich soup of minerals, which... 256 00:21:16,280 --> 00:21:18,430 It's like an espresso. 257 00:21:18,480 --> 00:21:22,670 Tiny flecks of rock and minerals that the river picked up 258 00:21:22,720 --> 00:21:27,110 over its wandering course and swept along with the flow. 259 00:21:35,040 --> 00:21:37,550 All rivers carry some silt, 260 00:21:37,600 --> 00:21:40,670 but the Nile has the benefit of starting in Ethiopia, 261 00:21:40,720 --> 00:21:44,270 where the rock is young and volcanic. 262 00:21:45,320 --> 00:21:48,270 This forms the richest of silts. 263 00:21:48,320 --> 00:21:53,230 140 million tonnes of the stuff are carried by the Nile down river 264 00:21:53,280 --> 00:21:55,710 to Egypt each year. 265 00:21:55,760 --> 00:21:58,590 Every year, the seasonal flood covered the fields 266 00:21:58,640 --> 00:22:04,510 and left behind nutrient-rich silt that fertilised the crops. 267 00:22:04,560 --> 00:22:08,630 The more silt, the more food was produced. 268 00:22:08,680 --> 00:22:10,790 It was the size of the flood - 269 00:22:10,840 --> 00:22:15,870 and with it the bounty of silt - that the Nilometer was used to predict. 270 00:22:17,960 --> 00:22:23,030 So, simply by measuring the height of the Nile, the Egyptians were able 271 00:22:23,080 --> 00:22:27,590 to forecast food production and, with it, the profits of the farmers. 272 00:22:29,160 --> 00:22:33,310 Each year, they used this information to set tax levels. 273 00:22:33,360 --> 00:22:38,190 So the wealth and the might and the splendour of Ancient Egypt 274 00:22:38,240 --> 00:22:42,110 is all down to a simple twist of geographical fate. 275 00:22:42,160 --> 00:22:45,670 In fact, Ethiopia itself gets almost no benefit 276 00:22:45,720 --> 00:22:48,870 from that fertile soil washed from its highlands. 277 00:22:48,920 --> 00:22:52,630 It's even said that its greatest export is the silt 278 00:22:52,680 --> 00:22:57,630 that it sends down the Nile, silt that made the Pharaohs rich. 279 00:23:00,320 --> 00:23:03,790 But the ebb and flow of the Nile 280 00:23:03,840 --> 00:23:08,390 had more far-reaching implications for the Egyptian people than mere taxes. 281 00:23:14,360 --> 00:23:19,150 Intriguingly, it may be that where access to water is limited, 282 00:23:19,200 --> 00:23:21,310 that actually determines 283 00:23:21,360 --> 00:23:26,030 the way a society is organised and even its use of slavery. 284 00:23:29,800 --> 00:23:32,150 Where water is in short supply - 285 00:23:32,200 --> 00:23:35,110 or from a single source, as it is in Egypt - 286 00:23:35,160 --> 00:23:40,870 then you need a highly structured society to get the best out of it. 287 00:23:44,080 --> 00:23:47,470 For large-scale irrigation, you need bureaucrats to decide 288 00:23:47,520 --> 00:23:49,190 where to dig the water channels. 289 00:23:49,240 --> 00:23:52,070 You need teams of working men - slaves, really - 290 00:23:52,120 --> 00:23:54,110 to do the actual hard work of digging. 291 00:23:54,160 --> 00:23:58,470 And once the channels are in place, you need farmers with money enough 292 00:23:58,520 --> 00:24:00,590 to buy the water it's delivered. 293 00:24:00,640 --> 00:24:03,710 So right away you've got three tiers of society, 294 00:24:03,760 --> 00:24:06,590 and I haven't even mentioned the Pharaohs. 295 00:24:08,280 --> 00:24:12,510 So the rigid, hierarchical structure of Egyptian society 296 00:24:12,560 --> 00:24:16,150 wasn't just dictated by the Pharaohs. 297 00:24:16,200 --> 00:24:20,230 It also emerged because the Egyptians had only one water source - 298 00:24:20,280 --> 00:24:21,750 the Nile. 299 00:24:27,600 --> 00:24:29,470 5,000 years ago, 300 00:24:29,520 --> 00:24:33,350 it wasn't just the Ancient Egyptians who noticed the value of rivers. 301 00:24:35,240 --> 00:24:40,390 Other great civilisations were also forming along the banks of rivers. 302 00:24:41,960 --> 00:24:43,790 In Mesopotamia, 303 00:24:43,840 --> 00:24:49,030 the Sumerian civilisation flourished between the Tigris and the Euphrates. 304 00:24:50,960 --> 00:24:56,270 Further east, the Harappan civilisation formed by the Indus. 305 00:24:58,160 --> 00:25:03,350 And early Chinese civilisations were emerging along the Yellow River. 306 00:25:07,640 --> 00:25:11,950 But not all early farmers were content to settle by rivers. 307 00:25:12,000 --> 00:25:14,910 Others learned to exploit new sources of water, 308 00:25:14,960 --> 00:25:17,190 in the unlikeliest places. 309 00:25:19,320 --> 00:25:22,270 Like the Sahara Desert, in Libya. 310 00:25:31,600 --> 00:25:35,270 These are the remains of the ancient city of Garama, 311 00:25:35,320 --> 00:25:40,710 which about 2, 500 years ago was the centre of a powerful empire. 312 00:25:40,760 --> 00:25:44,470 Today, it's a bit of a maze, but from up here 313 00:25:44,520 --> 00:25:48,710 you can see the shapes of the buildings, the way the streets interconnect. 314 00:25:54,640 --> 00:25:58,630 You get a real sense of how this place must have worked in its prime. 315 00:26:08,000 --> 00:26:10,710 This was the home of the Garamantians... 316 00:26:12,280 --> 00:26:15,430 ...which, for me, are a rather forgotten people. 317 00:26:15,480 --> 00:26:19,470 They've been eclipsed in the history books by their showy contemporaries, 318 00:26:19,520 --> 00:26:21,310 the Greeks and the Romans. 319 00:26:21,360 --> 00:26:27,150 The Garamantians dominated the Sahara Desert for almost 2,000 years. 320 00:26:27,200 --> 00:26:31,830 They were the society that first brought civilisation to the desert. 321 00:26:35,280 --> 00:26:39,750 Far from just scraping by in this harsh landscape, 322 00:26:39,800 --> 00:26:42,430 the Garamantes were flourishing. 323 00:26:44,080 --> 00:26:47,630 They grew crops such as cereals and grapes. 324 00:26:47,680 --> 00:26:49,990 They kept horses and pigs. 325 00:26:50,040 --> 00:26:53,350 Clearly, they needed large amounts of water. 326 00:26:56,040 --> 00:26:59,670 So where did they find it, here in the middle of the desert? 327 00:27:03,520 --> 00:27:08,190 Now, this is the key to the Garamantians' incredible success. 328 00:27:08,240 --> 00:27:12,070 It's vertical holes that are sunk deep into the ground... 329 00:27:13,520 --> 00:27:19,190 ...40 to 50 metres - that's about 150 feet. 330 00:27:19,240 --> 00:27:21,110 And the purpose of them was pretty simple - 331 00:27:21,160 --> 00:27:23,630 it was to bring water up from below ground. 332 00:27:23,680 --> 00:27:27,350 In this environment, it must have seemed like it was almost magic. 333 00:27:27,400 --> 00:27:31,710 In fact, the Garamantians had discovered groundwater. 334 00:27:33,600 --> 00:27:37,590 Beneath the surface of the Sahara is a surprising part 335 00:27:37,640 --> 00:27:39,550 of the great water cycle - 336 00:27:39,600 --> 00:27:42,670 a massive store of groundwater. 337 00:27:46,440 --> 00:27:49,510 This is water that has seeped into the ground 338 00:27:49,560 --> 00:27:53,390 and has collected in porous layers of rock. 339 00:27:53,440 --> 00:27:55,670 The water came from the period 340 00:27:55,720 --> 00:28:00,950 thousands of years before, when the Sahara was lush and wet. 341 00:28:02,520 --> 00:28:07,390 Some of that water percolated into the rocks below and remained there, 342 00:28:07,440 --> 00:28:10,390 despite the dramatic drying above... 343 00:28:12,000 --> 00:28:16,070 ...until the Garamantes found it. 344 00:28:16,120 --> 00:28:18,990 You kind of dig them down until you hit the water table 345 00:28:19,040 --> 00:28:21,350 and then you just keep doing the same thing. 346 00:28:21,400 --> 00:28:25,990 There's one after another, after another, all in a whole line. 347 00:28:27,160 --> 00:28:31,270 But these holes aren't wells - they're maintenance shafts. 348 00:28:31,320 --> 00:28:35,310 They reach down to tunnels which carried the water. 349 00:28:35,360 --> 00:28:39,270 The point is, right up there at the end is where the water source is, 350 00:28:39,320 --> 00:28:43,550 so the water flows naturally from the escarpment up there, underground, 351 00:28:43,600 --> 00:28:46,390 down to the kind of oasis over there. 352 00:28:46,440 --> 00:28:49,070 Now, that's where the Garamantians' city was. 353 00:28:49,120 --> 00:28:52,470 What they could have done is they could have dug wells down 354 00:28:52,520 --> 00:28:56,950 and lifted the water out, but that's a lot of work for very little return. 355 00:28:57,000 --> 00:29:00,190 Much better to use gravity to channel the water 356 00:29:00,240 --> 00:29:03,190 in an underground tunnel straight to where they need it. 357 00:29:04,760 --> 00:29:08,390 That was the Garamantians' real ingenuity. 358 00:29:12,360 --> 00:29:16,430 The Garamantes had managed to tap the same water 359 00:29:16,480 --> 00:29:21,150 that the early Saharans had enjoyed thousands of years earlier. 360 00:29:21,200 --> 00:29:24,630 By mining groundwater, the Garamantians managed 361 00:29:24,680 --> 00:29:29,870 to turn the clock back on the Sahara - they made the desert bloom again. 362 00:29:32,320 --> 00:29:35,630 But the human struggle to pin down water 363 00:29:35,680 --> 00:29:38,470 is forever balanced on a knife edge. 364 00:29:38,560 --> 00:29:42,230 Get that balance wrong and you pay the price. 365 00:29:43,280 --> 00:29:49,910 For all their ingenuity, the Garamantes over-exploited their groundwater. 366 00:29:49,960 --> 00:29:55,110 Eventually it ran out, and so did their civilisation. 367 00:29:56,480 --> 00:29:59,910 Now all that remains are the bats. 368 00:30:06,080 --> 00:30:11,190 Today, modern Libyans have tapped into this same groundwater supply, 369 00:30:11,240 --> 00:30:15,950 by using pumps to reach deeper than the Garamantes could. 370 00:30:16,000 --> 00:30:21,510 But just like their ancient predecessors, they're exploiting a finite resource. 371 00:30:21,560 --> 00:30:25,750 At most, it will last only another 50 years. 372 00:30:28,040 --> 00:30:32,590 But water in this most inaccessible stage of the water cycle 373 00:30:32,640 --> 00:30:34,870 is found in many other places. 374 00:30:36,440 --> 00:30:40,830 It's at its most spectacular in Tallahassee, in Florida. 375 00:30:42,920 --> 00:30:48,190 Here, divers are just beginning to explore a mysterious series of caves 376 00:30:48,240 --> 00:30:54,270 called a karst system, carved out by groundwater over millions of years. 377 00:30:57,120 --> 00:31:01,550 This is one of the planet's least known frontiers. 378 00:31:02,600 --> 00:31:05,270 When they began, these divers had no idea 379 00:31:05,320 --> 00:31:07,270 of the extent of the cave network. 380 00:31:10,680 --> 00:31:15,070 To explore these caves, they've made the longest dives in history, 381 00:31:15,120 --> 00:31:18,870 travelling more than ten kilometres from the cave entrance. 382 00:31:21,680 --> 00:31:26,390 They're sometimes underwater for 24 hours at a time. 383 00:31:31,200 --> 00:31:33,430 Their efforts have revealed 384 00:31:33,480 --> 00:31:37,190 one of the world's largest underwater cave systems. 385 00:31:38,760 --> 00:31:43,230 It's part of a huge store of groundwater, of varying depths, 386 00:31:43,280 --> 00:31:48,710 that underlies all of Florida and reaches into neighbouring states. 387 00:31:48,760 --> 00:31:51,230 And it's not just the USA. 388 00:31:51,280 --> 00:31:55,950 There's groundwater in the most unexpected places. 389 00:31:56,000 --> 00:32:00,870 More than 30% of all the fresh water on Earth is under our feet. 390 00:32:03,080 --> 00:32:09,910 Looked at this way, our apparently solid planet is more like a sponge. 391 00:32:17,200 --> 00:32:19,110 In our early history, 392 00:32:19,160 --> 00:32:24,630 the need for reliable supplies of water led us to rivers and groundwater. 393 00:32:27,800 --> 00:32:32,430 But as humans spread across the planet, they learned to exploit 394 00:32:32,480 --> 00:32:36,150 the vagaries of the water cycle in many different ways. 395 00:32:40,280 --> 00:32:43,030 The key was adaptation. 396 00:32:47,600 --> 00:32:49,750 Take rain. 397 00:32:54,320 --> 00:32:58,670 A familiar occurrence in many parts of the world. 398 00:32:58,720 --> 00:33:02,230 But this is rain at its most extreme - 399 00:33:02,280 --> 00:33:04,070 the monsoon. 400 00:33:13,320 --> 00:33:17,270 The significance of the monsoon isn't the human discomfort 401 00:33:17,320 --> 00:33:21,630 but how the people here have learned to live with it. 402 00:33:22,720 --> 00:33:25,830 I'm travelling to the very epicentre of the monsoon, 403 00:33:25,880 --> 00:33:28,310 a place called Cherrapunjee, 404 00:33:28,360 --> 00:33:33,150 which holds the world record for the highest rainfall in a single year. 405 00:33:45,200 --> 00:33:48,390 I thought I knew rain. 406 00:33:48,440 --> 00:33:51,590 If you're from the west of Scotland, you've met rain before, 407 00:33:51,640 --> 00:33:53,670 but this is different, it's different rain. 408 00:33:53,720 --> 00:33:56,510 It's hard to explain. It's the sheer intensity of it - 409 00:33:56,560 --> 00:33:58,150 it just comes barrelling down. 410 00:33:58,200 --> 00:34:01,110 But also, the raindrops are massive. 411 00:34:01,160 --> 00:34:04,870 You feel as if you could fill an egg cup with them, which means that, 412 00:34:04,920 --> 00:34:06,670 within minutes, you're just soaked. 413 00:34:06,720 --> 00:34:09,430 It's pointless with a hood and all the rest of it - I'm soaked. 414 00:34:09,480 --> 00:34:12,030 What I really need is a brolly, like this chap. 415 00:34:13,600 --> 00:34:15,230 Very wet! 416 00:34:16,480 --> 00:34:17,590 Wet. 417 00:34:17,640 --> 00:34:19,630 Just watch it, it's very slidy. 418 00:34:23,160 --> 00:34:27,030 Back in west Scotland, where I'm from, the average annual rainfall 419 00:34:27,080 --> 00:34:31,750 is nearly a metre, and that might horrify a Californian, 420 00:34:31,800 --> 00:34:35,870 but here in Cherrapunjee, the annual average rainfall 421 00:34:35,920 --> 00:34:41,310 is more than ten times that - between 11 and 12 metres. 422 00:34:41,360 --> 00:34:44,430 That's nearly the height of a four-storey building. 423 00:34:49,760 --> 00:34:55,310 Streams turn to rivers, and rivers turn to torrents. 424 00:35:00,640 --> 00:35:04,750 When you live with so much water, you have to adapt... 425 00:35:04,800 --> 00:35:07,030 just to get around. 426 00:35:07,080 --> 00:35:11,030 And that's exactly what the local Khasi people have done. 427 00:35:20,680 --> 00:35:22,910 Look at this! 428 00:35:22,960 --> 00:35:26,510 Isn't this fantastic? Look at it! 429 00:35:26,560 --> 00:35:32,470 It's a living bridge - look, you can see all these roots coming down. 430 00:35:32,520 --> 00:35:34,830 The texture of them is beautiful. 431 00:35:41,280 --> 00:35:45,910 I mean, this entire structure is built of growing rubber tree. 432 00:35:51,040 --> 00:35:52,830 It's just mad when you follow it! 433 00:35:55,120 --> 00:35:58,790 You can see that this is the perfect union of the tree and the villagers. 434 00:35:58,840 --> 00:36:01,230 The locals have kind of trained the roots, 435 00:36:01,280 --> 00:36:04,510 kind of guided them through, knitted them together. 436 00:36:07,720 --> 00:36:10,790 What they've done here is they've grabbed some rootlets like this 437 00:36:10,840 --> 00:36:13,310 and taken it round. And look, here it is... 438 00:36:13,360 --> 00:36:16,630 this set of rootlets here. That's incredibly strong. 439 00:36:16,680 --> 00:36:18,390 It's an anchor for the bridge. 440 00:36:20,680 --> 00:36:25,390 Ordinary bridges would rot under the relentless drenching of the monsoon. 441 00:36:25,440 --> 00:36:30,710 What's clever about these root bridges is they get stronger as they get older. 442 00:36:30,760 --> 00:36:34,950 So wide! I mean, a whole village could get through here. 443 00:36:45,240 --> 00:36:49,430 Surprisingly, the intensity of the monsoon rain is all down 444 00:36:49,480 --> 00:36:51,510 to a basic property of water. 445 00:36:56,880 --> 00:37:01,630 Compared to other substances, water takes a lot of energy to heat up. 446 00:37:05,200 --> 00:37:08,430 So the land and the ocean react very differently 447 00:37:08,480 --> 00:37:11,510 to the rising temperatures of early summer. 448 00:37:14,520 --> 00:37:18,550 During these months, India's land surface heats up much more 449 00:37:18,600 --> 00:37:20,870 than the surrounding Indian Ocean. 450 00:37:22,760 --> 00:37:26,470 The high temperature reduces the density of the air, 451 00:37:26,520 --> 00:37:29,070 creating low pressure. 452 00:37:29,120 --> 00:37:33,070 That sucks moist ocean air onto the land, 453 00:37:33,120 --> 00:37:35,590 which brings rain. 454 00:37:39,520 --> 00:37:42,710 It's because the whole system is driven by the sun's heat 455 00:37:42,760 --> 00:37:44,870 that the rains come in the summer. 456 00:37:47,040 --> 00:37:49,910 But it also means that the monsoon 457 00:37:49,960 --> 00:37:53,350 only lasts for three months of the year. 458 00:37:53,400 --> 00:37:57,510 For the rest of the time, there's virtually no rain. 459 00:38:05,720 --> 00:38:08,190 The people of India have adapted, 460 00:38:08,240 --> 00:38:11,590 as much as they can, to these extremes of the monsoon. 461 00:38:12,840 --> 00:38:14,430 I think it's this way. 462 00:38:14,480 --> 00:38:17,350 It's great, you have to use your elbows in here. 463 00:38:23,840 --> 00:38:27,310 But outsiders are not always so sensitive to its rhythms. 464 00:38:32,240 --> 00:38:36,750 Here in India, the changing strength of the monsoon year on year 465 00:38:36,800 --> 00:38:41,310 had really tremendous impacts on the country's political fortunes. 466 00:38:41,360 --> 00:38:44,990 That's especially true of its recent colonial past, 467 00:38:45,040 --> 00:38:46,790 the story of which was played out 468 00:38:46,840 --> 00:38:50,550 against a backdrop of water abundance and scarcity. 469 00:38:50,600 --> 00:38:53,710 Clearly there are lots of reasons to explain the fate 470 00:38:53,760 --> 00:38:55,790 of British colonial rule in India, 471 00:38:55,840 --> 00:39:00,310 but one of the least explored and most intriguing is water. 472 00:39:03,880 --> 00:39:06,790 In the 19th century, the failure of the British 473 00:39:06,840 --> 00:39:11,350 to manage India's water supply had significant consequences... 474 00:39:11,400 --> 00:39:13,630 for them and for the Indian people. 475 00:39:22,360 --> 00:39:26,150 Perhaps it was naivety, perhaps it was because they were outsiders, 476 00:39:26,200 --> 00:39:30,310 perhaps it was their inability to cope with extreme weather, 477 00:39:30,360 --> 00:39:34,110 but the British never really got to grips with the monsoon. 478 00:39:39,120 --> 00:39:42,270 For thousands of years, people here have been developing ways 479 00:39:42,320 --> 00:39:44,150 to deal with the monsoon. 480 00:39:44,200 --> 00:39:48,230 And this was one of the most important - it's a huge open well 481 00:39:48,280 --> 00:39:52,190 that was dug down deep enough to reach groundwater. 482 00:39:52,240 --> 00:39:57,470 When the rains came, the water was filtered through the surrounding ground 483 00:39:57,520 --> 00:40:01,230 and held in the well like a gigantic bucket. 484 00:40:01,280 --> 00:40:04,030 But these stepwells, as they were known, 485 00:40:04,080 --> 00:40:06,390 were more than water collectors. 486 00:40:06,440 --> 00:40:10,550 The genius of this design was it turned the mundane need for water 487 00:40:10,600 --> 00:40:12,790 into a social ritual. 488 00:40:12,840 --> 00:40:17,470 People didn't just come here to dip for water - they gossiped, 489 00:40:17,520 --> 00:40:19,910 they bathed, they even worshipped. 490 00:40:23,840 --> 00:40:27,950 Over 3,000 stepwells were built, up until the 19th century. 491 00:40:29,000 --> 00:40:32,910 For millions, they were the main source of water. 492 00:40:37,360 --> 00:40:40,310 Despite the fact that structures like this helped the Indian people 493 00:40:40,360 --> 00:40:43,150 survive droughts, the British didn't like it. 494 00:40:43,200 --> 00:40:46,150 They were concerned that people bathing in the same water they drank from 495 00:40:46,200 --> 00:40:48,230 was bad news. 496 00:40:48,280 --> 00:40:51,310 So on health grounds, they shut them down. 497 00:40:51,360 --> 00:40:53,350 I mean, they may have had a point, 498 00:40:53,400 --> 00:40:56,990 and they solved that issue by bringing in piped water, 499 00:40:57,040 --> 00:40:59,830 but at the same time, they imported another problem 500 00:40:59,880 --> 00:41:01,950 that was much, much worse. 501 00:41:04,040 --> 00:41:05,590 It's a little-known fact, 502 00:41:05,640 --> 00:41:09,750 but the British built canals on a colossal scale across India, 503 00:41:09,800 --> 00:41:12,430 more than 57,000 kilometres of them - 504 00:41:12,480 --> 00:41:16,390 perhaps their biggest engineering achievement anywhere. 505 00:41:19,480 --> 00:41:23,190 Yet the British didn't realise that, even more than stepwells, 506 00:41:23,240 --> 00:41:27,510 these huge bodies of standing water were a health hazard - 507 00:41:27,560 --> 00:41:32,230 the perfect environment for malaria to breed and spread. 508 00:41:37,360 --> 00:41:41,950 Given the lack of sensitivity the British showed to the Indian climate, 509 00:41:42,000 --> 00:41:46,350 it's perhaps ironic that the monsoon played a significant role 510 00:41:46,400 --> 00:41:49,270 in undermining British rule in India. 511 00:41:56,640 --> 00:42:00,630 At the end of the 19th century, the monsoon rains failed. 512 00:42:06,120 --> 00:42:09,390 For a decade, there were repeated droughts. 513 00:42:09,440 --> 00:42:12,990 Crops were ruined, and there were terrible famines. 514 00:42:15,080 --> 00:42:18,270 But the British failed to respond effectively - 515 00:42:18,320 --> 00:42:22,990 in fact, they even continued to export rice. 516 00:42:23,040 --> 00:42:27,150 This indifference to the rhythms of the monsoon fuelled popular anger 517 00:42:27,200 --> 00:42:32,390 against colonial rule, and the independence movement grew rapidly. 518 00:42:41,480 --> 00:42:43,950 Today, the stepwells are being repaired. 519 00:42:46,800 --> 00:42:49,510 Pumps accessing groundwater are used to protect 520 00:42:49,560 --> 00:42:51,630 against the unreliable monsoon. 521 00:42:53,640 --> 00:42:58,830 And that's made India the largest user of groundwater in the world. 522 00:43:07,800 --> 00:43:11,470 Adapting to the water cycle has meant the difference between success 523 00:43:11,520 --> 00:43:14,150 and failure for many civilisations. 524 00:43:19,400 --> 00:43:23,830 But there was another strategy that also brought success... 525 00:43:24,840 --> 00:43:28,750 ...and that was to take control of the water cycle. 526 00:43:34,440 --> 00:43:38,710 There was one early civilisation above all others that took control 527 00:43:38,760 --> 00:43:42,310 of the planet's most dramatically changing source of water. 528 00:43:43,960 --> 00:43:46,790 They mastered the monsoon. 529 00:43:51,480 --> 00:43:53,390 They were the Khmers, 530 00:43:53,440 --> 00:43:56,230 and from the 9th century, they dominated the area 531 00:43:56,280 --> 00:43:59,470 we now know as Cambodia. 532 00:43:59,520 --> 00:44:02,710 And this was their greatest achievement... 533 00:44:02,760 --> 00:44:06,870 the legendary temple complex of Angkor. 534 00:44:13,800 --> 00:44:16,910 You get a real sense of the age of this place here, 535 00:44:16,960 --> 00:44:19,550 cos this was built over 1, 200 years ago. 536 00:44:21,240 --> 00:44:24,630 In a few places, like here, you can see it's showing the age. 537 00:44:24,680 --> 00:44:27,950 Look, the faces have all gone, 538 00:44:28,000 --> 00:44:33,070 but, look at this, that looks as if it could have been carved just yesterday. 539 00:44:42,240 --> 00:44:44,590 Angkor was built to honour the Hindu gods 540 00:44:44,640 --> 00:44:48,590 and it symbolised the extraordinary success of the Khmers. 541 00:44:53,560 --> 00:44:56,510 In a way, this place is a monument to something else - 542 00:44:56,560 --> 00:45:01,230 the Khmers' ability to harness the power of the monsoon. 543 00:45:12,120 --> 00:45:14,470 The Khmers were first drawn to this region 544 00:45:14,520 --> 00:45:18,310 by the Tonlé Sap lake and the river that feeds it. 545 00:45:22,000 --> 00:45:26,270 Today, it's home to a floating, permanent community, 546 00:45:26,320 --> 00:45:28,510 replete with all the necessary amenities. 547 00:45:36,400 --> 00:45:38,550 All life here is lived on the river - 548 00:45:38,600 --> 00:45:44,230 the whole village, houses, shops, churches, schools, everything. 549 00:45:48,080 --> 00:45:50,550 A hardware store! 550 00:45:54,800 --> 00:45:58,150 Everybody's watching telly. They're all watching soap operas, 551 00:45:58,200 --> 00:46:00,070 or just chilling out. 552 00:46:01,280 --> 00:46:04,470 People settle here today for the same reason 553 00:46:04,520 --> 00:46:07,870 the Khmers did over 1,000 years ago - 554 00:46:07,920 --> 00:46:12,830 the unusual behaviour of the lake around monsoon time. 555 00:46:12,880 --> 00:46:17,070 Each year when the monsoon rains fall, the land around here 556 00:46:17,120 --> 00:46:22,830 just can't drain fast enough, and this lake, Tonlé Sap, swells enormously. 557 00:46:22,880 --> 00:46:27,590 It more than trebles in size, becoming, for just a few months, 558 00:46:27,640 --> 00:46:31,110 the largest freshwater lake in Southeast Asia. 559 00:46:38,720 --> 00:46:43,270 And every year, the water brings with it a spectacular bounty. 560 00:46:48,880 --> 00:46:53,990 Fish! Loads of them, nibbling away at your toes in this murky water. 561 00:46:57,160 --> 00:47:00,110 So many, that when it floods, the Tonlé Sap lake 562 00:47:00,160 --> 00:47:03,910 becomes the richest source of freshwater fish in the world. 563 00:47:07,240 --> 00:47:10,470 Back in the 9th century, the Khmers realised 564 00:47:10,520 --> 00:47:15,590 that this annual influx of fish and water offered a glittering opportunity. 565 00:47:19,520 --> 00:47:23,750 They set about building a fishing industry here, and with the profits, 566 00:47:23,800 --> 00:47:26,150 they built the temples of Angkor. 567 00:47:29,480 --> 00:47:34,030 But as it grew, the Khmer kingdom faced a stumbling block. 568 00:47:35,280 --> 00:47:40,710 When the monsoon finished each year, the fish and water would vanish. 569 00:47:40,760 --> 00:47:45,230 So each year, the inhabitants were plunged into drought and hunger. 570 00:47:45,280 --> 00:47:48,950 The Khmer rose to the challenge magnificently. 571 00:47:49,000 --> 00:47:53,270 They decided that rather than be at the whim of the monsoon, 572 00:47:53,320 --> 00:47:55,670 they would make it work for them. 573 00:48:11,320 --> 00:48:13,110 This is part of a vast network 574 00:48:13,160 --> 00:48:17,030 of irrigation tunnels that crisscross the whole of Angkor. 575 00:48:17,080 --> 00:48:20,150 When the Khmer started digging these in the 9th century, 576 00:48:20,200 --> 00:48:22,470 people had seen nothing like them. 577 00:48:22,520 --> 00:48:25,790 This was plumbing on a grand scale. 578 00:48:35,960 --> 00:48:38,910 From the air, it's still visible today. 579 00:48:41,360 --> 00:48:45,830 Over 1,000 years ago, the Khmers managed to divert a river 580 00:48:45,880 --> 00:48:47,910 by 80 kilometres. 581 00:48:49,720 --> 00:48:51,270 They built canals 582 00:48:51,320 --> 00:48:56,350 that extended over an area of 1,000 square kilometres 583 00:48:56,400 --> 00:48:57,950 and dug reservoirs 584 00:48:58,000 --> 00:49:02,990 that could hold up to 600 million cubic metres of monsoon water. 585 00:49:05,520 --> 00:49:10,950 With this system, the Khmers seized control of the planet's water cycle. 586 00:49:11,000 --> 00:49:14,910 They turned the seasonal rainfall of the monsoon into a reliable, 587 00:49:14,960 --> 00:49:17,830 all-year-round water supply. 588 00:49:19,880 --> 00:49:24,150 It was an enormous achievement, enabling Angkor at its peak 589 00:49:24,200 --> 00:49:27,910 to support a population in excess of one million. 590 00:49:32,800 --> 00:49:35,950 Thanks to their control of water, 591 00:49:36,000 --> 00:49:40,150 the Khmers had built the largest pre-industrial city in the world. 592 00:49:51,160 --> 00:49:54,150 The Khmer hung on until the 15th century, 593 00:49:54,200 --> 00:49:58,630 which was when the kingdom of Angkor finally went to the wall. 594 00:49:58,680 --> 00:50:01,070 They were victims of their own success. 595 00:50:01,120 --> 00:50:03,390 Their population went through the roof, 596 00:50:03,440 --> 00:50:07,230 and they simply outstripped their resources, including - 597 00:50:07,280 --> 00:50:10,670 despite all that incredible engineering - 598 00:50:10,720 --> 00:50:12,910 including the water supply. 599 00:50:12,960 --> 00:50:14,670 I guess that there are limits 600 00:50:14,720 --> 00:50:17,670 to what even the mighty monsoon can sustain. 601 00:50:26,200 --> 00:50:30,030 Today, we control water on a massive scale. 602 00:50:33,280 --> 00:50:40,110 The world's reservoirs now hold over 10,000 cubic kilometres of water. 603 00:50:40,160 --> 00:50:45,670 That's five times as much water as in all the rivers on Earth. 604 00:50:47,800 --> 00:50:52,110 And because most of it is pooled in the more populated northern hemisphere, 605 00:50:52,160 --> 00:50:56,350 away from the equator, the extra weight has slightly changed how the Earth 606 00:50:56,400 --> 00:50:58,070 spins on its axis. 607 00:50:58,120 --> 00:51:00,830 It's caused the Earth's rotation to speed up, 608 00:51:00,880 --> 00:51:06,150 shortening the day by 8 millionths of a second in the last 40 years. 609 00:51:14,800 --> 00:51:18,550 Today, we take our control of water for granted. 610 00:51:18,600 --> 00:51:22,590 Modern civilisation couldn't exist without it. 611 00:51:24,000 --> 00:51:28,110 But there's still only a finite amount of water to go around. 612 00:51:28,160 --> 00:51:31,110 In many parts of the world, 613 00:51:31,160 --> 00:51:36,590 scarcity has led to a bitter struggle for control over the available supply. 614 00:51:36,640 --> 00:51:40,270 And that's true in even the wealthiest countries. 615 00:51:41,880 --> 00:51:47,070 Today, Los Angeles is a city with every luxury and convenience. 616 00:51:52,520 --> 00:51:55,150 Yet not so long ago, at the turn of the last century, 617 00:51:55,200 --> 00:51:58,350 Los Angeles was struggling. 618 00:51:59,480 --> 00:52:04,870 LA's problem was its location, hemmed in on three sides by desert 619 00:52:04,920 --> 00:52:07,470 and on the fourth by ocean. 620 00:52:07,520 --> 00:52:11,270 So it lacked the most basic requirement for city life - 621 00:52:11,320 --> 00:52:13,750 a reliable water supply. 622 00:52:16,480 --> 00:52:20,510 So it came up with a plan to get the water it so needed. 623 00:52:27,280 --> 00:52:30,790 400 kilometres to the north of the growing city, 624 00:52:30,840 --> 00:52:33,830 nestled within the Sierra Nevada mountain range, 625 00:52:33,880 --> 00:52:36,710 was a place called Owens Valley. 626 00:52:37,880 --> 00:52:43,070 It was a verdant place, where people were settling and building farms. 627 00:52:43,120 --> 00:52:46,310 At the heart of it was plentiful water - 628 00:52:46,360 --> 00:52:49,310 a wide river feeding a huge lake. 629 00:52:54,200 --> 00:52:58,710 This valley must have seemed like the answer to Los Angeles' prayers. 630 00:52:58,760 --> 00:53:01,550 There was enough water here to easily supply 631 00:53:01,600 --> 00:53:03,510 over one million people. 632 00:53:03,560 --> 00:53:05,430 There was only one problem... 633 00:53:05,480 --> 00:53:07,150 it didn't belong to them. 634 00:53:07,200 --> 00:53:11,150 It belonged to the farmers of Owens Valley. 635 00:53:11,200 --> 00:53:14,070 It would have to be taken by stealth. 636 00:53:24,640 --> 00:53:27,070 It wasn't long before men appeared in the valley, 637 00:53:27,120 --> 00:53:29,190 masquerading as investors. 638 00:53:31,080 --> 00:53:35,990 They offered to buy up farmland at seemingly irresistible prices, 639 00:53:36,040 --> 00:53:38,710 just to get the water rights that went with it. 640 00:53:40,280 --> 00:53:44,190 It wasn't technically illegal, but it was certainly shady. 641 00:53:45,280 --> 00:53:47,510 And it worked. 642 00:53:47,560 --> 00:53:51,190 In 1913, after six years of construction, 643 00:53:51,240 --> 00:53:53,030 an aqueduct was opened. 644 00:53:53,080 --> 00:53:54,550 And this is it. 645 00:54:07,760 --> 00:54:10,750 In a way, this aqueduct was a triumph, 646 00:54:10,800 --> 00:54:13,470 certainly as far as Los Angeles was concerned. 647 00:54:13,520 --> 00:54:16,350 It allowed millions of people 200 miles down there 648 00:54:16,400 --> 00:54:19,510 to live in a growing and vibrant city. 649 00:54:19,560 --> 00:54:22,430 But that's not how people here saw it. 650 00:54:24,120 --> 00:54:28,070 The Owens Valley farmers didn't give up without a struggle. 651 00:54:30,440 --> 00:54:32,670 A kind of loose resistance movement started, 652 00:54:32,720 --> 00:54:36,390 and they would take over places like this and open the sluice gates, 653 00:54:36,440 --> 00:54:39,510 allowing the water to pour back down into Owens Valley. 654 00:54:39,560 --> 00:54:43,590 And regularly they'd dynamite the aqueduct. 655 00:54:46,120 --> 00:54:47,630 But the city rebuilt it, 656 00:54:47,680 --> 00:54:50,750 and a game of cat and mouse continued 657 00:54:50,800 --> 00:54:53,630 for three more dynamite-filled years. 658 00:54:53,680 --> 00:54:58,510 Eventually, the police clamped down with a "shoot to kill" policy, 659 00:54:58,560 --> 00:55:00,390 and the rebellion fizzled out. 660 00:55:00,440 --> 00:55:03,110 The city had won. 661 00:55:07,920 --> 00:55:12,910 Today, the Los Angeles Aqueduct is just part of a giant network 662 00:55:12,960 --> 00:55:17,470 of pipes and aqueducts all serving one of the world's great cities. 663 00:55:36,240 --> 00:55:39,630 But, back in Owens Valley, the lake has all but vanished, 664 00:55:39,680 --> 00:55:42,590 and the river is barely a trickle. 665 00:55:44,840 --> 00:55:49,230 The story of Owens Valley is not an isolated case. 666 00:55:49,280 --> 00:55:53,790 Today, there are conflicts over water taking place all around the world. 667 00:55:58,600 --> 00:56:01,750 Israel, the Palestinians, Syria and Jordan 668 00:56:01,800 --> 00:56:04,350 dispute access to the River Jordan. 669 00:56:06,520 --> 00:56:11,310 Egypt, Sudan and Ethiopia quarrel over the waters of the Nile. 670 00:56:15,080 --> 00:56:17,590 On the Indus river, 671 00:56:17,640 --> 00:56:22,430 India and Pakistan are in conflict over dams built on the river's tributaries. 672 00:56:22,480 --> 00:56:25,550 And these are only some of the more well-known examples. 673 00:56:30,120 --> 00:56:33,310 10,000 years ago, we lived at the whim 674 00:56:33,360 --> 00:56:36,190 of the unpredictable water cycle. 675 00:56:38,720 --> 00:56:42,310 Since then, we have harnessed the power of rivers 676 00:56:42,360 --> 00:56:44,630 to advance our civilisations. 677 00:56:48,680 --> 00:56:51,470 We have extracted groundwater from the depths 678 00:56:51,520 --> 00:56:53,110 of the most unlikely places. 679 00:56:55,520 --> 00:57:00,550 And we have learned to redirect and store water on a massive scale. 680 00:57:07,000 --> 00:57:11,470 Today, we have unprecedented power over the planet's water. 681 00:57:13,040 --> 00:57:15,350 But one thing hasn't changed - 682 00:57:15,400 --> 00:57:19,910 there's still only a finite amount of water on Earth. 683 00:57:27,200 --> 00:57:28,670 It seems to me 684 00:57:28,720 --> 00:57:32,430 that water is the Achilles heel of our modern civilisation. 685 00:57:32,480 --> 00:57:34,270 It's the one resource, 686 00:57:34,320 --> 00:57:35,550 more than any other, 687 00:57:35,600 --> 00:57:38,350 with the potential to limit our ambitions. 688 00:57:40,360 --> 00:57:44,630 The fundamental limits of the water cycle are still there. 689 00:57:44,680 --> 00:57:49,310 But the lesson of history is that the most successful civilisations 690 00:57:49,360 --> 00:57:51,790 learn to adapt to those limits. 691 00:57:53,480 --> 00:57:56,390 So the problem is more with us. 692 00:57:56,440 --> 00:58:01,550 Now, that prospect may find you gloomy or, like me, more optimistic. 693 00:58:01,600 --> 00:58:06,350 But either way, at least the future's in our hands. 694 00:58:09,160 --> 00:58:12,310 Next time, the contradictory role of the deep Earth. 695 00:58:14,320 --> 00:58:17,990 It drove great technological breakthroughs, 696 00:58:18,040 --> 00:58:20,710 but its gifts... 697 00:58:20,760 --> 00:58:22,390 came at a price.