WEBVTT NOTE language en-US 00:00:00.000 --> 00:00:04.240 The oceans hold many mysteries we're still trying to unravel. 00:00:04.240 --> 00:00:06.735 But while lots of us want to know what secrets 00:00:06.790 --> 00:00:08.200 lie in its deepest points, 00:00:08.200 --> 00:00:11.240 there are things just as strange in the shallows. 00:00:11.240 --> 00:00:15.299 And sometimes these bizarre formations can be clearly visible without 00:00:15.358 --> 00:00:17.240 diving below the surface at all. 00:00:17.240 --> 00:00:21.390 This is unveiled, and today we're answering the extraordinary question. 00:00:21.390 --> 00:00:24.887 What are these unexplained undersea sinkholes and what do they 00:00:24.944 --> 00:00:25.790 mean for Earth? 00:00:25.790 --> 00:00:27.710 Do you need the big questions answered? 00:00:27.710 --> 00:00:31.268 Are you constantly curious? Then why not subscribe to unveiled 00:00:31.325 --> 00:00:33.964 for more clips like this one and ring the bell 00:00:34.022 --> 00:00:36.030 for more thought provoking content? 00:00:36.030 --> 00:00:38.599 The sea, from the choppy waves on the surface to 00:00:38.653 --> 00:00:39.670 the deepest depths, 00:00:39.670 --> 00:00:42.110 plays host to many strange phenomena. 00:00:42.110 --> 00:00:45.597 It's long proven dangerous not only to sailors venturing into 00:00:45.654 --> 00:00:46.740 treacherous waters. 00:00:46.740 --> 00:00:48.860 But also to people living on the coast. 00:00:48.860 --> 00:00:52.257 Many cultures have had flood myths dating back thousands of 00:00:52.315 --> 00:00:52.660 years, 00:00:52.660 --> 00:00:55.991 including the oldest piece of literature that survives to this 00:00:56.045 --> 00:00:56.260 day, 00:00:56.260 --> 00:00:58.740 the Mesopotamian Epic of Gilgamesh. 00:00:58.740 --> 00:01:01.007 A flood myth is also a major part of the 00:01:01.063 --> 00:01:03.500 Book of Genesis and The Legend of Atlantis. 00:01:03.500 --> 00:01:06.180 People fear the power and might of the sea, 00:01:06.180 --> 00:01:09.820 and that could be why these vast underwater sinkholes are 00:01:09.884 --> 00:01:11.800 so extraordinarily unsettling. 00:01:11.800 --> 00:01:15.162 The most famous underwater sinkhole is the great blue hole 00:01:15.220 --> 00:01:18.118 just off the coast of Belize in Central America in 00:01:18.176 --> 00:01:20.320 a small atoll called Lighthouse Reef. 00:01:20.320 --> 00:01:22.760 Because of its position in an otherwise shallow, 00:01:22.760 --> 00:01:24.560 idyllic corner of the Caribbean, 00:01:24.560 --> 00:01:27.247 with other parts of the reef being used as popular 00:01:27.301 --> 00:01:28.000 diving spots, 00:01:28.000 --> 00:01:31.120 the great blue hole has attracted a lot of attention 00:01:31.120 --> 00:01:33.360 due to its depth of over 400 feet. 00:01:33.360 --> 00:01:37.064 It looks significantly darker than the pale coral reef and 00:01:37.127 --> 00:01:38.660 shallow water around it. 00:01:38.660 --> 00:01:42.023 It's also home to diverse sea life and the entrance 00:01:42.089 --> 00:01:44.660 to an extensive underwater cave system. 00:01:44.660 --> 00:01:46.900 The spot is popular with scuba divers, 00:01:46.900 --> 00:01:49.020 although it isn't without danger. 00:01:49.020 --> 00:01:51.500 3 divers have disappeared inside, 00:01:51.500 --> 00:01:54.260 although only two bodies have ever been found. 00:01:54.260 --> 00:01:57.074 Another famous blue hole like this is just north of 00:01:57.129 --> 00:01:57.460 Dahab, 00:01:57.460 --> 00:02:00.347 Egypt, on the coast of the Red Sea that separates 00:02:00.406 --> 00:02:01.820 Egypt from Saudi Arabia. 00:02:01.820 --> 00:02:03.660 Simply called the blue hole, 00:02:03.660 --> 00:02:05.580 it's much less striking to look at, 00:02:05.580 --> 00:02:09.461 but significantly more dangerous. As many as 200 divers have 00:02:09.525 --> 00:02:10.560 perished within, 00:02:10.560 --> 00:02:13.772 despite its unassuming appearance. If you didn't know what it 00:02:13.824 --> 00:02:15.720 was and looked at it from the coast, 00:02:15.720 --> 00:02:18.840 you'd have no idea it was over 300 feet deep. 00:02:18.840 --> 00:02:21.511 The likely cause for the deaths is that the blue 00:02:21.567 --> 00:02:23.960 hole has a tunnel entrance called the arch, 00:02:23.960 --> 00:02:27.080 and in the past inexperienced divers would get lost and 00:02:27.137 --> 00:02:30.200 tragically lose their lives trying to swim through it. 00:02:30.200 --> 00:02:32.240 There are many more blue holes too, 00:02:32.240 --> 00:02:35.654 including one named the Dragon Hole in the South China 00:02:35.717 --> 00:02:35.970 Sea. 00:02:35.970 --> 00:02:39.410 The deepest anywhere in the world at nearly 1000 feet. 00:02:39.410 --> 00:02:42.176 And there are multiple blue holes around the Bahamas in 00:02:42.226 --> 00:02:42.930 the Caribbean. 00:02:42.930 --> 00:02:45.650 In fact, it's thought that there may be over 1000, 00:02:45.650 --> 00:02:48.930 and so far we've only explored around 200 of them. 00:02:48.930 --> 00:02:51.730 But what are these submarine sinkholes? 00:02:51.730 --> 00:02:54.050 Is there something more sinister going on? 00:02:54.050 --> 00:02:56.570 Well, interestingly, many of these sinkholes, 00:02:56.570 --> 00:02:58.130 including the one in Belize, 00:02:58.130 --> 00:03:01.639 were created thousands of years ago when planet Earth was 00:03:01.700 --> 00:03:03.670 in the grip of the last Ice Age. 00:03:03.670 --> 00:03:05.990 Which lasted from around 00:03:05.990 --> 00:03:07.790 115,000 to 00:03:07.790 --> 00:03:11.110 11,500 years ago, peaking 20,000 years ago. 00:03:11.110 --> 00:03:14.006 Back then, sea levels were much lower because so much 00:03:14.060 --> 00:03:15.590 of Earth's water was frozen. 00:03:15.590 --> 00:03:19.010 It's believed by scientists that most of these holes were 00:03:19.070 --> 00:03:20.990 created through carst processes. 00:03:20.990 --> 00:03:23.702 In a nutshell, this is when certain types of rock 00:03:23.757 --> 00:03:25.030 are dissolved by water. 00:03:25.030 --> 00:03:26.910 Usually this involves carbonate rock, 00:03:26.910 --> 00:03:30.570 often limestone being slowly dissolved by rain. 00:03:30.570 --> 00:03:33.098 Back when these regions of the planet were still high 00:03:33.145 --> 00:03:34.290 above the water surface, 00:03:34.290 --> 00:03:36.869 as we mentioned, the great blue hole exists in the 00:03:36.921 --> 00:03:38.210 heart of Lighthouse reef, 00:03:38.210 --> 00:03:40.810 and Lighthouse Reef is a carbonate platform, 00:03:40.810 --> 00:03:43.646 so it's made of those very same carbonate rocks that 00:03:43.701 --> 00:03:45.010 are so easily dissolved. 00:03:45.010 --> 00:03:47.646 In fact, most of the most notable coral reefs in 00:03:47.701 --> 00:03:48.250 the world, 00:03:48.250 --> 00:03:51.180 including the Great Barrier Reef in the Coral Sea of 00:03:51.236 --> 00:03:52.250 the South Pacific, 00:03:52.250 --> 00:03:56.311 are on carbonate platforms made-up of the ancient skeletons of 00:03:56.376 --> 00:03:57.490 simple organisms. 00:03:57.490 --> 00:04:00.328 During the Ice Age, rainfall was all that was needed 00:04:00.382 --> 00:04:03.874 to dissolve the carbonate and create these enormous cave systems 00:04:03.929 --> 00:04:06.330 that were then filled in by water as the ice 00:04:06.330 --> 00:04:09.921 melted. Thankfully, this is much less dramatic than the mechanisms 00:04:09.976 --> 00:04:12.370 that have been making new sinkholes on land. 00:04:12.370 --> 00:04:16.186 Sometimes called exploding craters, it's been a popular news topic 00:04:16.243 --> 00:04:19.308 in recent years as new sinkholes appear in the Arctic 00:04:19.365 --> 00:04:19.770 Circle, 00:04:19.770 --> 00:04:22.920 largely Siberia. As permafrost melts, 00:04:22.920 --> 00:04:26.000 methane deposits have been expanding and exploding. 00:04:26.000 --> 00:04:30.166 They create huge underground voids that then collapse into dangerous 00:04:30.227 --> 00:04:30.840 sinkholes, 00:04:30.840 --> 00:04:34.720 some of which are still growing and threatening nearby communities. 00:04:34.720 --> 00:04:38.469 This is a frightening consequence of global climate change driven 00:04:38.527 --> 00:04:39.680 by human activities, 00:04:39.680 --> 00:04:43.258 while underwater sinkholes are also the result of warming after 00:04:43.315 --> 00:04:44.280 the last Ice Age, 00:04:44.280 --> 00:04:47.842 this was part of a natural and much more gradual 00:04:47.916 --> 00:04:48.510 process. 00:04:48.510 --> 00:04:51.270 Having said that, some blue holes do seem to have 00:04:51.326 --> 00:04:54.030 formed through means other than carst processes. 00:04:54.030 --> 00:04:56.550 When sinkholes are produced through carst processes, 00:04:56.550 --> 00:04:59.550 it also creates many additional smaller tunnels, 00:04:59.550 --> 00:05:02.030 sometimes building mass cave systems. 00:05:02.030 --> 00:05:05.310 However, not every blue hole has these smaller tunnels. 00:05:05.310 --> 00:05:08.230 Some are simply deep shafts in the shallows. 00:05:08.230 --> 00:05:11.370 Some studies suggest they may have been created by other 00:05:11.426 --> 00:05:13.950 mechanisms like the sheer force of the tides, 00:05:13.950 --> 00:05:16.050 but more research needs to be done. 00:05:16.050 --> 00:05:18.650 That's part of what makes the blue hole so enticing, 00:05:18.650 --> 00:05:22.250 not just to divers looking for a unique otherworldly experience, 00:05:22.250 --> 00:05:24.610 but for scientists the world over. 00:05:24.610 --> 00:05:27.370 So what can we learn by studying blue holes? 00:05:27.370 --> 00:05:29.790 Well, one major thing is that they can teach us 00:05:29.841 --> 00:05:32.930 about what the world was like long before human exploration. 00:05:32.930 --> 00:05:35.338 Some of these holes are so deep and have been 00:05:35.392 --> 00:05:36.730 home to life for so long, 00:05:36.730 --> 00:05:39.730 that the bases are full of preserved fossils. 00:05:39.730 --> 00:05:42.450 Remember, these blue holes form in coral reefs, 00:05:42.450 --> 00:05:45.370 some of the most diverse places on our planet. 00:05:45.370 --> 00:05:46.890 They're not void of life, 00:05:46.890 --> 00:05:50.250 despite being dark and strange to us here on land. 00:05:50.250 --> 00:05:53.124 Animals live out their entire lives in and around these 00:05:53.176 --> 00:05:53.490 holes, 00:05:53.490 --> 00:05:54.810 and then when they die, 00:05:54.810 --> 00:05:56.170 they sink to the bottom, 00:05:56.170 --> 00:05:58.650 creating layers of long dead creatures. 00:05:58.650 --> 00:06:00.450 This sounds a little macabre, 00:06:00.450 --> 00:06:04.775 but studying those fossils with submersibles and careful excavation can 00:06:04.835 --> 00:06:07.150 tell us so much about ocean diversity. 00:06:07.150 --> 00:06:10.237 And what kind of ecosystems have formed in these unique 00:06:10.293 --> 00:06:10.910 conditions? 00:06:10.910 --> 00:06:14.150 Bizarrely, even tortoise fossils have been found in those holes, 00:06:14.150 --> 00:06:17.790 which is especially interesting since tortoises all live on land. 00:06:17.790 --> 00:06:21.030 The strange conditions in the holes are what keep fossils 00:06:21.087 --> 00:06:22.110 so well preserved. 00:06:22.110 --> 00:06:25.358 It's thought that the diversity of life in these cavities 00:06:25.415 --> 00:06:28.550 is many times greater than even coral reefs themselves, 00:06:28.550 --> 00:06:31.449 and much weirder, though it may not initially be as 00:06:31.506 --> 00:06:34.632 exciting as finding the remains of a huge dinosaur down 00:06:34.689 --> 00:06:35.030 there. 00:06:35.030 --> 00:06:39.409 Scientists have also discovered unique bacterial colonies that feed on 00:06:39.472 --> 00:06:39.910 sulfur. 00:06:39.910 --> 00:06:41.390 The holes are full of sulfur. 00:06:41.390 --> 00:06:43.883 Because of the large number of dead animals that have 00:06:43.930 --> 00:06:46.188 been living and dying down there for such a long 00:06:46.235 --> 00:06:46.470 time, 00:06:46.470 --> 00:06:50.309 these remains release sulfur, making the water toxic for lots 00:06:50.372 --> 00:06:51.190 of creatures. 00:06:51.190 --> 00:06:53.350 Certain kinds of bacteria, on the other hand, 00:06:53.350 --> 00:06:57.288 thrive in these conditions, showing again the sheer stubbornness of 00:06:57.347 --> 00:07:00.110 life on Earth to adapt and keep going even when 00:07:00.169 --> 00:07:02.520 everything says it should be killed off. 00:07:02.520 --> 00:07:05.116 Could exploring the blue holes then be a way to 00:07:05.171 --> 00:07:08.320 prepare for investigating alien ecosystems in the future? 00:07:08.320 --> 00:07:10.840 Saturn's largest moon, Titan, for instance, 00:07:10.840 --> 00:07:13.040 has lakes of liquid hydrocarbons, 00:07:13.040 --> 00:07:17.566 specifically ethane and methane. Could natural processes create blue holes 00:07:17.627 --> 00:07:21.480 and liquid hydrocarbons similar to their occurrences in oceans? 00:07:21.480 --> 00:07:23.600 Some of Titan's lakes are vast, 00:07:23.600 --> 00:07:25.677 and if any life has ever been able to form 00:07:25.727 --> 00:07:26.320 on the moon, 00:07:26.320 --> 00:07:28.150 it could have done so there. 00:07:28.150 --> 00:07:31.750 If bacteria can thrive on poisonous oceanic sulfur, 00:07:31.750 --> 00:07:35.222 then maybe it could do so on poisonous oceanic methane 00:07:35.287 --> 00:07:36.830 and ethane just as well. 00:07:36.830 --> 00:07:38.870 In this way, perhaps the blue holes are, 00:07:38.870 --> 00:07:41.430 like other extreme environs on our home planet, 00:07:41.430 --> 00:07:44.587 a great way to prepare for studying distant moons in 00:07:44.647 --> 00:07:46.590 search of extraterrestrial life. 00:07:46.590 --> 00:07:49.594 What's your verdict? Would you ever dive into a deep 00:07:49.652 --> 00:07:50.230 blue hole, 00:07:50.230 --> 00:07:52.430 or would you stay safely on the coast? 00:07:52.430 --> 00:07:54.811 And how much time and effort do you think we 00:07:54.865 --> 00:07:56.380 should spend exploring them? 00:07:56.380 --> 00:07:59.100 Ultimately, the blue holes are fascinating, 00:07:59.100 --> 00:08:03.324 relatively small and partially isolated biomes that have been able 00:08:03.388 --> 00:08:07.612 to create unique microbal environments in the many millennia since 00:08:07.676 --> 00:08:09.660 the last Ice Age began to thaw. 00:08:09.660 --> 00:08:14.420 And that's the mystery behind these unexplained underwater sinkholes. 00:08:14.420 --> 00:08:16.780 What do you think? Is there anything we missed? 00:08:16.780 --> 00:08:18.180 Let us know in the comments, 00:08:18.180 --> 00:08:20.915 check out these other clips from unveiled and make sure 00:08:20.965 --> 00:08:23.700 you subscribe and ring the bell for our latest content.