Earth 100 Million Years Ago, Antarctica, Everest, and Megaquakes: Exploring Our Planet Through "What If You Went There?"

The video opens by conveying the scale of 100 million years. If you walked nonstop for 100 million years, you would cover a total of a little over 3 trillion km, enough to travel between Earth and the Sun more than 10,000 times. On the other hand, if Earth's history were compressed into a single year, 100 million years ago would fall on December 23, making it a very recent event on Earth's timescale.

One hundred million years ago was the middle of the Cretaceous period, which lasted from about 145 million to 66 million years ago. Humanity's ancestors are thought to have been mouse-like mammals living in the shadows of the dinosaurs. The video first says there is no need to worry about breathing. Oxygen levels are estimated to have been about the same as today's roughly 21%, or slightly higher, and carbon dioxide levels at several hundred to 1,000 ppm, only somewhat higher than outdoor air today. Since there were eras when carbon dioxide exceeded 2,000 ppm, the video says this period would be comfortable by comparison.

Apart from the atmosphere, however, things were very different. The breakup of the supercontinent Pangaea had been underway since about 180 million years ago, and North America is recognizable. South America and Africa had also begun to separate, and the shapes of their coastlines show that they once fit together perfectly. But India was an island, and Australia and Antarctica were still connected. Because one reason modern Antarctica is cold is that the continent sits isolated near the pole, where heat cannot easily reach it, the video says Antarctica 100 million years ago may have been covered in greenery. Today, Antarctica's ice is said to account for about 70% of Earth's fresh water. Without that ice at the time, sea levels were far higher—by one account, 200 m higher than today. It was also an era when a shallow sea was forming in the middle of North America. Temperatures were high as well: compared with today's average of about 15°C, some studies with extreme projections put it near 30°C. Volcanic activity was also intense.

The Tyrannosaurus, Triceratops, and Stegosaurus that many people hope to see did not exist 100 million years ago. Stegosaurus lived about 150 million years ago and had already gone extinct, while Tyrannosaurus and Triceratops would not appear for tens of millions of years. The video uses this to emphasize that dinosaurs evolved over an enormous span of time.

A famous dinosaur from 100 million years ago is Spinosaurus. It measured 14 m long and had a 1.5 m sail-like structure on its back. Its purpose is still not well understood, but the leading theories are that it was used for regulating body temperature or for courtship displays. Its main diet is thought to have been fish, though it may also have eaten other dinosaurs. Others included the 13 m Giganotosaurus, which may have been larger than Tyrannosaurus; Carcharodontosaurus, with shark-like teeth; and Argentinosaurus, a sauropod 30–35 m long. Birds, a type of dinosaur, already existed as well.

The video also introduces creatures that look comical by modern standards. The pterosaur Pterodaustro is said to have had a sharply curved beak lined with around a thousand fine teeth, and is thought to have eaten small creatures at the water's edge like a flamingo. The 10 m crocodile Stomatosuchus had a lower jaw that bulged like a pelican's, and it has been suggested it may have had no teeth. In the seas there were giant bivalves 1 m across and sharks with wing-like fins. Nigersaurus, a herbivorous dinosaur with a mouth like a nail clipper, snakes that still had hind legs, and crocodiles that walked on two legs are also thought to have appeared around this time.

Among small animals, the video explains that the emergence of bees and butterflies was significant, because insects appeared that carried pollen as they fed. Flowering plants appeared about 130 million years ago, and while they were increasing by 100 million years ago, gymnosperms such as pines were still dominant. Perhaps because insects took over pollination and plants no longer needed to scatter huge amounts of pollen, flowering plants spread rapidly afterward and now make up about 90% of all plants. However, 100 million years ago there were no grasses yet, so grasslands did not exist. The ground is thought to have been covered in mosses and ferns.

This era also produced oil and the limestone that is the raw material of chalk. About 60% of the world's oil is thought to have formed in the Cretaceous, and the period created such distinctive strata that its Japanese name, "hakua," means chalk. The video sums up that Earth 100 million years ago would look like a different planet that had followed an Earth-like evolutionary path, and closes by noting that the mammals of the time lived in hiding, never knowing when a dinosaur might attack.

Antarctica can reach temperatures of -50°C and wind speeds of 80 m per second, and because NASA uses Antarctic bases to train for long-duration missions, it is also called the "white Mars." According to the video, Antarctica became locked in ice only very recently in Earth's history. About 200 million years ago, Antarctica formed the continent of Gondwana together with South America, Africa, India, and Australia. Partly because carbon dioxide levels were high, there were periods when it was relatively warm and covered in forests. When Gondwana broke apart and Antarctica was left behind near the pole, the Antarctic Circumpolar Current formed around it, blocking heat from other ocean currents. This current is thought to have formed no later than 30 million years ago.

Today there are many tour packages, and getting to Antarctica itself is not that difficult. Tourist ships departing from Argentina, Chile, Australia, and New Zealand are the most practical option, but most land on the Antarctic Peninsula at the northern tip of the continent. The northernmost part of the peninsula lies at about 63° south, which, translated to the Northern Hemisphere, is farther from the pole than settlements in Greenland. Flights are an option for reaching the interior, and some packages even reach the South Pole. With enough preparation, simply going there and spending time is not that much of an ordeal.

Summer (December–January) on the Antarctic Peninsula has an average high of 1°C and an average low of -2°C, roughly like midwinter in Japan. But Antarctica is windy, and in summer the wind blows especially hard in the early morning. Winds can reach 40 m per second, on par with a typhoon, and speeds above 80 m have been recorded. Even when the temperature is around 0°C, the wind chill drops considerably because so much heat is stripped from the body. In winter on the Antarctic Peninsula, the average high falls below -10°C and the average low is around -16°C. Because of the polar night, when the sun never rises, without artificial light you would be enduring blizzards in total darkness.

In the interior, altitude also becomes a problem. Most of the Antarctic continent is covered by an ice sheet several kilometers thick, and its average elevation is far higher than that of other continents. The South Pole sits at an elevation of about 2,830 m, and the average winter air pressure drops to about 675 hPa. This is because the atmosphere contracts in the extreme cold, and it is comparable to the summit of Mount Fuji, which is about 900 m higher. The South Pole's annual average temperature is -49°C; even its highest recorded temperature was below freezing, and its lowest was -83°C.

There are even colder places. The lowest temperature ever recorded on Earth, -89.2°C, was measured at Vostok Station, about 1,300 km from the South Pole. This is thought to be because its elevation of 3,488 m is higher than the South Pole's. Temperatures are normally measured 1.25–2 m above the ground, but in Antarctica it is colder near the surface, and satellite observations show many spots below -90°C, with one result of -98°C. Taking wind into account, the video says it would not be surprising for the wind chill to fall below -100°C.

In the cold, the human body constricts blood vessels in the hands, feet, and elsewhere to reduce heat loss, but at extremely low temperatures this leads to frostbite. Near the South Pole, the low pressure reduces the oxygen you can take in, so the body cannot efficiently produce energy to keep warm. Humidity can drop as low as 0.03%, so you lose moisture with every breath and become dehydrated easily, which reduces blood flow and raises the risk of frostbite. The video says that, taken together, it would not be surprising to get frostbite in just a few minutes. With severe frostbite, much like food accidentally left in the freezer, ice crystals destroy cells, and even after thawing, bodily fluids may leak out and the tissue may never recover.

Hypothermia also looms. According to one researcher, the human body is not designed to function below -40°C, and a single error of judgment can be fatal. A study calculating the heat the body produces through metabolism versus the heat it loses found that at -50°C, unless you are very heavily dressed, your body temperature would drop below 30°C within a few hours, leading to death without rescue. The video ends by noting that during the age of exploration in the early 20th century, some people collapsed along the way and were buried in snow, and their bodies likely still lie in Antarctica today.

At 8,848 m, Everest stands on the border between Nepal and Tibet. In terms of climbing technique alone, many mountains are harder than Everest, but the video points out that this view focuses only on the technical side, and that the danger stemming from the altitude itself is far too great.

The cost is also huge. The climbing permit on the Nepal side, which most people use, was $11,000 in 2023, more than 1.5 million yen at the April 2023 exchange rate. An increase to $15,000 was scheduled for 2025. You also need travel costs to get there, equipment such as tents, boots, and cold-weather gear, and multiple oxygen tanks. Since the Nepali government prohibits solo climbs, a Sherpa guide is also mandatory. The total is said to be at least around 5 million to 10 million yen, and it can exceed 30 million yen.

It also takes time. Base Camp, the first staging point, is at about 5,400 m, where air pressure is about half that at sea level. If you were suddenly placed at this pressure, you would quickly run short of oxygen and might even pass out. Climbers can endure it because the body detects the oxygen shortage, produces more red blood cells, and raises the heart rate. These responses, however, take days to weeks. So climbers repeatedly ascend temporarily from camp to higher altitudes and return the same day for many days, acclimatizing as they work toward the summit. It is not unusual for the climb from Base Camp to the summit to take more than a month.

Even so, about 600 people reached the summit in 2023, roughly 250 climbers and roughly 350 Sherpas. The cumulative number of people who have summited, as far as records go, exceeds 6,000. The favorable window is limited to a few weeks in late May, and even then, summit attempts are only possible on days with good weather, so in some years it gets so crowded that lines form near the summit. Air pressure at the summit is about one-third of that at sea level, the average May temperature is -25°C, and winds frequently exceed 20 m per second. Outside the window, storms are frequent, and even during the favorable period conditions tend to deteriorate from evening on, so summiting after 2 p.m. is generally avoided. In recent years, there have been many deaths among climbers who attempted it without enough experience, and this has become a problem.

It is generally thought that the body cannot acclimatize above about 7,500–8,000 m, and this region is called the "death zone." The body is described as slowly dying there, and the iron rule is to keep your time there as short as possible. Oxygen tanks are effective, but relying on them too heavily is also a mistake, since the body may be unable to cope with the thin air when a tank runs empty.

Staying too long leads to high-altitude pulmonary edema or high-altitude cerebral edema. When the heart works hard to pump blood amid the oxygen shortage, blood pressure rises, fluid accumulates in the lungs, and oxygen becomes even scarcer. High-altitude cerebral edema occurs when blood vessels in the brain dilate in an effort to deliver oxygen, causing the brain to swell; it begins with headaches and mood changes and ends with impaired consciousness, leaving you unable to stand. Above 8,000 m, helicopter rescue is difficult, so rescuers must rely on manpower. But because this environment is life-threatening for other climbers too, the video reports that, controversial as it is, people who appear to need rescue are often passed by.

Because the area near the summit is below freezing nearly year-round, bodies freeze and barely decompose. Many of the climbers who died on Everest are said to remain as they looked in life. Where rescue is difficult, recovering bodies is difficult too, so many are simply left in place and are sometimes seen by later climbers. For the Sherpas, whose ancestors have lived around the Himalayas for generations, Everest is a sacred place. Voices criticizing the many foreigners' bodies left there are growing louder, but partly due to economic circumstances, no fundamental solution has been reached, the video says.

Niagara Falls, on the border between the United States and Canada, has a maximum drop of 57 m—not as tall as the world's top waterfalls—but the video describes its power as among the greatest in the world. Until about 12,000 years ago, northeastern North America was covered by glaciers 2–3 km thick. When they melted at the end of the Ice Age, water collected in basins and formed the prototypes of the Great Lakes, and a step between Lake Erie and Lake Ontario to the east, created by differences in how easily the rock eroded, became the precursor of the falls.

The cliff of the falls has been continually retreating due to erosion, estimated at 1–1.5 m per year over the past 560 years. The rate has now slowed because hard rock forms the cliff and water is diverted for hydroelectric power, but it is still about 30 cm a year. At this pace, the falls are expected to reach Lake Erie and disappear in about 50,000 years.

Niagara Falls consists of three falls, in order of size: Horseshoe Falls (the Canadian Falls), American Falls, and Bridal Veil Falls. Horseshoe Falls has a drop of 57 m, and the other two drop around 20–30 m, but according to the video, it is actually the two lower falls that are more dangerous. American Falls and Bridal Veil Falls have no plunge pool, and rocks lie at the bottom, so there is a very high chance you would strike them and die.

Horseshoe Falls is not safe either. One study, based on things like bone fractures and lung damage, found that the limit for a person to plunge into water unharmed is about 100 km/h, or a height of a little over 50 m, so the 57 m Horseshoe Falls is right at the limit or slightly beyond it. Just before the falls, the current can reach 40 km/h, so you would be swept over at its mercy. As the iron rules for falling into water from a height, the video lists taking a big breath, straightening your body just before entry so you go in feet-first vertically, and clenching your anus tightly. It adds, however, that few people could calmly do this while being swept along at high speed.

The challenges continue after the fall. The water in the plunge pool is mixed with so much air that light cannot reach deep, and you lose track of which way is up. The current is so fast that you risk being pinned against rocks and drowning. Getting away from the plunge pool and surfacing is the safest course, but that is difficult with broken bones or internal injuries. The water temperature exceeds 20°C in summer, but in other seasons it is too cold to swim, and in winter it is close to 0°C. People are said to be able to move properly in cold water for only a few minutes. On the other hand, during the day there are tourists watching, and tour boats go close to the plunge pool, so there are quite a few cases of people who survived thanks to someone calling for help or a rescue. The final hurdle is the law. Going over the falls is prohibited by local law, and a man who went over in 2003 was fined $2,260 (more than 200,000 yen) and banned from entering Canada.

The Nankai Trough is a trench about 4,000 m deep on the seafloor south of Shikoku. The Philippine Sea Plate sinks beneath the Eurasian Plate by a few centimeters each year, and when the landward plate being dragged down can no longer withstand the strain and springs back, an earthquake occurs. Because the plates keep moving, major earthquakes have occurred repeatedly. In the past, megaquakes have struck at intervals of roughly 100–150 years. The most recent were the 1944 Showa Tonankai earthquake and the 1946 Showa Nankai earthquake, and as of the video's production, 75 years had already passed. The interval has been as short as 90 years and as long as more than 200 years, so the video says it would not be strange to think we are already in the period when one could occur.

According to the Earthquake Research Committee's assessment using a time-predictable model, the probability of an M8–9 class earthquake occurring within the next 30 years was put at 60–70%. However, this calculation dates from 2013, and the probability is thought to increase by about 1% each year. Nankai Trough earthquakes are diverse—sometimes linked with a Tonankai rupture, for example—making it difficult to estimate the source region. Sediment analysis also suggests that earthquakes larger than the largest on record may have occurred. The largest-class event is projected at M9.1; it has a longer recurrence interval, and its probability is put at less than one-tenth that of an M8-class quake.

A Cabinet Office study group, reasoning that the probability of the next earthquake being the largest class is not zero, has estimated damage assuming a largest-class event. Seismic intensity would be 6-lower or higher from western Kanagawa Prefecture to Miyazaki Prefecture, with some coastal areas reaching intensity 7, and nearly everywhere west of Fukushima Prefecture would experience intensity 4 or higher. This estimate does not include long-period ground motion. The video points out that long-period shaking easily resonates with high-rise buildings, so even in Nagoya and in the Tokyo metropolitan area, where shaking is relatively mild, upper floors could suffer major damage.

The most frightening threat is the tsunami. Overlaying the tsunami heights estimated for 11 scenarios, waves would exceed 10 m along the Pacific coast from Shizuoka Prefecture to Kagoshima Prefecture, with some areas from the Shizuoka coast to Shikoku exceeding 20 m. Because the source region is close to land, the tsunami would arrive in as little as 2 minutes in the fastest-hit areas. In the Great East Japan Earthquake, the earliest arrival was 25 minutes.

Deaths could reach as many as 323,000, about 16.4 times those of the Great East Japan Earthquake. About 59,000 people are projected to die from shaking and building collapse, and about 224,000 from the tsunami. Up to 2,386,000 buildings would be completely destroyed or burned, with the number completely destroyed 17.5 times that of the Great East Japan Earthquake. In the hardest-hit areas, about 90% of lifelines would stop. Water service would be cut for 34.4 million people, and 32.1 million would lose sewer service. Power outages would affect 27.1 million customers, with 90% losing power in the three Tokai prefectures and the Kinki region. Gas supply would be cut to 1.8 million households, and 9.3 million landlines would be unable to make calls. Economic damage would amount to 169.5 trillion yen, or 214 trillion yen including 44.7 trillion yen in losses from reduced economic activity—more than twice the national budget. The government conducts continuous monitoring and, if anomalies are detected, investigates and releases related information. Citing the saying "preparation prevents worry," the video encourages viewers to get prepared.

On the earthquake directly beneath the Tokyo metropolitan area, which is home to about 30% of Japan's population, the video first explains the two types of earthquakes: subduction-zone earthquakes, in which strain built up by a subducting plate is released (the Great East Japan Earthquake), and inland earthquakes, in which strain underground moves a fault (the Great Hanshin-Awaji Earthquake). A quake directly beneath Tokyo is said to be possible in either form, and the video lists four types.

The first is an inland earthquake caused by a fault in the Kanto interior; the largest of these is the 33 km Tachikawa fault zone, which runs from Fuchu in Tokyo to Hanno in Saitama Prefecture. If it moved, an M7.4 earthquake would strike the densely populated western part of Tokyo, with an estimated 6,300 deaths. This fault zone is said to move on a 10,000–15,000-year cycle, and it last moved 20,000–13,000 years ago. The video likens this to being "overdue." Unknown active faults may also be hidden. The second is the Northern Tokyo Bay earthquake, which is projected to produce the maximum intensity of 7, centered on the eastern part of Tokyo's 23 wards, where the ground is weak. The third is a subduction-zone earthquake on the Sagami Trough, which caused the Great Kanto Earthquake, said to occur on a 200–400-year cycle. Since only about 100 years have passed since the Great Kanto Earthquake, it was thought that another would not occur for some time, but recent research has found that only about half of the source region ruptured in the Great Kanto Earthquake, leaving the rest "unruptured." Detailed predictions are not yet possible. The fourth is the Tokai earthquake. It has occurred on a 100–150-year cycle, but 160 years have passed since the last one, the Ansei Tokai earthquake of 1854.

In a Northern Tokyo Bay earthquake, the shitamachi (old downtown) districts would experience intensity 7. Intensity 7 is the same strength as the Great Hanshin-Awaji Earthquake: TVs and pianos would fly and slam into walls, people would be unable to move, and panic would leave them unable even to think. Sixty percent of buildings built before 1981 without seismic retrofitting would collapse instantly; up to about 610,000 buildings are projected to be completely destroyed and up to about 23,000 people killed. In the 23 wards, up to about 50% of households would lose power and water, the subway would be halted for a week, and JR lines for about a month. Major roads would also become impassable.

In a large city, fire is especially terrifying. In the 1923 Great Kanto Earthquake, 90% of the roughly 100,000 victims died in fires. Even today, areas packed with old wooden houses along roads less than 4 m wide are dangerous. When fires make the ground extremely hot, large updrafts form and create tornado-like flames called fire whirls. In central Tokyo, there are fears that winds funneled between buildings could spawn fire whirls one after another and make them larger. In the worst case, 16,000 people could die in fires, and blazes could spread with roads impassable and no way to put them out.

In an ocean earthquake, tsunami damage could be severe because many areas around Tokyo Bay are "zero-meter zones" lying below sea level. The video says a 2 m tsunami could flood up to about 20 km from the coast, and a 6 m tsunami up to about 40 km. Tsunamis traveling up rivers and flooding into underground shopping centers are also possible. In the heavily reclaimed bayside areas, liquefaction would be severe, tilting buildings and pushing up manholes, cutting off roads. If lateral spreading—where the ground moves sideways by tens of meters, buildings and all—were to split and breach levees, residents of zero-meter zones would need to evacuate to high ground immediately, even without a tsunami. Human-caused harm, such as looting amid the chaos, would also occur. Saying that even if science cannot stop earthquakes, we can learn from the past, the video recommends checking hazard maps from the Cabinet Office and local governments.

Yellowstone, the world's oldest national park, has geysers and vividly colored hot springs, as well as hot springs so dangerous that entry is prohibited. The video introduces a real accident. On June 7, 2016, 23-year-old Colin Scott visited Norris Geyser Basin, one of the most volcanically active areas in the park, with his sister Sable. Despite warning signs, the two left the boardwalk and were looking for a hot spring at a temperature suitable for bathing. While Sable filmed on her smartphone, Colin, who was making his way down a slope to test the water temperature, slipped and fell into a spring. Rescuers found his body submerged in the spring, but with daylight fading and lightning approaching, the recovery was called off for the day. By the next day, the body was gone. Park official Veress stated that a significant amount of the body had dissolved in a very short time. Because the water was highly acidic and very hot, organic matter broke down extremely quickly. The footage has been kept private out of consideration for the family. Since 1870, at least 22 people on record have died in accidents involving hot springs or geysers.

The park lies in a caldera thought to be the remnant of a supervolcano eruption, and magma is still being supplied underground. Hot water mixes underground with rainwater and snowmelt, creating geysers and hot springs. The hot springs fall broadly into three categories. The first, the neutral chloride type, is rich in sodium and chloride and has a pH of 6.7–9.5, which is not a dangerous value. But temperatures are often 90–94°C, and since water boils at around 93°C in the park, which sits above 2,000 m, many of these springs are at a boil. Most of the violent geysers and blue hot springs are of this type, and jumping in would give you severe burns over your entire body within seconds.

The second, the acid sulfate type, is the most dangerous. Found from the northern to northeastern part of the caldera, these springs form when hydrogen sulfide in steam and volcanic gases escaping from magma reacts with oxygen to form sulfuric acid, producing strongly acidic water of pH 2 or lower, comparable to stomach acid. Some springs are said to be even more acidic. The water dissolves even the surrounding rock, turning it into clay minerals, which create Yellowstone's distinctive multicolored pools. With water temperatures sometimes exceeding 90°C, falling in would denature the body's proteins, which would then be dissolved by the acid. The video explains that, with convection at the bottom of the pool bringing a continuous supply of fresh, strongly acidic water, a human body would vanish in less than a day. The third, the calcium carbonate type, consists of water that has passed through sedimentary rock containing limestone and gypsum, and forms stepped travertine terraces. These are relatively cool and calm, but some are exceptionally hot, and the video says you should not approach them, also for the sake of nature conservation.

The roughly 2,500 km Ganges flows from northern India through Bangladesh into the sea. Because its source is Himalayan ice, its flow is stable year-round, and it is rich in nutrients, so agriculture has thrived there since ancient times. More than 400 million people live in its basin, meaning more than 1 in 20 people in the world benefit from the river. In Hinduism, the Ganges is considered to be the goddess Ganga herself and is believed to wash away all impurity. People scoop the water with their hands and pour it over their heads as they bathe. At the same time, the Ganges is also said to be one of the most polluted rivers in the world.

The outcome varies greatly depending on where you fall in. At Gangotri, near the source, glacial water untouched by pollution rushes by, and being swept away risks drowning or hypothermia. The video focuses on the holy city of Varanasi. It is believed that being cremated here and having your ashes scattered in the river frees you from the cycle of rebirth, so many people come to spend their final days. On the riverbanks, bodies are cremated with firewood, and by one account the fires have not gone out for hundreds of years. But because there are so many people and not enough money for cremation, bodies are sometimes put into the river after being only partly burned, and large numbers of bodies enter the river every day.

The problem is microbes. Sewer systems are also inadequate: while the standards for coliform bacteria are 50 or fewer per 100 ml for drinking water and 500 or fewer for bathing, a 2019 survey found 22,000–34,000. This is an indicator of how much human feces is entering the river. About 60% of the world's people who defecate in the open are said to live in India. Cholera is a particular concern: the bacteria release a toxin in the intestines, flooding the gut with large amounts of water and electrolytes and causing severe diarrhea. Symptoms can appear within 2 hours of infection, and it can kill within days; fluid loss can reach as much as 20 L a day, with stool resembling the water used to rinse rice. Severe dehydration causes sunken eyes and progresses to muscle cramps and multiple organ failure. There is also a risk of typhoid and hepatitis A. There is reportedly also a theory that because there are so many bacteria, viruses that infect bacteria also flourish, suppressing the rate at which people fall ill.

However, the video also says that not everyone gets sick. Only about 1 in 10 people with cholera become seriously ill, and locals who bathe every day may have a degree of immunity. On the other hand, it says you often hear of Japanese travelers being hospitalized for about a week. This is thought to be traveler's diarrhea caused by E. coli, Campylobacter, and the like. As for the rumor that wearing a condom would help, the video dismisses it, saying it is ineffective because most pathogens act on the digestive system. Upstream in Kanpur, chromium and other substances used in tanning leather are discharged untreated, and some places give off a foul smell. The video sums up the causes of the pollution as a lack of infrastructure and resources, with sewage treatment unable to keep up with population growth, and problems with residents' awareness. There are also reports that water quality improved dramatically in 2020 due to the effects of a certain infectious disease, so it is physically possible to address the problem, but neither awareness nor infrastructure will change quickly, and it will be a long effort, the video says.

Belize, in Central America, is a small country about 1.2 times the size of Shikoku, and it is home to the largest coral reef in the Northern Hemisphere. Within it lies the Great Blue Hole, a deep circular hole about 300 m across and up to about 120 m deep. The Dragon Hole in the South China Sea, at 300 m deep, is deeper, and the Blue Hole in Dahab, Egypt, where diving accidents are common, is also well known, but the Great Blue Hole is the largest in the world by diameter, and seen from above it is nearly a perfect circle. It is thought to have formed when the ceiling of a limestone cave collapsed during the Ice Age, when sea levels were low, and seawater flowed in as sea levels rose at the end of the Ice Age. It is one of the world's top diving spots, but regular scuba diving can reach only a few tens of meters at most.

Diving from the center of the hole, you might at best encounter large sharks if you are lucky, and there is nothing particularly unusual all the way to the bottom. Diving from the rim, there are coral reefs on a gentle slope, where you can see brightly colored small fish, sea anemones, cleaner shrimp, sea turtles, and shallow-water sharks. But at around 15 m the slope turns into a cliff, and marine life drops off sharply. At 20–30 m, the water temperature falls abruptly from around 30°C to about 26°C, and below that boundary visibility improves dramatically. This is said to be because there is little water movement inside the hole.

At 30–45 m on the southern wall is a cave entrance where stalactites and stalagmites form a shape like a shark's mouth. The largest stalactite is 12 m long and 60 cm in diameter. Since stalactites and stalagmites cannot form underwater, they are evidence that this area was once above ground. Stalactites, which should hang straight down, are tilted by about 5–12 degrees, thought to be because crustal movement tilted the land sometime after the first stalactites began forming 200,000 years ago. At 50 m, there is a cave on the western wall extending horizontally for about 50 m, and it reportedly contains sea turtle bones.

At around 90 m, the scene changes completely. The water temperature drops to about 22°C, and a layer of hydrogen sulfide like a brown fog covers the entire hole. Below it, almost no light reaches, and there is no oxygen. On the seafloor at 120 m lies a "conch graveyard" strewn with shells, where conchs and hermit crabs that fell in suffocated and their remains were left behind. It is thought that the decomposition of organic matter falling from above used up the oxygen, and that hydrogen sulfide released by bacteria that break down organic matter without oxygen has accumulated around 90 m. In a 2018 exploration, the bodies of two people who had gone missing were found, along with plastic bottles and cameras. These too will eventually be covered by mud falling from above, and the Great Blue Hole itself is expected to fill with mud someday and become shallows.

Humans have reportedly reached the bottom only twice, including the 2018 exploration. Finally, while noting that it is a rumor with no scientific basis, the video says there may be a tunnel connecting to the Belize mainland. It then ends by asking viewers whether they would explore the seafloor or slowly be buried in mud.