What if Mount Fuji Erupts Tomorrow?

0:00 This might just be the most iconic mountain on the entire planet.

0:04 At 3,776 meters, it is Japan's highest peak.

0:10 It's a national park and a UNESCO World heritage site.

0:13 The mountain is mentioned in Japan's oldest stories, poems, and haikus,

0:18 and has been depicted thousands of times in Japanese art.

0:22 Both Shinto and Buddhism consider it a sacred home of the gods.

0:26 And for nearly a thousand years, observers have made pilgrimages to the summit.

0:31 They've built temples and shrines and marked the first climbing routes.

0:35 Today, more than 300,000 people follow these routes to the summit each year,

0:40 while millions more visit the mountain's lower slopes.

0:44 Fuji is a volcano.

0:46 It's erupted about 180 times in the last 5,600 years,

0:51 averaging once every few decades.

0:55 For the last 319 years, Fuji has been quiet.

1:01 The cities in its shadow haven't.

1:04 Tokyo has grown into one of the largest cities in the entire world,

1:08 more than 30 million people.

1:10 The edge of the city is just 50 kilometers from the volcano.

1:14 These are hazard maps showing how far lava,

1:18 lava bombs, pyroclastic flows, and volcanic mudflows could reach.

1:23 This one is the scariest, showing how far away ash could fall.

1:28 We'll look at these in more detail later on.

1:31 Fuji is just one of more than a hundred volcanoes in Japan.

1:35 Volcanoes are usually found along tectonic plate boundaries,

1:39 and this is the intersection of six different plates.

1:43 This is one of the most environmentally hazardous places on the planet.

1:47 Japan has some of the world's biggest earthquakes and tsunamis.

1:51 monsoons and typhoons cause flash floods and mudslides in the mountains,

1:56 and yet more than 120 million people live here.

2:01 Check out this graph of disaster-related fatalities in Japan

2:05 with two huge earthquakes in 1995 and 2011.

2:10 Notice how, at the start, there's consistently thousands of fatalities a year,

2:15 and then, around 1960, that drops to just a couple hundred.

2:19 Japan has led the world in figuring out how to mitigate disasters,

2:25 typhoons, floods, earthquakes, tsunamis, and even volcanic eruptions.

2:31 I wanna talk about what Fuji's next eruption could mean for Tokyo,

2:36 but I also wanna introduce you to volcanoes

2:38 that are even larger and much more active, one is erupting currently.

2:44 Every single one has people living concerningly close.

2:48 We'll learn about how people manage to live right in their shadows.

2:52 And perhaps more importantly, why?

2:57 (gentle music) Check this out.

3:20 This is Aso.

3:22 This is Japan's largest volcano and one of its most active.

3:26 On top of the crater rim here,

3:28 there's all these little concrete shelters, so you can,

3:32 I guess, hide out so a lava bomb or something doesn't fall on you.

3:35 What's nuts about Aso is that it's not just

3:38 that steaming cauldron over the edge of this crater.

3:42 It's this whole area.

3:43 These distant mountains back here, that's the edge of the caldera.

3:48 One of the country's most active volcanoes, very scary-looking at the top,

3:52 but still has thousands of people living within the crater, but that's Japan.

4:00 I have quite a few volcanoes to show you here,

4:03 and this is just the first of three videos from Japan.

4:07 This was a big trip.

4:08 It took an entire month to film everything.

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5:48 Fuji is, obviously, Japan's most iconic mountain, that perfect cone shape,

5:56 but if you look at it from the right angle, you'll notice there's this big bite,

6:01 just this chomp taken right out of the side of the mountain.

6:05 October 28th, 1707, a magnitude 8.6 earthquake strikes Japan,

6:11 generating a tsunami that kills over 5,000 people.

6:15 At the start of December, smaller earthquakes are felt around Fuji.

6:19 On December 16th, a cloud of ash rises 20

6:22 kilometers above the summit and pumice falls from the sky.

6:27 By the end of the day,

6:28 falling lava bombs have burned temples and villages on the volcano slopes,

6:38 I'm inside of the chop.

6:43 The eruption lasted 16 days and released about 800 million cubic meters of ash.

6:49 The worst ashfall disaster in Japan's history.

6:53 About a million people lived in what is now Tokyo,

6:57 nearly four centimeters of ash and debris fell across the city,

7:01 blocking out the sun and causing respiratory illness.

7:04 Some parts of the city received as much as 15 centimeters,

7:08 accumulating on rooftops and eventually caving them in.

7:12 Farms near the volcano were buried under as much as three meters of ash.

7:17 Crops failed and a decade of famine followed.

7:21 During heavy rains, ash washed into rivers, forming volcanic mudflows,

7:26 fast-moving walls of ash and debris,

7:28 destroying farms and villages in their path.

7:32 Because the eruption left behind so much ash,

7:34 mudflows continued for nearly 200 years.

7:38 And in the worst affected areas, farmland is still unusable today.

7:46 The eruption came outta Fuji's south flank rather than its summit,

7:51 forming this crater.

7:52 (gentle music) Volcanic eruptions almost always begin with earthquakes,

8:12 and the earthquakes are almost always caused by the volcano.

8:16 Fuji 1707 eruption is a very rare case

8:20 where geologists believe the earthquake actually caused the eruption,

8:25 essentially plugging the magma chamber and allowing pressure to build.

8:32 In 1707, Japan was almost completely isolated from the rest of the world.

8:38 Sailing into the open ocean or even building

8:41 a ship capable of doing so was banned,

8:44 but small boats were often swept out to sea by storms or ocean currents.

8:49 Some returned with reports of a large white column rising from the sea,

8:54 a small island absolutely crawling with white birds.

9:03 (birds chirping and squawking) Fuji is the product

9:14 of the Pacific plate sliding beneath the Philippine plate.

9:19 Follow that plate boundary into the ocean

9:21 and you'll find a chain of volcanic islands.

9:26 Miyake Island is one of Japan's most active volcanoes,

9:30 erupting once every 20 or so years.

9:33 In 1983, lava flows destroyed forests and villages.

9:37 And in 2000, a large crater formed at the summit

9:40 and began emitting dangerous amounts of sulfur dioxide.

9:44 The entire island was evacuated and residents couldn't return until 2005.

9:50 Today, those on the island are required to carry

9:53 gas masks in case the volcano's sulfur emissions spike.

9:57 In the mid 1800s, Japan's isolation ended.

10:00 Explorers began sailing to distant islands, including Torishima,

10:05 a small volcanic island 500 kilometers south of Tokyo,

10:10 and the most important breeding ground for the short-tailed albatross.

10:14 In 1887, a Japanese businessman founded

10:17 the Tamaoki Company to harvest feathers from Torishima.

10:21 And by 1900, 300 people lived in a company town on the island.

10:27 Workers hunted more than a hundred birds a day.

10:29 And by 1902, more than 500 million had been killed.

10:34 That same year, the volcano erupted,

10:36 burying the town and all of its inhabitants.

10:40 Tamaoki himself was away,

10:42 so he returned and rebuilt the town and the business the next year.

10:48 in 1922, with albatross populations too low

10:51 to sustain the business, the company closed.

10:54 And in 1933, with fewer than a hundred left, hunting was banned.

11:00 Eruptions in 1939 and 1941, once again, buried much of the island under lava.

11:13 (horn blaring) In November of 1791,

11:41 a swarm of earthquakes was felt in the coastal town

11:45 of Shimabara at the foot of the Unzen Volcano in southern Japan.

11:50 In January, a column of ash rose from the summit.

11:54 And in February, lava began flowing down its slopes.

11:58 On May 21st, a large section of the mountain collapsed.

12:02 The resulting landslide swept much of Shimabara into the bay,

12:06 creating a tsunami that hit the city across the water.

12:10 15,000 people were killed in Japan's deadliest volcanic eruption on record.

12:17 The debris from the landslide can be seen today.

12:21 It added about four square kilometers to the coast and dozens of small islands.

12:34 In 1989, earthquakes were once again recorded near Shimabara.

12:41 (gentle music) An unstable lava dome grew at the summit,

12:54 reaching more than 120 meters tall.

12:58 Pieces of the dome broke off, sending pyroclastic flows down the volcano slopes,

13:04 fast-moving avalanches of extremely hot ash, gas, and debris.

13:10 (volcano rumbling) On June 23rd,

13:19 an unusually large pyroclastic flow killed 43 people,

13:24 most of them scientists and news reporters, including Katia and Maurice Krafft,

13:31 the world's most famous volcanologist at the time, and probably still today.

13:39 Over the next several years,

13:41 more than 9,000 pyroclastic flows reached further down the volcano slopes,

13:46 blocking roads and destroying homes.

13:53 This is a school right at the base

13:55 of the volcano here that was burned by pyroclastic flows.

13:59 It's been left in its mostly original state, hasn't really been cleaned up.

14:05 (gentle pensive music) (gentle pensive music continues) (birds

14:48 chirping) Sakurajima is widely considered Japan's most active volcano.

15:18 Look at it on a map,

15:20 you'll see this big island rising right out of Kagoshima Bay,

15:23 several craters at the summit, river channels running down its slopes,

15:28 and villages on the island, home to more than 4,000 people.

15:35 Just four kilometers across the bay,

15:36 you'll see Kagoshima City, home to more than 600,000.

15:41 January 12th, 1914, Sakurajima erupts, pyroclastic flows bury three villages,

15:49 and a magnitude seven earthquake kills 35 people in Kagoshima City.

15:54 Over the next few weeks,

15:55 lava flows bury five more villages and one connects the island to the mainland.

16:01 By the time the eruption ends,

16:03 half of the more than 20,000 people living on the volcano have left for good.

16:09 Smaller eruptions happened in 1939 and 1946.

16:13 And in 1955, Sakurajima began erupting almost nonstop.

16:19 In 1985, 474 eruptions were recorded at the summit.

16:24 In 2009, activity increased again, reaching nearly a thousand eruptions a year.

16:30 Most eruptions look similar, an explosion at the summit sends ash into the sky,

16:35 which then rains down onto the volcano slopes.

16:38 If the explosion is big enough or the wind is blowing west,

16:42 the ashfalls in Kagoshima city.

16:45 18 river channels flow down the mountain slopes and into the ocean,

16:50 averaging a 20% grade, really steep.

16:53 Japan is rainy, especially here in the South.

16:56 And during heavy rains,

16:57 all of that ash is washed into these channels, forming mudflows.

17:03 As the volcano became increasingly active,

17:06 mudflows became larger and more frequent,

17:09 damaging or destroying homes, hotels, highways, and bridges.

17:14 In the 1940s, the local government began

17:17 building flood controlled dams on Sakurajima's rivers.

17:21 Let's say you have a section of river a hundred

17:23 meters long that drops 20 meters at 20% grade.

17:27 Add a 10-meter drop in the middle,

17:29 and now each of these sections has a 10% grade, slowing down mudflows.

17:35 This was effective, but it was expensive,

17:38 and construction wasn't happening fast enough.

17:43 In 1974, mudflows killed nine people.

17:47 Eight of them were workers building flood control dams on the mountain slopes.

17:52 (mudflow rumbling) Kagoshima needed a lot more dams on Sakurajima,

18:03 and they needed them built more safely.

18:06 So people pressured Japan's national government.

18:09 And in 1976, construction began

18:12 on the world's most ambitious volcano control project.

18:18 I found my way into the mud flow channel.

18:21 Here's the volcano back here, and here's the dam.

18:24 You can see it's got these big teeth that'll catch big

18:27 boulders and keep them from moving down and destroying stuff downstream.

18:32 Flood control dams have been installed on all 18 of Sakurajima's rivers.

18:37 Sensors are installed along the river's upper reaches.

18:41 Some are like trip wires.

18:43 A mud flow breaks the wire and workers downstream are alerted and evacuated.

18:48 Others are like pendulums, activated when a mud flow flips them up.

18:53 Mudflows are also monitored using surveillance cameras and LIDAR sensors,

18:58 which can estimate the size of debris flows.

19:01 Machine learning is used to analyze the results and identify patterns.

19:06 In 2009, a remote control helicopter was developed, used to take measurements,

19:12 gather ash samples, and install monitoring equipment close to the craters.

19:18 Along the base of the volcano, you'll find 32 concrete shelters.

19:23 Inside each of the shelters,

19:24 they have this kind of hazard map, shows you where you are, what your risks are,

19:31 and gives you a little flow chart showing you how to evacuate.

19:34 Along the waters edge are 20 evacuation ports for evacuees to gather,

19:39 and 20 ferries ready to take them to the city.

19:42 The city has about a hundred ash

19:44 sweepers to clean the streets after large eruptions, sort of like snowplows.

19:50 Most cities provide resonance with trash cans, maybe recycling bins.

19:54 In Kagoshima, you also get these yellow bags to collect ash and deposit

19:59 it at one of more than 6,000 collection sites throughout the city.

20:03 Many of the houses near the volcano have roofs, window sills,

20:07 and gutters that are designed to prevent ash from accumulating.

20:11 Not too different from an A-frame cabin designed for snow.

20:15 The city distributes pamphlets and instructional videos to residents

20:19 and requires schools to teach students about volcano safety,

20:23 sometimes including visits from volcanologists and field trips to the volcano.

20:28 The Sakurajima Volcanic Prevention Council meets every two months.

20:32 It includes the city government,

20:34 the prefectural government and weather observatory,

20:37 the Regional Development Bureau, and university research centers.

20:40 Experts from Kagoshima are familiar faces

20:43 at national and international volcano conferences,

20:46 sharing their system with the rest of the world.

20:54 The system isn't just allowing a large

20:57 city to function next to an erupting volcano.

21:00 It's preparing for a worst case scenario.

21:03 Large eruptions, like the one in 1914, happened at least every few centuries.

21:09 An eruption that size could drop up to a meter of ash in Kagoshima City,

21:15 cutting off water and electricity.

21:25 After World War II, Japan's infrastructure was left weakened,

21:29 and repeated major disasters killed thousands each year.

21:32 The beginning of the 1960s saw a push to rebuild with disasters in mind.

21:38 New legislation building codes were passed,

21:41 and billions of dollars were invested.

21:44 They built flood control projects all across the country,

21:47 developed the world's best earthquake-resistant architecture,

21:51 coordinated new warning systems, and implemented an annual disaster prevention

21:56 day to share information and practice evacuations.

22:04 Building hundreds of dams throughout Japan has had an environmental cost,

22:09 one that we'll talk about in more detail in the next video.

22:21 This area, Southern Kyushu, is one of Japan's richest regions for archeology.

22:27 Thanks largely to its frequent volcanic eruptions.

22:31 A big eruption drops a clean layer of ash that's easy to spot in an excavation.

22:37 Ash layers are easy to date and can

22:40 often be distinguished from ash from other eruptions.

22:44 So if an archeological site is sandwiched between two ash layers,

22:48 we can infer its age.

22:50 Big eruptions also drop ash over large areas.

22:54 Really helpful for synchronizing the age of sites across an entire region.

23:01 The most famous site in this region is the Uenohara site,

23:05 a large village buried under ash

23:07 from an eruption of Sakurajima about 9,500 years ago.

23:12 With artifacts dating back over 10,000 years,

23:15 it is one of Japan's first villages, capturing the start of a transition

23:20 from hunting and gathering to sedentary life.

23:26 In 2024, researchers from Kobe University published an insane paper.

23:32 They found a layer of volcanic debris

23:34 covering the sea floor around the Kikai Caldera,

23:38 a massive underwater volcano south of Kyushu.

23:42 They were able to determine the debris' age, around 7,300 years,

23:47 and the size of the eruption that made it.

23:50 Between 332 and 457 cubic kilometers,

23:56 the largest eruption of the last 10,000 years.

24:00 The eruption sent pyroclastic flows 80 kilometers across the ocean

24:05 and into Southern Kyushu and dropped ash on most of Japan.

24:10 Right above that ash layer, the number of villages here nose dives.

24:14 The villages then gradually return with a new style of pottery,

24:20 hinting at a culture changed by the volcano.

24:23 Evidence of the eruption changing Japan's society has

24:26 been found as much as 800 kilometers away,

24:29 preserved beneath the exact same layer of ash.

24:33 (gentle music) After Unzen's eruption,

24:44 the lava dome was still unstable and mudflows continued,

24:48 so a series of dams and channels was built.

24:52 To build in such hazardous terrain,

24:54 they designed unmanned construction vehicles that could be controlled remotely.

25:00 Unmanned construction has spread throughout Japan.

25:04 It's being used to build similar

25:06 infrastructure on other active volcanoes and allowing

25:09 cities to more quickly and safely

25:11 rebuild following natural disasters of all varieties.

25:17 Japan's volcanic islands are an incredible biological resource,

25:21 allowing scientists to study how life evolves in isolation.

25:25 Some species are only found on individual islands or island chains.

25:31 Numerous birds are only found in the Izu Islands.

25:35 And despite its frequent eruptions,

25:37 Miyakejima provides one of their best habitats.

25:41 Torishima sits right in the path of incoming typhoons,

25:44 so weather observatory was built in 1947.

25:48 In 1951, the meteorologists found something,

25:52 a small group of short-tailed albatross.

25:56 They began working to save the species,

25:58 quickly joined by biologists and conservationists.

26:02 And by the 1970s, the population was slowly growing.

26:07 By the 2000s, over a thousand birds were living on the island.

26:11 Researchers could begin introducing them to other islands nearby,

26:16 mitigating the risk of extinction when Torishima erupts again.

26:20 The most recent eruption was in 2002.

26:24 Thankfully, minor.

26:28 Even on the mainland, Japan's volcanoes act as biological islands.

26:33 Not only is Aso here Japan's largest volcano,

26:37 it also supports one of its only natural grasslands.

26:41 This is a very rare ecosystem here

26:43 in Japan and also just a really beautiful one.

26:47 Volcanoes are one of many factors that have made Japan environmentally singular.

26:53 The country is home to hundreds of endemic species of all varieties,

26:57 some only found on individual mountains.

27:01 In the next video, we'll visit a sacred dormant volcano that provides

27:05 some of the best habitats for one of Japan's most unique endemic animals.

27:10 Volcanoes provide water with the abundant melt water from Fuji,

27:14 supporting thousands of farms on its lower slopes,

27:17 and rich volcanic soil has created much of Japan's best farmland.

27:22 Japan has more than 13,000 hot springs, or onsens,

27:27 heated by the same forces that drive the volcanoes,

27:30 with some in use for more than a thousand years.

27:34 In some parts of Japan,

27:35 geothermal plants provide renewable energy and power entire cities.

27:40 And Japan has barely tapped into their potential geothermal resources.

27:45 And of course, in a country where art, folklore,

27:49 and spirituality are inseparable from the mountains,

27:53 the cultural value of volcanoes is immeasurable.

28:17 (gentle music) In 2000, earthquakes were once again recorded under Fuji.

28:36 Japan's government assembled a committee of scientists to evaluate

28:39 the risks and issue new guidance to 44 million people,

28:45 basically asking what would happen if the 1707 eruption happened today.

28:53 Anyone living within range of lava flows

28:56 and pyroclastic flows would have to evacuate.

28:59 That's about 750,000 people,

29:02 and another 470,000 would need to evacuate due to heavy ashfall.

29:08 A large eruption would drop nearly two centimeters of ash across

29:11 much of Tokyo and 16 centimeters on the city's southwestern edge.

29:18 Ash would contaminate tap water and stick to electrical wires,

29:22 causing widespread power and cell outages.

29:26 Roads would be impassable, trains would become inoperable,

29:31 and air travel would be impossible.

29:35 Heavy ashfall would accumulate on rooftops and could even collapse buildings,

29:40 with large ones particularly vulnerable.

29:43 And after the eruptions end,

29:45 the city would be left with the logistical nightmare

29:48 of cleaning up about 500 million cubic meters of ash.

29:54 If the eruption happened during the winter,

29:56 it would melt the mountains snowpack,

29:58 generating mudflows that could reach some small cities.

30:04 The 1707 eruption was one of Fuji's largest,

30:07 so its next eruption will likely be much smaller.

30:11 But Japan's experts still recommend that local

30:14 governments find places to store ash.

30:17 The power lines and railways be improved to function during ashfall,

30:22 and that residents keep emergency supplies on hand,

30:25 food, water, goggles, and dust masks.

30:30 No one knows when the world's most iconic volcano will erupt again.

30:36 How many people will be living in its shadow when it does,

30:39 or how many will have to leave?

30:42 But what is clear from Fuji's last eruption,

30:46 from Sakurajima's daily explosions, from buried buildings and bird islands,

30:52 and even from the largest eruption of the last 10,000 years,

30:57 is that they won't be gone for long.

31:00 (gentle music)