This is the world's largest rainfall simulator, located in Tsukuba, Japan.

Now, I know that it just looks like a warehouse with a lot of sprinklers, but this building is incredibly important.

The science conducted here keeps tens of millions of people safe, and it's only becoming more and more vital.

We were given exclusive access to tour the facility, and they even let us experience what it's like to stand under the most intense rainfall ever recorded.

This is insane.

Nope, nope, nope, nope, nope, nope, nope, nope.

Japan is a land of natural disasters.

Most people will think of earthquakes, tsunamis, and volcanoes, but it also experiences massive typhoons, which are like hurricanes that pick up energy from the Pacific Ocean and dump meters of rain on the islands.

Yakushima Island near the southern part of Japan is one of the wettest places on Earth, receiving up to 10 meters of rain every year.

For comparison, the Amazon rainforest gets only about three meters of rain per year.

That's three times less than Yakushima Island.

Most of the time, rain isn't a problem.

The water evaporates or seeps into the soil and is then absorbed by plants or enters the underground aquifer systems.

But too much rain in a short period, like after a typhoon can lead to serious problems.

Problems that scientists working at NIED use the rainfall simulator to study and hopefully prevent.

The giant warehouse is equipped with 550 nozzles attached to the roof.

The scientists can control the intensity of the rain from 15 millimeters per hour to 300 millimeters per hour.

The most rainfall ever measured in one hour occurred on the 22nd of June, 1947, when 305 millimeters of rain fell on the town of Holt, Missouri.

So this warehouse can simulate the most intense rainfall anyone has ever experienced.

In a way that makes it the rainiest place on Earth.

And honestly, I'm having a good time here in sunny Australia.

So I sent Veritasium producer Petr in my stead.

They're gonna turn on the rain really soon.

So I'm running out to grab my rain jacket, I'm ready.

Wow, this is so much rain.

All around me, there is exactly 300 millimeters of rain falling every hour.

We've only turned it on about five minutes ago, and there's just so much rain.

There's already puddles, this is genuinely, absolutely wild.

I don't think I've ever experienced anything like this before.

The nozzles at the simulator contain four holes of varying diameters, so they can produce raindrops of different sizes, and the size of the raindrop affects how fast it falls.

You've likely experienced the kind of rain with the big droplets that pelt your face or walked through a drizzle where small drops hover like mist.

Because the smaller the raindrop, the slower it falls.

There are two forces that act on a raindrop, gravity and air resistance.

And the larger raindrops have a higher weight to surface area ratio, and so they have a higher terminal velocity, so they will be falling faster.

A raindrop that's one millimeter in diameter falls at only about two meters per second, while a three millimeter raindrop falls at six meters per second.

Air resistance is also why raindrops aren't shaped like cartoon raindrops.

They are closer to spherical, but a bit flatter on the bottom where they encounter oncoming air.

If a raindrop gets too big, it flattens out, caves in in the middle and briefly resembles a little parachute before it breaks up into smaller droplets.

Flooding is something that Japan takes very seriously and for good reason.

In July, 2018, there were floods all over the country due to the rainfall from Typhoon Prapiroon.

Some regions of the country received nearly two meters of rain in just 10 days, and the resulting floods were enough that more than 8 million people had to be evacuated from their homes.

Over 200 people died, and the damage to property from the flooding was more than 1 trillion yen, nearly $10 billion.

Tokyo, the capital city of Japan, is very vulnerable to flooding.

There are more than a hundred rivers crisscrossing the city of nearly 40 million people.

To prevent the rivers from overflowing, there are pipes and tunnels under the city leading to an enormous water storage tank.

In October, 2019, typhoon Hagibis dumped over 200 millimeters of rain onto the city in under 48 hours.

The underground system diverted 12 million cubic meters of water and prevented an estimated $1.7 billion in damage.

But flooding isn't the only problem.

Japan is a very mountainous country with many towns and villages situated in valleys.

So combined with the heavy rainfall, this creates the perfect conditions for another hazard.

(man screaming)

The researchers at NIED have identified over 700,000 places where landslides are a significant threat to homes.

But landslides are so complicated.

There are so many factors that affect if a landslide will occur, how large it could be, how fast it will move, and how much damage it will do.

Not just the slope angle or the amount of new rain, but the type of soil, the minerals present, and the vegetation growing on top of the slope.

This is footage from a landslide that occurred in Norway in June of 2020.

There's barely any slope angle here, but the whole area sits on a layer of quick clay, an incredibly unstable clay layer, which when exposed to intense rainfall loses its structural integrity, becoming a liquid.

In the case of this landslide, there were no casualties, though several homes were swept out to sea.

The complexity of the physics of landslides is why the work done at the large scale rainfall simulator is so important.

To study landslides, the whole warehouse has a secret, it moves.

The rainfall simulator can be in one of five positions.

Velocity is about one meter per one minute.

One meter per minute.

Wow, it moves on these railway tracks.

Every time you move it, you need to grab this gigantic wrench and undo these bolts.

This is the pipe that connects all the water up to the sprinklers.

There's a number of these kind of openings in the pipe along the line.

So when you move it, you just connect it to a different part.

So this is the landslide testing facility?

Yes, this is a 30 degree slope, you can see this.

This is a shallow landslide.

Now you can put the soil here, about one meter.

You would put all the sand and all the soil, and then you'd bring the building over.

You'd move it over here.

And then you'd start raining on it.

Yes.

20 degrees this slope, is 20 degrees.

Right, so you have a 30, a 40, and a 20, 20, 30, 40.

Yes.

So what causes landslides?

Well, a slope will slide when the force of gravity pulling it down becomes greater than the force of friction holding it up.

There's a misnomer out there that water makes the materials slipperier, so it rains and it soaks into the ground and it makes it slipperier.

That's not true, water's actually an anti lubricant for many materials, including quartz, which is our most common mineral in soils and on earth.

So water doesn't make soil slipperier, but soil is porous.

There are grains and there are pores between those grains.

And as it rains, the water seeps into the soil, the pores become filled with water, and as it keeps raining, the water pressure in the pores increases, which decreases the friction between the grains.

For a slope that was already at risk of sliding, this decrease in friction is what ultimately leads to the slope breaking apart and sliding downhill.

One thing that our mathematical models are not very well developed at yet is determining is it going to slide slowly or is it going to slide or maybe flow rapidly?

That's a tough one, that's, so experimentation's a great way of getting at that.

And you know, the scaler effects are huge.

So many people have a little model in their lab, you know, a small scale flume, but now you're dealing with materials, you know, you're starting to get off scale.

A large scale stimulation device like they have in Japan is really important.

So how can you prevent landslides?

Well, there are a few things you can do, like using steel beams and mesh to anchor the slope.

Or you can dig up the top layer of the soil to decrease the slope angle.

And when it's expected that a slope will slide after too much rain, engineers drill holes into the slope and place pipes to drain the water out.

Another solution is to create catchment zones and diversion channels, big holes in the ground that will catch or divert the landslide before it hits a residential area.

Trees are incredibly effective at preventing landslides.

Not only do their roots provide great anchors, they draw the water up from the soil to be evaporated away, which effectively drains the soil and decreases the water level.

(chainsaw operating)

Where steep slopes have been deforested for the timber industry, landslides have become more common.

Over the last few decades, there has been a tenfold increase in landslides in the forests of British Columbia.

Humans are really good at causing landslides and we're shaping the land a heck of a lot more quickly than geologic processes do.

So when we excavate over here or add more weight or more landslide or more material over here, and that triggers landslides very, very regularly.

Study in Seattle from Seattle, Washington a few decades ago, showed that I think more than 85% had at least partial human trigger, if you will.

While the focus of the work is primarily on landslide study and prevention, the simulator is also used to test how drones fly in rainy and windy conditions.

It's also used to test self-driving cars.

The data is used to improve the hardware and software of how the cars detect various objects.

The two main ways that self-driving cars detect other cars, traffic lights and pedestrians is with cameras or LIDAR sensors.

In both cases, rain can decrease the visibility and accuracy of these sensors.

So testing them in a perfectly repeatable environment helps engineers develop solutions to these weather challenges.

In Japan, the rainfall condition is changing.

And now in this century, the rainfall condition is so many heavy rainfall in so much short time.

As the world heats up due to humanity's addiction to fossil fuels, extreme weather events are becoming more common.

Compared to 30 years ago, the number of rain events in Japan with an intensity of 50 millimeters per hour have become 40% more common.

Rainfall at double that intensity has become 70% more common.

Climate change will increase the rate of flooding and the occurrence of landslides in the future, which is why the work done at the world's largest rainfall simulator is becoming more and more important.

But I hope that humanity focuses its efforts on addressing the root causes of climate change and not just on the mitigation of its negative impacts.

The best way to minimize landslide damage is prevention, and it's the same for cybersecurity.

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So I wanna thank NordVPN for sponsoring this part of the video, and now back to the world's largest rainfall simulator.

All right, I'm gonna try it without an umbrella.

I have a very nice, very nice jacket.

I'm not sure how it's gonna hold up, but we're gonna try.

(Petr screaming)

Oh my God, this is insane.

Nope, nope, nope, nope, nope, nope, nope, nope.

My jacket is waterproof, so I was fine.

But my jeans are absolutely soaked through.