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This is Unexplainable. I'm Noam Hassenfeld. And this week, a question that seems simple, but it's surprisingly hard to answer.
How tall is the tallest mountain in the world? It turns out the official height of Mount Everest is constantly changing. Laura Bolt is a producer for Vox Video. So back in December, I was listening to the radio and a news story came on about a new official height for Mount Everest. Today, China and Nepal, agreeing for the first time, announced that Mount Everest, which straddles both countries, just got higher.
And it really struck me. I didn't think that the height of Everest was something that was not a fixed thing. So I started doing some research and I called up one of the Nepali surveyors who took the most recent measurement of Everest. His name's Kimlal Gautam. And he was actually the surveyor who had the difficult task of climbing to the top of Everest.
He told me it was a really challenging job. He's actually summited Everest once before, and that's part of the reason why they chose him to do that part of the measurements. He told me that it was his duty. This scientific job was his responsibility. So Kim Lal climbs to the top of Mount Everest.
How does he measure it? So he takes a GPS receiver to the top and he's waiting to receive satellite pings, as many as possible. So he lingers there for a while. We have to go to the top of the world and we have to sit up there and we have to record the satellite data at the top. So people who have climbed to the top of Everest have described it as the roof of the world. You have a panoramic view of the Himalayas.
but they are typically just turning around and going back down. Maybe they'll take a selfie at the top. And that's because anywhere above 26,000 feet on any mountain in the world is known as the death zone. Oxygen is extremely low, so the risk of death up there is far higher. But Kim Lal, because his job was to be receiving as many satellite pings as possible, he lingered there for several hours.
That's negative 45 degrees Fahrenheit in the middle of the night. He actually lost the big toe on his left foot due to frostbite. But he had a pretty good attitude about it. I think this is a token of love given...
He told me it was a token of love from Mount Everest, and it seemed like it was a sacrifice he was willing to make.
But it turns out that Kim Lowell getting to the top is actually the easiest part of measuring Mount Everest. Wait, how is that possibly the easy part? It's because the harder part and the reason Mount Everest's height is constantly changing is figuring out where the bottom is. What do you mean the hard part is figuring out the bottom? Well, the bottom is sea level. And that's what we use as the base measurement for practically any height on Earth.
And sea level is a really hard problem that scientists have been working on for over 100 years. It's even harder in a mountain range like the Himalayas because it's nowhere near the sea. And so you sort of have to calculate where sea level would be if it was there. And the reason Everest's height keeps changing is because
They're still using some of the same methods to figure out where sea level is that they were using in the 1800s when they first took a measurement of Everest. At the time, India was under British rule, so British and Indian surveyors used the closest sea that they had access to, which was the Bay of Bengal in Calcutta.
They had to first of all define sea level. This is Roger Billum. He's a professor of geology at the University of Colorado. So they measured sea level for a month and engraved a stone rock in the harbor. They had to take that sea level measurement at the sea and figure out a way to get it to the Himalayas.
which was hundreds of miles away at the Nepal border. Yeah, I assume the ground isn't flat here, right? How do you draw a totally level, straight line all the way to Everest? You have to walk. Okay. Starting from that mark in the ground, they would put basically a ruler 10 feet high, and they would look with a special telescope at this from a distance of,
about 50 feet away. And then they'd turn around and point the telescope at the next ruler, which was also 50 feet away. And what they were doing is comparing the elevations of the two rulers. So now they would move to another point, and then another, and another, and another.
And each time they could move only about 100 feet in this measurement, transferring height. They had to cover an area of hundreds of miles and they were on foot. So this took them years and years to do.
So this is basically trying to extend sort of a pretend line from the sea all the way to the bottom of Mount Everest? Yes, except that they couldn't really get to the bottom of Mount Everest. Nepal was closed off to foreigners at the time, so British and Indian surveyors had to take the height measurement from as close to Everest as they could get, which was over 100 miles away at the Nepal border.
And back then, before they had GPS to go to the top of the mountain, how did they actually get the height of Everest? So once they calculated where sea level was at that point...
they used an instrument called a theodolite. It's a distant ancestor to what you might see engineers using today. It looks a little bit like a telescope, and it basically just measures angles. So this is trigonometry 101. If you know two angles to a triangle and you know one side, then you can basically finish the triangle. So
So they're solving a trig problem where one side of the triangle is the height of Mount Everest? Yes, exactly. So how accurate was the measurement of Everest from back in the 1800s? And how does that compare to the one that Kim Law's team just made? It was remarkably really, really close. So the first measurement taken in the 1800s was 29,002 feet.
And the measurement they took in 2020 was 29,032 feet. So they were only off by 30 feet, which is really incredible if you think about it. Yeah, that's nuts. It's wild. Yeah. But measurements of Everest have fluctuated pretty wildly over the years since that first measurement was taken. They fluctuated anywhere from 100 feet to a few feet because sea level
sea level is a really complicated measurement to take. And that's not just because it takes a long time to walk from the nearest sea to the base of a mountain. How is it complicated to measure sea level? Is it just factoring in, I don't know, like high tide, low tide, things like that? Yeah, tides are definitely part of it, but it's more complicated than that. Along the east coast of the United States, there's about a meter, a meter and a half of difference between
Florida, and Maine. I talked to Dan Roman, who's a scientist at NOAA, which is the National Oceanic and Atmospheric Administration. It's a government agency. And he explained to me that sea level isn't as uniform as you might think.
So first of all, the ocean varies from place to place. So the Gulf Stream is warm, and all that warm water sits a little bit higher than that cold water from the Labrador Current that's denser and saltier and sits lower. He kind of sees the world's oceans like a big hot tub. Where's the water level? Well, on this side it's kind of high, on this side it's kind of low.
So you want to turn off the jets and figure out where the water would be across the entire hot tub. Well, you can't turn the jets off in this hot tub. It's the ocean.
So to factor in all of these variations, scientists use global mean sea level. So like the average sea level around the world? Yes. It's what we talk about when we talk about global warming, when we say sea levels are rising. They are talking about global mean sea level, and that's going up, you know, a fraction of an inch every year. Okay. So is that the only reason why it's hard to figure out sea level? No, not even close. Okay. Okay.
There's two other things that make measuring sea level more complicated than you might think. Okay. One is that the Earth actually has a bulge at its equator. So the Earth is kind of like a ball, but it's squished a little on the poles because it spins, and so you get this centrifugal forces. Sea level is actually higher along the equator than it is along the poles. So sea level changes based on the shape of the Earth.
you know, this bulge at the equator. What's the second thing that scientists have to take into account? So they also have to take gravity into account. Okay. What that means is that large objects like mountains exert their own gravity and they actually pull the sea towards them a bit. So when you're imagining measuring a mountain range like the Himalayas that is nowhere near the sea, you're imagining where the sea would be if it ran under the Himalayas. It's an imaginary ocean.
And because the mountain, I guess, exerts a bunch of gravity, if the sea were under Mount Everest, like if that translation were perfect, the sea level would actually be slightly higher there because Mount Everest is pulling it up. Exactly. So as you can see, it's an extremely complicated mathematical problem that you have to solve just to figure out how tall something is.
So Mount Everest's height has changed over the decades because, one, it's really hard to translate sea level all the way to the mountain. But two, it's like really hard to figure out what that sea level even is. Yes, but there is something else going on that has nothing to do with how hard it is to calculate sea level anymore.
And that's that the height of Mount Everest is literally changing. What do you mean? So the Himalayas are growing every year. The rocks at the top of the world are actually ocean rock. And that means that they were formed over 50 million years ago. When India slammed into the rest of Asia, that all got scraped up and it's being pushed up to the top of the roof of the world. And that collision is technically still happening. It's still going up. It is increasing.
geologically speaking, a train wreck that is ongoing. That creates a lot of tectonic plate friction in the area. And that's why there's a lot of earthquakes near the Himalayas. And I'm sure you remember that in 2015, Nepal suffered a horrible earthquake that killed thousands of people. So it was really in response to that earthquake that they decided to retake the measurement because earthquakes can actually release some of that tectonic pressure and make mountain ranges shorter.
So that's why Kim Law and his team climbed all the way to the top of Everest? Yes, but that was just to take the GPS measurement from the top. Right, right. In order to figure out where the bottom was, they had to figure out where the gravity field was by taking gravity measurements. They had to figure out where the bulge was, and they had to transfer sea level using telescopes.
So then is this measurement that Kim Law's team just got, is this the right measurement? Is this the true height of Everest? Well, it is for now because we've all agreed that 29,032 feet is the official height of Everest. So in that sense, sure. But it is definitely going to change again.
because scientists are still figuring out where the bottom of Everest is, essentially. The Nepal government still had to spend years transferring sea level from the sea to the mountain
using some of the same methods they used in the 1800s. And they had to do all these gravity measurements along the way to make sure that they had it right. So it's all sea level's fault? Yes. I feel like I've turned you into a sea level skeptic. Yeah, yeah. I mean, sea level is an important measurement for a lot of reasons. It's important for the shipping industry, shipping.
It's important for measuring the impact of global warming, how much higher sea level is around the world. But in figuring out a precise height of something above sea level, especially if it's not near the sea, it's just a really hard base measurement to use.
So if you wanted to take a measurement of Everest that wasn't constantly in flux like this, then you might have to use something other than sea level. But if we did that, Everest might not be the tallest mountain in the world. Coming up after the break, how at least three different mountains other than Everest have a claim to being the tallest mountain in the world, depending on how you measure them. Support for Unexplainable comes from Greenlight.
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Okay, Unexplainable, we're back. Laura, in the first half of the show, you were talking about why it's so hard to measure Mount Everest and how it comes down to the bottom of the mountain, not the top, actually. And it's complicated because of this whole mess that is how scientists measure sea level. So how else could we measure a mountain? So I was talking to Dan Roman about this. He's the scientist from NOAA who I interviewed.
And he explained to me that it's really about perspective. When you want to measure something, you need to determine what you're measuring from. How high is it? Above what? And if you change where you're measuring from, where the base measurement is, there are some new contenders for the tallest mountain in the world. All right, let's hear about contender number one.
There's a Hawaiian mountain, Mauna Kea, that is actually taller than Everest if you're doing a simple base-to-peak measurement. And that's because a majority of Mauna Kea is...
underwater. If you took away the water and you looked at how high that volcano is sitting, it is very impressive. If you measure Mauna Kea like you would a person, you know, from your feet to your head, from the bottom to the top of the mountain, it would be the tallest mountain in the world.
Okay, that seems like a pretty simple way to measure a mountain. What's contender number two? So there's actually another mountain that would be taller than Everest if we do a simple base-to-peak measurement, and it's not because this mountain is underwater.
What mountain is that? That would be Denali in Alaska. Okay, that's the mountain formerly known as McKinley? Yes, exactly. Everest actually sits on top of a plateau. So the base of Everest is already pretty high above sea level. So it sort of gets a head start in that sense. Okay. But Denali, where that mountain meets the land, is much closer to sea level. So the base to peak is actually much higher than Everest.
Like how much taller? If we do a simple base-to-peak measurement, Denali would actually be taller than Everest by something like 6,000 feet. Oh, wow. Okay, that's, like, huge. Yeah, and it would look taller if you stood at the base. So you can look up and get a crick in your neck trying to look up at the top of it, whereas, you know, when you're looking at the Himalayas, you see the front part of the Himalayas behind that sticking up. Maybe if you're lucky on a clear day, you might see Mount Everest.
So this is sort of a semantic question of like, what is a mountain, right? Yes.
Do we count the, like, Himalayan plateau that Everest is sitting on? Is that part of Mount Everest? Or does Mount Everest sort of start on top of that plateau? Yeah, it could be looked at that way. It's because we've chosen sea level as the base measurement that we do include that plateau in the official height of Everest. But there's also another way to think about height that's not necessarily semantic, and it's if we chose a different base measurement.
So something that would be simpler than sea level is the center of the Earth. Sure. And that's where the bulge at the equator comes in. The Earth's not a round ball. It is squashed 20 kilometers on the poles. So that means it's relatively wider by 20 kilometers on the equator. And that equals 65,000 feet. Wow. That's like two Everest.
Okay. And so a mountain in Ecuador, Mount Chimborazo, on paper it is much shorter than Everest. But because of that bulge, it's actually the closest mountain to space. Okay. So if you want to stand on a peak that is the closest to the stars, you should go to Chimborazo and not to Everest. Okay, so we've got these three contenders besides Everest for tallest mountain, Mount Achaea, Denali, Chimborazo. Okay.
I gotta say, if I'm thinking about all of these, I'm not as swayed by Chimborazo being sort of the closest to space, because I don't really think that we feel that 20-kilometer bulge if we're near the equator. But I am kind of swayed by Denali. I mean...
the mountain that feels the tallest. It's the mountain that looks the tallest if you're nearby. I feel like just like we don't take into account that 20-kilometer head start in Chimborazo, we're probably not taking into account the Himalayan plateau that sits under Everest, right? I don't know. It feels like Denali has
has a good claim to being the tallest mountain in the world. Yeah. I mean, if you think about sea level being a human-centric measurement, that's why we chose it. I mean, that makes a lot more sense than choosing the center of the Earth as a base measurement. We want to know what the experience of climbing a mountain would be like. By that measure, base to peak does matter. And the experience of
Climby Denali versus Everest would reflect that. Yeah, I mean, it feels like Everest only has all this importance because of sea level, which is almost arbitrary. Like, it doesn't represent either what's the actual highest thing on Earth or what feels like the highest thing compared to the land around.
It's just sort of this weird problem we've created for ourselves. Yes, we've definitely put ourselves in a bind because we have chosen sea level as a base measurement. It's extremely hard to measure sea level. But Dan, the scientist I spoke to, is working on a new model for sea level that should make it easier. How would that model work?
So remember that sea level is by no means a static thing. It changes all over. Right. Because of temperature, because of the salinity in the water.
because of the bulge at the equator, and because of all the little gravity fluctuations around the world. Right, okay. So Dan is basically working on a model that accounts for all of these things, and it would essentially create a universal zero that everyone could use for sea level. And it will help us get rid of all of this really tedious stuff
surveying that people were doing, you know, in the 1800s and frankly still doing today, which is why it took years to take the latest measurement. And so essentially Dan is trying to make sea level more of a universal value that's pegged to something objective, like this model of all the little gravity fields on Earth? Yeah, exactly. And once Dan gets this true universal sea level,
I assume Everest's height is going to change again, right? Yeah, it'll change. But even Dan doesn't know in which direction. I don't know if it'll go up or down, but it will change most likely. At the end of the day, like, is this just trivia? Is this just something to, like, impress your friends at the bar with?
to be like, oh, hey, you know, Denali is actually the tallest mountain in the world from base to summit. We had this actually, we had our geospatial summit, and this is one of the trivia questions. I mean, how much does all of this measuring mountains matter? It's a good question. I mean, I think it matters a little and a lot. I mean, especially for people who really care about superlatives or conquering the highest thing in the world. Yeah.
A lot of people have died. Hundreds of people have died climbing Everest. People pay tens and thousands of dollars to climb this mountain. It only costs a few hundred dollars to climb Chimborazo if you wanted to. People just want to climb whatever is the tallest mountain.
And that was made obvious in the 80s when there was a measurement taken of K2, which is in Pakistan. I picked up the paper and here was a headline saying that K2, which we'd all thought was the second highest mountain on Earth,
was going to be the first, number one. That measurement actually made K2 taller for a second than Everest. This whole range might be about 247 meters or about 800 feet higher. Not only K2, but some of the other peaks in the surrounding area might be
be higher than Mount Everest. That measurement eventually got debunked. They found out there were a lot of things wrong with it. But there was a huge frenzy, and a lot of people said, well, I guess I got to go climb K2 now. Edith was very excited because it's different to climb the highest peak in the world than the second peak in the world.
So is this ultimately about tourism? Not necessarily. I mean, it gives communities an immense sense of pride in having something like Everest in their country as well. And I could tell when I got a chance to speak with the surveyors who took the latest measurements, one of those being Kim Lal, the man who climbed to the top of Everest.
He told me that they were very proud. That after this devastating earthquake, that it was their responsibility to clarify the
the question around the height of Mount Everest. Half the mountain is in China and half the mountain is in Nepal. And this was the first time that they had taken a measurement on their side of the mountain. And for Kim Lal, even though he suffered a lot during the climb and he lost a toe,
And even though the measurement is probably going to change again, I could tell that he was really, really proud to have taken part in something so monumental for Nepal. I'm so proud and I feel so happy that I've really completed my duty. This episode was reported by Laura Bolt and produced by Noam Hassenfeld, who wrote the music for it, too.
We had edits from Meredith Hodnot, Jillian Weinberger, and Brian Resnick. This episode was mixed and sound designed by Christian Ayala and fact checked by me, Mandi Nguyen. Bird Pinkerton is out there measuring the universe. Liz Kelley Nelson is the VP of Vox Audio and Lauren Katz heads up our newsletter. Speaking of the newsletter, if you haven't signed up yet, you can do that at vox.com/unexplainable. Each week it has lots of cool stories that didn't quite make it into the show and just other weird, interesting things that we're thinking about.
We'll also have a link this week to an Everest video that Laura produced for Vox. It's called Why Mount Everest's Height Keeps Changing. Unexplainable is part of the Vox Media Podcast Network, and we'll be back in your feed next Wednesday.