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If you asked people what the tallest mountain on Earth is, most would respond with Mount Everest, which is on the border of Nepal and Tibet. Everest is the highest peak in the Himalayan mountain range, rising 8,849 meters (29,032 feet) above sea level.

Everest is also commonly seen as one of the high points of human conquest. Scaling the mountain and reaching its peak was once known as one of the most significant challenges a human could undertake.

However, according to Joe Hanson, PhD, host of PBS’s “Be Smart,” Everest may not be the tallest mountain on Earth. In a video called “Why No One Can Agree on What's REALLY the Tallest Mountain,” he shows that height is in the eyes of the beholder when it comes to mountains.

Hanson is a science writer, biologist, and educator whose work has been published in WIRED, Nautilus, Scientific American, Texas Monthly, and other publications.

“Everest checks in at 8,848.86 meters tall today. But we still don't really know if that's right. Because on a planet that isn't perfectly round wrapped in a crust that keeps moving, measuring a mountain turns out to be way harder than you think,” Hanson opens the video.

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Hanson says that the title of tallest mountain on Earth has changed more than a few times over the last 300 years. In the 18th century, Mount Chimborazo in Ecuador was considered the tallest, but in 1908, that honor switched to Dhaulagiri in Nepal. Thirty-nine years later, that honor was taken by Kangchenjunga on the border of Nepal and India, until Everest usurped it just five years later.

There are two major problems with definitively ranking the tallest mountains on Earth. First, there is no universally accepted rule on what a mountain is or how one is defined. Second, mountains aren't the massive unchanging things that they appear to be.

What is the tallest mountain on Earth?

If you count the submerged part beneath the water, Hawaii's Mauna Kea is 20% taller than Everest. If you just measure base-to-summit, then Denali in Alaska is the tallest. "Everest only takes the title because most of the time, we measure mountains from sea level," Hanson says.

Everest is considered the tallest mountain because we measure from sea level, but that’s not the most reliable place to start. Due to Earth's gravity and shape, sea level varies across the globe, creating different elevations across the various oceans and seas. Scientists average these variations to create a “mean” sea level, the baseline for measuring mountain heights.

“But these days, the commonly accepted view is to measure a mountain's height above mean sea level. So Everest gets the title of tallest, despite other mountains having pretty strong claims to the throne,” Hanson says. “So, to summit all up, it's pretty easy to figure out where a mountain ends, but not everyone agrees on where a mountain starts. So when it comes to figuring out what's really the tallest mountain, maybe first we should get to the bottom of that.”

The funny thing is that even if Everest is the tallest mountain on Earth, it may not be that way forever; according to Hanson, Nanga Parbat in Pakistan is growing faster than Everest and could eclipse its famous neighbor in the next couple hundred thousand years. So, enjoy your time in the spotlight, Everest. In a few hundred thousand years, you may be downgraded to number two.