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Every spring,

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hundreds of adventure-seekers dream[br]of climbing Qomolangma,

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also known as Mount Everest.

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At base camp, they hunker down for months

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waiting for the chance to scale [br]the mountain's lofty, lethal peak.

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But why do people risk life and limb[br]to climb Everest?

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Is it the challenge?

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The view?

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The chance to touch the sky?

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For many, the draw is Everest's status[br]as the highest mountain on Earth.

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There's an important distinction[br]to make here.

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Mauna Kea is actually the tallest[br]from base to summit,

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but at 8850 meters above sea level,

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Everest has the highest altitude [br]on the planet.

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To understand how [br]this towering formation was born,

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we have to peer deep [br]into our planet's crust,

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where continental plates collide.

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The Earth's surface [br]is like an armadillo's armor.

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Pieces of crust constantly move over,

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under,

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and around each other.

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For such huge continental plates,[br]the motion is relatively quick.

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They move two to four [br]centimeters per year,

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about as fast as fingernails grow.

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When two plates collide,

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one pushes into or underneath the other,[br]buckling at the margins,

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and causing what's known as uplift[br]to accomodate the extra crust.

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That's how Everest came about.

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50 million years ago, the Earth's [br]Indian Plate drifted north,

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bumped into the bigger Eurasian Plate,

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and the crust crumpled,[br]creating huge uplift.

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Mountain Everest lies at the heart[br]of this action,

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on the edge of the Indian-Eurasian [br]collision zone.

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But mountains are shaped by forces[br]other than uplift.

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As the land is pushed up, [br]air masses are forced to rise as well.

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Rising air cools, causing any water[br]vapor within it to condense

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and form rain or snow.

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As that falls, [br]it wears down the landscape,

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dissolving rocks or breaking them down[br]in a process known as weathering.

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Water moving downhill carries[br]the weathered material

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and erodes the landscape,

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carving out deep valleys and jagged peaks.

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This balance between uplift and erosion[br]gives a mountain its shape.

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But compare the celestial peaks[br]of the Himalayas

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to the comforting hills of Appalachia.

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Clearly, all mountains are not alike.

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That's because time [br]comes into the equation, too.

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When continental plates first collide,[br]uplift happens fast.

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The peaks grow tall with steep slopes.

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Over time, however, gravity and water[br]wear them down.

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Eventually, erosion overtakes uplift,

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wearing down peaks [br]faster than they're pushed up.

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A third factor shapes mountains: climate.

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In subzero temperatures, some snowfall[br]doesn't completely melt away,

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instead slowly compacting [br]until it becomes ice.

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That forms the snowline, which occurs[br]at different heights around the planet

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depending on climate.

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At the freezing poles,[br]the snowline is at sea level.

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Near the equator, you have to climb[br]five kilometers before it gets cold enough

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for ice to form.

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Gathered ice starts flowing under[br]its own immense weight

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forming a slow-moving frozen river[br]known as a glacier,

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which grinds the rocks below.

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The steeper the mountains, [br]the faster ice flows,

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and the quicker it carves [br]the underlying rock.

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Glaciers can erode landscapes[br]swifter than rain and rivers.

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Where glaciers cling to mountain peaks,[br]they sand them down so fast,

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they lop the tops off[br]like giant snowy buzzsaws.

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So then, how did the icy Mount Everest[br]come to be so tall?

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The cataclysmic continental clash[br]from which it arose

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made it huge to begin with.

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Secondly, the mountain lies [br]near the tropics,

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so the snowline is high,[br]and the glaciers relatively small,

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barely big enough to widdle it down.

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The mountain exists in a perfect storm[br]of conditions

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that maintain its impressive stature.

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But that won't always be the case.

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We live in a changing world [br]where the continental plates,

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Earth's climate,

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and the planet's erosive power

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might one day conspire to cut [br]Mount Everest down to size.

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For now, at least, it remains legendary[br]in the minds of hikers,

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adventurers,

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and dreamers alike.