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If you visit a cave on a hot summer day, you might come away with the impression that it's somewhat cold underground. Actually, nothing could be further from the truth. The Earth's temperature increases steadily the deeper you go. Indeed, mine workers in deep, South African gold mines must learn to work in surroundings that are over one hundred twenty degrees—all year round.
在炎熱的夏日，如果你去山洞一游，離開時你可能會覺得地下有些寒冷。實際上，沒有事物能夠背離真理過遠。往地心走得越深，地球的溫度會穩步上升。在南非深邃的金礦里勞作的礦工必須學會常年在溫度超過120度的地下環境工作。
The temperature at the Earth's center is estimated to be about seven thousand degrees fahrenheit! Where did all that heat come from? Surprisingly, much of the Earth's heat is left over from when the planet originally formed, over four billion years ago. The Earth was made by the collision of billions of planetesimals, chunks of rock and ice much like today's asteroids and comets. Gravity pulled these planetesimals together, and the young Earth grew slowly, like a snowball, as more and more pieces collided with it.
據估計地心的溫度大約有7000華氏攝氏度！這一切熱量從何而來？令人驚訝的是，地心的大多數熱量大多是40億年前地球起源初期遺留下來的。地球是由數十億星子，和類似于今天的小行星和彗星的大塊巖石和冰碰撞形成。引力將這些星子集中在一起，因此初生的地球就像滾雪球一樣，吸引越來越多的碎片與它相撞。
Each planetesimal would have been moving pretty quickly as it crashed into the new planet, and all that speed was converted into another form of energy—heat. This heat increased even more as the material in the Earth's interior compressed and settled under the new planet's growing mass. If all this happened so long ago, why do we still feel the heat left over from Earth's childhood? It's for the same reason that a cave stays cool in the middle of summer: The Earth is a remarkable insulator. Heat from the Earth's core takes billions of years to escape to the surface.
當星子墜向地球時，移動速度非常快，而且速度會轉化為另一種形式的能量——熱。當這些物質在地球內部受到擠壓，并被埋藏在質量日益增長的地球下時，會迸發更多的熱量。如果這一切都發生在很久以前，那么我們為什么仍然會感覺到地球初期遺留下來的熱量呢？這與盛夏洞穴仍然保持涼爽是同一個道理：地球是一個非凡的絕緣體。地心的熱量要經過數十億年才能從地表釋放出來。
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