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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.
在炎热的夏日，如果你去山洞一游，离开时你可能会觉得地下有些寒冷。实际上，没有事物能够背离真理过远。往地心走得越深，地球的温度会稳步上升。在南非深邃的金矿里劳作的矿工必须学会常年在温度超过120度的地下环境工作。
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.
据估计地心的温度大约有7000华氏摄氏度！这一切热量从何而来？令人惊讶的是，地心的大多数热量大多是40亿年前地球起源初期遗留下来的。地球是由数十亿星子，和类似于今天的小行星和彗星的大块岩石和冰碰撞形成。引力将这些星子集中在一起，因此初生的地球就像滚雪球一样，吸引越来越多的碎片与它相撞。
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.
当星子坠向地球时，移动速度非常快，而且速度会转化为另一种形式的能量——热。当这些物质在地球内部受到挤压，并被埋藏在质量日益增长的地球下时，会迸发更多的热量。如果这一切都发生在很久以前，那么我们为什么仍然会感觉到地球初期遗留下来的热量呢？这与盛夏洞穴仍然保持凉爽是同一个道理：地球是一个非凡的绝缘体。地心的热量要经过数十亿年才能从地表释放出来。
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