If you visit a cave on a hot summer day, you mightcome away with the impression that it's somewhatcold underground. Actually, nothing could be furtherfrom the truth. The Earth's temperature increasessteadily the deeper you go. Indeed, mine workers indeep, South African gold mines must learn to work insurroundings that are over one hundred twentydegrees—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 degreesfahrenheit! Where did all that heat come from? Surprisingly, much of the Earth's heat is leftover from when the planet originally formed, over four billion years ago. The Earth was madeby the collision of billions of planetesimals, chunks of rock and ice much like today's asteroidsand comets. Gravity pulled these planetesimals together, and the young Earth grew slowly, likea 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 evenmore as the material in the Earth's interior compressed and settled under the new planet'sgrowing mass. If all this happened so long ago, why do we still feel the heat left over fromEarth's childhood? It's for the same reason that a cave stays cool in the middle of summer: TheEarth is a remarkable insulator. Heat from the Earth's core takes billions of years to escape tothe 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。