So how much do we know about what's inside the Earth? Very little. Scientists are generally agreed that the world beneath us is composed of four layers—rocky outer crust, a mantle of hot, viscous rock, a liquid outer core, and a solid inner core.1 We know that the surface is dominated by silicates, which are relatively light and not heavy enough to account for the planet's overall density. Therefore there must be heavier stuff inside. We know that to generate our magnetic field somewhere in the interior there must be a concentrated belt of metallic elements in a liquid state. That much is universally agreed upon. Almost everything beyond that—how the layers interact, what causes them to behave in the way they do, what they will do at any time in the future—is a matter of at least some uncertainty, and generally quite a lot of uncertainty.
因此说，我们对地球的内部情况究竟了解多少？很少。科学家们普遍认为，我们脚底下的世界共分4层——一个岩石外壳，一个由炽热而又黏稠的岩石组成的地幔，一个液态的外核，以及一个坚实的内核。（有的人想要更详细的了解地球内部各个层次的厚度，我在这里提供几个数据，使用的是平均数：从0—40公里是地壳。从40—400公里是上层地幔。上层地幔和下层地幔中间的过渡地带位于400—650公里。从650—2700公里是下层地幔。从2700—2890公里是D层。从2890—5150公里是外核。从515—6370公里是内核。）我们知道，地面的主要成分是硅酸盐。硅酸盐比较轻，分量还不足以说明这颗行星的整体密度。因此，里面肯定还有较重的东西。我们知道，为了产生磁场，里面的什么地方肯定存在着一个浓缩的液态金属元素带。这一些是大家都承认的。除此以外，几乎一切——这几层结构是如何相互作用的，是什么原因它们才有了这种表现，未来的某个时刻它们会有什么动作，少说还都是个不大确定的问题，总的来说还都是个很不确定的问题。

Even the one part of it we can see, the crust, is a matter of some fairly strident debate. Nearly all geology texts tell you that continental crust is three to six miles thick under the oceans, about twenty-five miles thick under the continents, and forty to sixty miles thick under big mountain chains, but there are many puzzling variabilities within these generalizations. The crust beneath the Sierra Nevada Mountains, for instance, is only about nineteen to twenty-five miles thick, and no one knows why. By all the laws of geophysics the Sierra Nevadas should be sinking, as if into quicksand. (Some people think they may be.)
我们可以看到，即使是其中的一个部分——地壳——也是个争论得比较激烈的问题。几乎所有的地质学文字资料都会告诉你，地壳的厚度在海洋底下为5-10公里，在大陆底下约为40公里，在大山脉底下为65-95公里，但在这些一般规律之内还有许多令人费解的变异。比如，内华达山脉底下的地壳厚度只有大约30-40公里，谁也不清楚是什么原因。根据地球物理学的所有原理，内华达山脉应当在下沉，犹如陷入流沙那样。（有的人认为，这座山脉也许就是在下沉。）