Scientists are generally agreed that the world beneath us is composed of four layers, 科学家们普遍认为，我们脚底下的世界共分4层，

　　rocky outer crust, a mantle of hot, viscous rock, a liquid outer core, and a solid inner core. 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。

　　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. 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。

　　Therefore there must be heavier stuff inside. 因此，里面肯定还有较重的东西。

　　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. 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。

　　That much is universally agreed upon. 这一些是大家都承认的。

　　Almost everything beyond that—how the layers interact, what causes them to behave in the way they do, 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，

　　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. 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. 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。

　　Nearly all geology texts tell you that continental crust is three to six miles thick under the oceans, 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，

　　about twenty-five miles thick under the continents, and forty to sixty miles thick under big mountain chains, 在大陆底下约为40公里，在大山脉底下为65-95公里，

　　but there are many puzzling variabilities within these generalizations. 但在这些一般规律之内还有许多令人费解的变异。

　　The crust beneath the Sierra Nevada Mountains, for instance, is only about nineteen to twenty-five miles thick, and no one knows why. 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。

　　By all the laws of geophysics the Sierra Nevadas should be sinking, as if into quicksand. 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。

　　(Some people think they may be.) （有的人认为，这座山脉也许就是在下沉。）