The distance from the surface of Earth to the center is 3,959 miles, which isn't so very far. It has been calculated that if you sunk a well to the center and dropped a brick into it, it would take only forty-five minutes for it to hit the bottom (though at that point it would be weightless since all the Earth's gravity would be above and around it rather than beneath it). Our own attempts to penetrate toward the middle have been modest indeed. One or two South African gold mines reach to a depth of two miles, but most mines on Earth go no more than about a quarter of a mile beneath the surface. If the planet were an apple, we wouldn't yet have broken through the skin. Indeed, we haven't even come close.
从地面到地心的距离为6370公里。这不算太远。有人计算，要是朝地心打一口井，然后扔下一块砖头，它只要45分钟就能落到底(虽然到了那个地方它已经没有重量，因为地球的全部引力都在上面和四周，不在下面)。实际上，很少有人试图深入到地心。南非有一两个金矿井达到了3公里以上的深度，但地球上大多数矿井的深度不超过400米。假设地球是个苹果，我们还没有戳破它的皮。实际上，离戳破皮还远着呢。

Until slightly under a century ago, what the best-informed scientific minds knew about Earth's interior was not much more than what a coal miner knew—namely, that you could dig down through soil for a distance and then you'd hit rock and that was about it. Then in 1906, an Irish geologist named R. D. Oldham, while examining some seismograph readings from an earthquake in Guatemala, noticed that certain shock waves had penetrated to a point deep within the Earth and then bounced off at an angle, as if they had encountered some kind of barrier. From this he deduced that the Earth has a core. Three years later a Croatian seismologist named Andrija Mohorovicic was studying graphs from an earthquake in Zagreb when he noticed a similar odd deflection, but at a shallower level.
直到稍稍不到一个世纪以前，最知情的科学家所知道的地球内部的情况，比矿工知道的多不了多少──即，你可以在土里往下挖一段距离，然后碰上岩石，仅此而已。接着，1906年，一位名叫R·D·奥尔德曼的爱尔兰地质学家在审阅危地马拉一次地震的地震仪读数时，注意到有的冲击波渗入地球深处，然后以某个角度反弹回来，好像是遇到了什么障碍。他从而推断，地球有个地核。三年以后，克罗地亚地震学家安德烈·莫霍洛维契奇正研究萨格勒布一次地震的曲线图，突然注意到类似的转向，只是在较浅的层面上。