How do we know what it's like inside the Earth?

   Suppose you could dig a hole four thousand miles deep, straight to the center of the Earth. What would you find on the way? Although the deepest oil well only goes down about four miles, scientists have put together many kinds of information, and this is how they think the Earth is made from the surface to the center:
First comes a crust of ordinary rock. This is twenty or thirty miles thick in most places. Under the Pacific Ocean it is only a few miles thick.
   Next comes a layer of heavier rock about 1,800 miles thick.
   Inside this layer is the Earth's core. The core seems to be a sort of liquid metal, very hot and much heavier than any metals you have ever seen.
   Information about the Earth's inside comes from an instrument that was invented for another purpose. This instrument is the seismograph which makes a little wavy line on special paper every time it is joggled by an earthquake. Scientists study these wavy lines to find out when and where the Earth is quaking. They have also discovered that the lines can help reveal what kind of material the shocks of the earthquake passed through on their way from the quake to the seismograph.
   Will we ever have a look at the Earth's core itself? Probably not. But some day we will certainly know more about it and what it is made of.