Beginning of the Earth
How can we tell what the inside of the earth is like?
No scientific instrument has ever penetrated more than two or three miles into the earth, How, then, are we able to tell what the inside of the earth is like? The answer is: by the action of earthquakes.

Thousands of earthquakes occur every year in many parts of the world. Most of them are too slight to cause damage, but all of them send out shock waves that penetrate all through the earth, even the deepest parts.

These earthquake waves are of two basic kinds: P (primary) waves, and S (secondary) waves. P waves travel faster than S waves. P waves go through liquids, but S waves can't. The speed of both P and S waves vary according to the depth of the earth they have penetrated. Both behave differently when passing through different kinds of rocks.

When these waves come back to the surface, they are recorded on a delicate instrument called a seismograph. And it is by studying the speed and behaviour of the P and S waves - the distance they have travelled from their point of origin, the depths at which they have been reflected and bent, and the time it has taken for them to make their journey - that scientists are able to determine what the inside of the earth is like.
How can we tell what the inside of the earth is like?