Scientists believe that about a hundred billion years ago the earth, the sun, and all the planets of the solar system were nothing but a cloud of cold dust particles swirling through empty space.

Gradually, these particles were attracted to each other and came together to form a huge spinning disk. As it spun, the disk separated into rings, and the furious motion made the particles white-hot.

The center of the disk became the sun, and the particles in the outer rings turned into large fiery balls of gas and molten-liquid. Then they began to cool and condense and take on solid form. And at last, some four or five billion years ago , they became Earth, Mars, Venus and the other planets.

What is the earth like inside? 

Esentially, the earth is constructed something like a baseball. If you were to cut a baseball in two, you would see that it has a core of solid rubber. Wrapped around this inner core are a great many layers of heavy string. This string binding is solid, yet it is not as solid as the rubber core, for it will give and sometimes alter its shape under pressure. The outer covering is a thin layer of horsehide which holds in all the rest of the ball.

The solid-rock covering of the earth, called the crust, is between ten and thirty miles thick - much thinner in proportion than the horsehide covering of the baseball.

Under this crust is a tick layer of a different kind of rock, which is known as the earth's mantle. The rock of the mantle is solid, but it is solid in the same way that the string wrapping of the ball is solid. Under pressure it will move slightly and change its shape. The mantle extends to a depth of 1,800 miles.

Enclosed by the mantle is the earth's core. Unlike the center of a baseball, the center of the earth is made up of two parts: an outer core and an inner core. Both are composed of metal - mostly iron, with some nickel - but the outer core is liquid and the inner core is solid. At the very center of the core, the temperature is about 8,000 degrees - not much less than the the temperature on the surface of the sun.

The total distance from the earth's surface to its center averages approximately 3,960 miles.

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 the 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 (SIZE-mo-graf). And it is by studying the speed and behavior of P and S waves - the distance they have traveled 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.