Some scientists believe that the earth began billions of years ago as a huge ball of swirling dust and gases. As the earth grew larger, new materials piled up on the outside and squeezed the materials inside. Energy from all this activity was released as heat.

Eventually most of the iron in the earth melted and collected at the core, or center. This released more heat. The temperature at the core became hot—perhaps more than 9000°F (4,982°C). Lighter materials rose to the outside of the earth and cooled, forming the earth’s crust.

If you dig in your backyard, don’t worry about running into the earth’s core. You’d have to dig a hole 4,000 miles (6,437 kilometers) deep!

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