If you visit a cave on a hot summer day, you might come away with the impression that it's somewhat cold underground.
Actually, nothing could be further from the truth. The Earth's temperature increases steadily the deeper you go. Indeed, mine workers in deep, South African gold mines must learn to work in surroundings that are over one hundred twenty degrees-all year round. The temperature at the Earth's center is estimated to be about seven thousand degrees Fahrenheit! Where did all that heat come from?
Surprisingly, much of the Earth's heat is left over from when the planet originally formed, over four billion years ago. The Earth was made by the collision of billions of planetesimals, chunks of rock and ice much like today's asteroids and comets.
Gravity pulled these planetesimals together, and the young Earth grew slowly, like a snowball, as more and more pieces collided with it. Each planetesimal would have been moving pretty quickly as it crashed into the new planet, and all that speed was converted into another form of energy-heat. This heat increased even more as the material in the Earth's interior compressed and settled under the new planet's growing mass.
If all this happened so long ago, why do we still feel the heat left over from Earth's childhood? It's for the same reason that a cave stays cool in the middle of summer: The Earth is a remarkable insulator. Heat from the Earth's core takes billions of years to escape to the surface.