Pressure

Is probably not what I think it is, but it may be. In physics, they tell us that energy cannot be created or destroyed. Theoretically, energy in = energy out. So why, may I ask you, is the earth's center hot? Their is no combustion to heat it. Pressure is what heats the earth's center, making it give off thermal energy, with no end in sight. Probably indefinitely, or until earth is destroyed in one way or another. So, if we can harvest the earth's natural heat, we would have a near-indefinite source of energy. Right? Maybe I'm wrong, though.But if I am, I want one heck of a good explanation.