The heat is partly left over from the formation of the Earth. The other part comes from the decay of radioactive material throughout the entire planet.
This isn’t actually a huge amount of energy by itself, but the mantle and core are surrounded by a miles-thick blanket of rock that you’re currently sitting on. So it loses heat very very slowly because it’s very well-insulated: the amount of heat lost from the interior of the Earth is actually not much bigger than the amount of energy humans are currently using, meaning that humans are contributing almost as much to the Earth’s *surface* temperature through raw energy usage as the slow drain of heat from the core is.
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