Earth generates heat. The deeper you go, the higher the temperature. At 25km down, temperatures rise as high as 750°C; at the core, it is said to be 4,000°C. Humans have been making use of hot springs as far back as antiquity, and today we use geothermal technology to heat our apartments. Volcanic eruptions, geysers and earthquakes are all signs of the Earth’s internal powerhouse.
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Source: Phys.org