Temperature increases by roughly 3°C for every 100 meters of depth in the Earth's crust.

The geothermal gradient measures how fast Earth's temperature increases with depth. The global average is roughly 25–30°C per kilometer, though local geology creates significant variations. This heat comes from two sources: residual warmth left over from Earth's formation billions of years ago, and ongoing radioactive decay of isotopes such as uranium-238, thorium-232, and potassium-40.The Mponeng Gold Mine in South Africa, the world's deepest at over 4 kilometers below the surface, demonstrates this extreme. Rock temperatures there reach 66°C. To make such depths survivable, engineers pump ice slurry underground and run massive ventilation systems to cool the air to around 30°C. Without active cooling, the heat would become fatal within minutes.Earth's core reaches approximately 5,200°C—nearly as hot as the Sun's surface. This thermal energy travels toward the surface through conduction and convection. Scientists use specialized boreholes to monitor these temperature gradients and map geothermal potential, data crucial for developing geothermal power plants. Countries like Iceland and the United States already harness this internal heat to generate clean electricity.