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Deep beneath our feet, the heart of our planet glows with a heat that rivals the surface of the Sun, reaching temperatures of around 5,400 degrees Celsius. This incredible furnace is powered by two main sources. A significant portion of this heat is a remnant from Earth's violent formation about 4.5 billion years ago, when the planet was built from the collision of countless smaller bodies. The friction from this accretion and the subsequent sinking of heavier elements like iron to the center generated immense thermal energy that is still dissipating today. This primordial heat is supplemented by the continuous radioactive decay of elements such as uranium and potassium within the mantle and crust, which acts like a planetary-scale generator.
Scientists can't drill to the core to take its temperature directly, so they rely on indirect methods to understand this extreme environment. By studying how seismic waves from earthquakes travel through the planet, researchers can deduce the properties of its layers. It was Danish seismologist Inge Lehmann who, in 1936, first theorized that the Earth had a solid inner core distinct from its liquid outer core by analyzing these very waves. Modern laboratory experiments also recreate the core's conditions by squeezing tiny samples of iron between diamonds and blasting them with lasers, allowing scientists to test how materials behave under such immense pressures and temperatures.
One of the most fascinating aspects of the core is that while the outer core is a molten liquid, the inner core is a solid metal ball. This is not due to a cooler temperature, but rather to the staggering pressure at the very center of the Earth, which is more than 3 million times greater than at the surface. This intense pressure is so high that it raises the melting point of iron and nickel, forcing them into a solid state despite the sun-surface temperatures. The liquid outer core surrounding it is what generates Earth's protective magnetic field.