Earth's inner core is as hot as the Sun's surface — yet solid

More than 5,100 km beneath your feet sits Earth’s inner core, a ball of mostly iron and nickel. Its temperature reaches roughly 5,000 to 6,000 °C — comparable to the surface of the Sun. So why isn’t it a molten blob? The answer is crushing pressure: the core bears the weight of everything above it, over three million times the air pressure at sea level, which raises iron’s melting point enough to keep it solid even at solar-surface heat.

Nobody has seen the core, so how do we know it’s there? Earthquakes light it up. Seismic waves travel through the planet in two flavors — fast P-waves, which pass through anything, and S-waves, which cannot travel through liquid. In 1936 the Danish seismologist Inge Lehmann noticed P-waves arriving where they shouldn’t and deduced a distinct solid inner core nested inside the liquid outer core, the two bending and blocking waves in tell-tale ways.

That liquid outer core is what makes the planet livable. Heat escaping the inner core drives convection in the molten iron around it, and that churning generates the geodynamo — Earth’s magnetic field, the invisible shield that deflects the solar wind and keeps the atmosphere from being stripped away.

The inner core is also slowly growing, freezing outward as the planet cools and releasing the heat that powers the dynamo. Recent seismic studies suggest its rotation relative to the surface isn’t steady but may oscillate, occasionally slowing or reversing — a reminder that the deepest part of Earth is still, faintly, in motion.