Earth's Inner Core

planet Earth · extreme heat · mysteries

Impossibly hot metal. Earth's inner core is hotter than the surface of the Sun, yet it stays solid because of crushing pressure from all the layers above it pushing down with incredible force.

A hidden sphere. This inner core is about 1,230 kilometers across, which is roughly the same size as Earth's Moon and makes up about 20% of our planet's total radius.

Iron and nickel mostly. The inner core is almost entirely made of iron and nickel, the same metals humans use to build cars and skyscrapers, but compressed into an ultra-dense ball at Earth's heart.

Recently surprising science. For decades, scientists thought the inner core was completely solid and still, but in 2025 they discovered it might be slightly squishy and slowly changing shape over time.

Pressure creates hardness. Even though it's hotter than the Sun's surface, the inner core stays solid because the weight of Earth's entire crust, mantle, and outer core pressing down creates pressure so extreme that nothing can melt.