What's inside the Earth?

Earth is layered like an onion, sorted mostly by density. The outermost skin is the crust: thin oceanic crust runs about 5 to 10 kilometers deep, while continental crust reaches 30 to 50 kilometers. Beneath it lies the mantle, a nearly 2,900-kilometer-thick shell of hot, solid rock rich in silicon, magnesium, and iron. Deeper still is the outer core, a churning ocean of liquid iron and nickel beginning around 2,900 kilometers down. At the very center sits the inner core, a solid ball of iron and nickel roughly 1,220 kilometers in radius. Temperatures climb from mild at the surface to perhaps 5,200 degrees Celsius in the inner core, rivaling the Sun's surface. Earth's total radius is about 6,371 kilometers, yet everything below the crust remains forever out of reach.

No one has drilled anywhere near these depths; the deepest borehole, Russia's Kola Superdeep, reached only about 12 kilometers, barely nicking the crust. Instead, geologists read Earth's interior using seismic waves from earthquakes. Two wave types matter: P-waves pass through both solids and liquids, while S-waves cannot travel through liquid. When S-waves failed to appear on the far side of the planet, scientists concluded the outer core must be molten. In 1936, Danish seismologist Inge Lehmann noticed P-waves bending in a way that revealed a solid inner core within that liquid. The layers formed early, as a young molten Earth differentiated: dense iron sank toward the center while lighter rock floated upward, sorting the planet into the nested shells we detect today.

These hidden layers shape daily life. Swirling liquid iron in the outer core acts like a giant dynamo, generating Earth's magnetic field, which deflects harmful solar radiation and guides every compass needle. Heat escaping from the core and mantle drives slow convection currents that push tectonic plates, building mountains, opening oceans, and triggering earthquakes and volcanoes along plate boundaries. That same internal heat powers geothermal plants in places like Iceland. Volcanoes tap magma from the upper mantle and crust, delivering rare samples of the deep interior to the surface. Even Earth's rotation connects to the core, whose spin scientists track through decades of seismic recordings. Understanding these layers helps researchers forecast hazards, interpret the planet's history, and locate the mineral resources concentrated by these deep processes.

Myth: the mantle is a sea of molten lava. Reality: it is almost entirely solid rock that flows only very slowly, like stiff putty, over millions of years. Myth: lava erupts straight from the core. Reality: magma forms in the upper mantle and crust, far above the core, which stays sealed deep inside. Myth: the inner core must be liquid because it is even hotter than the outer core. Reality: crushing pressure raises iron's melting point so high that the inner core stays solid despite the intense heat. Myth: we have drilled into or directly sampled the mantle. Reality: every hole ever dug stops within the crust, and nearly everything we know about the deeper layers comes from indirect clues like seismic waves and laboratory experiments.