Which layer of Earth lies directly beneath the crust?

Beneath the thin crust lies the mantle, which accounts for the greatest share of Earth's volume and extends down roughly 2,900 kilometres to the core. It is rock rich in silicon, magnesium and iron, and although it is solid it is hot enough to creep very slowly over geological time, rather like extremely stiff putty. That slow churning is convection: hotter material rises, cools near the top, and sinks again, and the drag it exerts is the engine behind plate tectonics, driving continents apart, building mountains and feeding volcanoes. The boundary between crust and mantle is sharp enough that seismic waves change speed abruptly when they cross it, a discontinuity discovered by Andrija Mohorovicic and named after him. Below the mantle sits the outer core, which is genuinely molten iron and nickel, and its circulation generates Earth's magnetic field. The inner core, under immense pressure, is solid despite being even hotter.

No drilling project has ever reached the mantle, so most of what we know comes from earthquake waves and volcanic rock.