Earth's interior consists of four main layers - the inner core, outer core, mantle, and crust. The inner core is solid iron, the hottest part of Earth at 5,000 C. The outer core is liquid metal. The mantle is the largest layer, making up 67% of Earth and slowly flows like putty. It has an upper lithosphere and lower asthenosphere. The crust is the thinnest outer layer with ocean crust thinner than continental crust.