The core (C), beneath the mantle (B), is the deepest and hottest layer of the Earth. It is made almost entirely of metal. The core is made of two layers: the outer core, which borders the mantle, and the inner core. The inner core is shaped like a ball. Iron and nickel in the outer core form an alloy, or a mixture of metallic elements. The outer core is approximately 2,300 kilometers thick and very hot, between 4,000 and 5,000 degrees Celsius. The inner core is made mostly of iron. It is about 1,200 kilometers thick.