The inner core of the Earth is believed to be primarily composed of an iron-nickel alloy with some other elements. The inner core is the deepest and most central part of the Earth and is in a solid state. According to NASA, the inner core of nickel and iron has a radius of about 760 miles or 1,220 km. The intense pressure from the rest of the planet makes the nickel and iron solid, even though the inner core is much hotter than the outer core. The outer core, which overlays the inner core, is also composed of nickel and iron, but in liquid or molten form.