The Earth isn’t just a big rock, it is actually made up of layers. The four main layers of the earth are: the crust, mantle, outer core, and inner core. Three of these are solid and one liquid (magma). The deepest layer is a solid iron ball, about 1,500 miles in diameter and is nearly as hot as the surface of the sun.  Although this inner core is white hot, the pressure is so high the iron cannot melt.