Crust (6-30 miles)
Mantle (1800 miles)
Outer core (1400 miles)
Inner core (750 miles)

The core is the innermost zone of the earth. It is mainly iron with some nickel and cobalt. The inner core probably is solid, but the outer core may consist of the same elements in a molten form. The core is the most dense (heaviest) of the three zones.