Inner core, internal Earth structure. Artwork showing the appearance of the Earth's inner core. This is thought to be a solid iron-nickel alloy surrounded by an outer core of liquid iron and nickel. The internal structure of the Earth is divided into the outer crust, the upper and lower mantle, and the outer and inner core. For examples of the material composition of each layer of the Earth, see images C018/0208 to C018/0212.