The core is made up of 2 layers: inner core and outer core. It is the 3rd layer of Earth's make-up. It is 33% (1/3) of Earth's mass and is believed to be mostly made of iron and small amounts of nickel; almost no oxygen, silicon, aluminum, or magnesium is found in the core compared to the crust and mantle. Researchers say that the heavier substances from Earth sunk into the middle (core) while lighter substances, air and water, stayed on the surface.