Science fact of the day: Earth is comprised of four layers

Earth is composed of 4 distinct layers: an inner core made of iron and nickel; an outer core made of iron and nickel fluids; the mantle, which is the thickest layer and comprised of a hot, viscous mixture of molten rock; and the crust, which roughly 19 miles deep on land on average and around 3 miles deep on average at the bottom of the ocean.