The thickest layer of the Earth is the mantle, which is a whopping 2,900 km (1,802 miles) thick. The mantle is composed primarily of heavy metals, such as iron, nickel, and magnesium, and is dense, hot, and semi-solid. It makes up about 82% of the Earths volume. The Earths structure can be defined in several ways, but generally, it is seen as having a solid crust on the outside, an inner and an outer core, and the mantle in between. The crust is the thinnest layer, with an average thickness of about 40 km. The outer core is the thickest layer if the Earths core is considered as one big thing, but the idea of a separate outer and inner core is generally accepted.