There are four main layers of the Earth. The outer layer is the crust. The crust is about 25 miles thick under land, and 3-5 miles thick under water. Below the crust is the mantle. The mantle is about 1,800 miles thick, and 1,600-4,000 degrees! Below the mantle is the outer core. The outer core is 1,800 miles below the surface of the Earth, and is 1,400 miles thick. The inside of the Earth is made up of the inner core. This is a solid layer 800 miles thick. The inner core is 4,000 miles down.