The outermost layer of the Earth is the crust. In the oceans, it is about 4 miles thick. On the thickest continents, it extends some 40 miles into the Earth.
Below the crust is the mantle, a layer of weaker, hotter rock some 1,800 miles thick.
Beneath the mantle lies the outer core, a sea of liquid iron about 1,300 miles thick.
Finally, at the center of the Earth is the solid iron core, a sphere some 1,500 miles across.