The earth is made up of five layers: The crust, upper mantle, mantle, outer core, and inner core. These layers are determined by several different methods, such as using seismic waves that are sent into the earth and bounce back in a certain way according to the density or composition of the layer they bounce off of, people can measure the gravity in the earth, and experiment simulating pressure and heat in the Earth.
The crust: This is the thinnest layer, and yet the outermost layer, upon which we stand. This layer is composed mostly of silicates, typically quartz.. It has many mountains and low places, which is a good thing because if we did not have those and the Earth was totally flat, it would covered by water.
The Mantle: Still composed mostly of silicates, this layer is very rich in Iron and magnesium. The mantle is divided into two parts: the upper mantle and the mantle. The upper mantle is molten lava and is under very low pressure (in comparison to the other layers, of course: assuming you could survive the heat, you would still be crushed from all sides by the pressure) so the upper layer moves around a lot more and more fluid than the mantle, which is very thick from all the pressure. The undivided upper and lower mantle together are the thickest layer in the earth.
The Outer Core: As you go farther into the earth, the temperature increases, and so this layer is the most definite to kill you, though all the layers but the crust would melt you, or crush you, or both. Also, the denser substances sink, so the farther you go in, more of the denser elements are there also, so the outer core is comprised mostly of Iron and some Nickel, along with a few trace elements.
The Inner Core. Also known as the center of the Earth, this is no longer a layer but a ball now. The pressure is so enormous that it is not even liquid. This is comprised, like the outer core, of nickel and iron. Compared to the center of the Earth, Atlas’ job is like holding up a feather!
Even though the crust is only a fourth of the mantle, we have not been able to drill beyond it, thank goodness. It is just amazing the enormity of God’s creation.