The Earth's mantle is 2,900 kilometers (1,802 miles) thick. The mantle lies between the core and the outer crust layer and consists of four layers: lithosphere, asthenosphere, upper mantle and lower mantle.
The Earth's mantle can be discerned from its core and crust layers by measuring the spread of seismic waves travelling through it after seismic events such as earthquakes. Monitoring devices on different parts of the planet allow scientists to detect the arrival of the waves. Understanding how seismic waves travel through different materials allows this data to be used to study the mantle. These waves are either p-waves that represent vertical motion or s-waves for horizontal motion.