According to Indiana University, geologists observe the Earth's interior by using seismic waves to determine the depths of layers of molten and semi-molten material within Earth. Seismic stations positioned around the world record the strength of different types of waves in order to establish the structure of Earth's interior.
The two main types of waves used to observe Earth's interior are P-waves and S-waves. P-waves are pressure waves that can go through liquid and solid material, whereas S-waves can only go through solids. P-waves also travel faster than S-waves. P-waves and S-waves are also used to measure the distance of earthquakes from an observation station.
The inner core of the Earth is mostly composed of dense material. There are three main shells that make up the planet Earth: the crust, mantle and the core. Earth's interior is composed of the mantle and core, both of which are about equal in thickness. According to the U.S. Geological Survey, the core forms only "15 percent of the Earth's volume, whereas the mantle occupies 84 percent." The crust makes up only 1 percent of the total composition of the Earth. Geologists are improving their understanding of Earth's interior by doing laboratory experiments on rocks at high pressure and analyzing earthquake records on computers.