My answer is body waves. Unlike surface waves which travels only on the Earth's surface, body waves can get through earth's deepest layers or Earth's interior. There are two kinds/types of body waves: P-waves and S-waves. P waves are first to be recorded by seismograph and can travel on solid liquid gas. S-waves can only travel solid. These waves are very useful in determining Earth's interior thus speculating that some parts of the core is liquid (outer) and others.