The Earths core was discovered by Beno Gutenberg using data from seismographs. He found that P waves traveled through the entire body of the Earth but S waves did not. He found S waves were stopped at a boundary 2,900 km within the Earth. He knew that P waves were compression waves and they traveled through solids, liquids and gases. S waves are compression waves and they only travel through solid rocks. He published his research in 1913.