He focused on two particular waveforms produced by the tremors: primary (or P) and shear (or S) waves. These can be used to detect any areas of liquid (indicating a possible magma chamber) under the Earth"s surface because liquids block S waves and slow down P waves. The method is the same one scientists used more than 100 years ago to show that the outer core of the Earth, about 2,900km (1,800 miles) down, is liquid.