When an earthquake occurs, seismic shock waves travel out through the earth from the source of the event. The shock waves travel through the earth (body waves), or along the Earth's surface (surface waves), and can be recorded at remote monitoring stations. There are two types of body waves, P-waves and S-waves. S-waves cannot travel through the Earth's liquid core, which means that there is a limit on how far primary S-waves can travel. You can use this 'seismic shadow' to estimate the diameter of the Earth's core. This project shows you how.