Interpreting the waves recorded on seismograms in order to identify Earth structure is challenging. S waves do not travel along straight paths; instead, seismic waves are reflected, refracted, and diffracted as they pass through our planet. They reflect off boundaries between different layers, they refract (or bend) when passing from one layer to another layer, and they diffract around any obstacles they encounter (Figure 8.29). These different wave behaviors have been used to identify the boundaries that exist within Earth.