Fortunately, many earthquakes are large enough that their seismic waves travel all the way through Earth and can be recorded on the other side (Figure 8.29). This means that the seismic waves act like medical x-rays used to take images of a person's insides. There are about 100-200 earthquakes each year that are large enough (about Mw 6) to be well recorded by seismographs all around the globe. These large earthquakes provide the means to "see" into our planet and have been the source of most of the data that allowed us to figure out the nature of Earth's interior.