How do Scientists Know What’s Inside the Earth?
Like X-Ray Vision Through the Earth

In seismic tomography, scientists use earthquakes to see what is going on under the surface of the Earth, just like CAT scans let doctors see what is going on inside your body.
Question 1:
Why don't scientists just use x-rays to look inside the earth?
Question 2:
What do earthquakes have in common with x-rays?
This movie shows an old style seismograph recording an earthquake. As P-waves and S-waves from the earthquake pass by, they are recorded on a seismogram using a pen.
A modern EarthScope seismometer is digital and the size of a gallon paint can. It is usually buried several feet below ground and can record earthquakes from around the world. The seismogram is recorded as a digital computer file.