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Our transcription: By studying the travel times of both reflected and refracted seismic waves following great earthquakes, geophysicists can gradually piece together the general structure throughout Earth's interior. There's hundreds of thousands of earthquakes every year, and there's thousands of seismic stations scattered around the Earth, so we constantly listen for these earthquakes and constantly process them through our seismic stations and through our computers, and slowly we're building up an image of what actually is down inside the Earth, building up three-dimensional images. And I say "building up" because it's very much like building up an image on a television screen. As the television scanner sweeps across the image, you slowly see what it is you're looking at.
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