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The ground can shake.
How do we know what is under our feet?
Tap the picture. What happens if a wave goes through?Diagram · Model · Not to scaleSometimes we learn about an invisible place by watching a signal that travels through it.
We cannot go to the center. We can still ask what the shaking tells us.
We cannot go to the center. We can watch what passes through.
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There is more in this room, if you want it.