• Seismographs are located at various locations around the Earth...
• If an earthquake occurs at one location (e.g., at the south pole, as shown in this figure), P and S waves travel outward through the Eath's interior..
• Only P waves are detected on the opposite side of the Earth after an earthquake...

Since P waves travel through liquids, but S waves do not, the absence of S waves on the opposite side of the Earth means a liquid region exists in the Earth's interior!

Click here to study a virtual earthquake