Scientists came to know that the Earth's outer core is liquid primarily through the study of seismic waves generated by earthquakes. There are two main types of seismic waves relevant here: P-waves (primary or compressional waves) and S-waves (secondary or shear waves). P-waves can travel through both solids and liquids, but S-waves can only travel through solids. When an earthquake occurs, seismometers around the world detect these waves as they travel through the Earth. Scientists observed that S-waves do not travel through the outer core, creating a shadow zone where no S-waves are detected. This indicates that the outer core cannot be solid because S-waves are unable to travel through liquid. The only plausible state for the outer core is liquid, as gases can be ruled out due to the high density required. Additionally, the Earth's magnetic field supports the idea of a liquid outer core. The magnetic field is believed to be generated by the movement of molten iron in the outer core, consistent with dynamo theory. Seismic data, along with experiments simulating conditions inside the Earth and the behavior of iron under high pressure and temperature, collectively support the conclusion that the outer core is liquid. In summary:

• S-waves do not pass through the outer core, indicating it is liquid since S-waves only travel through solids.
• P-waves do pass through, confirming it is not a gas.
• The existence and nature of Earth's magnetic field suggest a moving, molten outer core.
• Laboratory experiments with iron under high pressure simulate the outer core's state.

These evidences together helped scientists determine that the outer core is liquid.