Answer:
The answer is in seismology
Explanation:
When there is an earthquake, it sends seismic waves throughout the planet. Seismologists record these vibrations.
It is as if we hit the planet with a giant hammer and listen to the sound on the other side.
In the 60s there was an earthquake in Chile that generated a lot of information. All seismic stations on Earth recorded the arrival of tremors caused by this earthquake.
Depending on the route that these vibrations take, they pass through different areas of the Earth and this affects how they are heard on the other side.
In the early days of seismology, researchers realized that some vibrations were lost. Secondary waves were expected to manifest on the other side of the Earth but did not appear.
The reason is simple: secondary waves can only reverberate through solid (non-liquid) material.
They would have run into something melted in the center of the Earth. When mapping the path of these waves, they realized that the rocks became liquid at 3,000 km deep.
In the 1930s, Danish seismologist Inge Lehmann noticed another type of waves: the so-called primary waves. These, surprisingly, travel through the center of the Earth and can be detected on the other side.
Lehmann found the explanation: the nucleus is divided into two layers. One is the inner layer that starts at 5,000 km deep and is solid. Only the outer layer is liquid.
But it is not only earthquakes that provide useful information on this subject: so do nuclear weapons since the detonation of a nuclear bomb also creates waves on Earth.
All this has allowed us to get an idea of how the structure of the Earth is: it has a liquid outer core, which begins about halfway to the center of the planet (about 2,400 km) and within it, a solid inner core with a diameter of 1,220 km.
