There is a potential difference of 300,000 volts between Earth's surface and the ionosphere. Furthermore, we are surrounded by super strong electromagnetic waves. This radio wave has the most powerful output on Earth. Schumann resonance.

"The Schumann resonances (SR) are a set of spectrum peaks in the extremely low frequency (ELF) portion of the Earth's electromagnetic field spectrum. Schumann resonances are global electromagnetic resonances, generated and excited by lightning discharges in the cavity formed by the Earth's surface and the ionosphere. " wikipedia

Schumann resonance has multiple spectra, but the lowest frequency is 7.83 Hz.

The relationship between the frequency and the wavelength of the electromagnetic wave is obtained by the following equation.

f = c / λ          c is the speed of light, λ is the wavelength

I tried it with this formula. The radius of the earth is 6371 km, the light speed of c is 299792458 m / s, and the circumference ratio is 3.141592.

Then, when the length of the ground surface is one wavelength, the frequency becomes 7.49 Hz. In the description of wikipedia, since it resonates between the ionosphere and the ground surface, it should be lower frequency.
Therefore, when calculating the radius at 7.83 Hz, it becomes 6096 km. This is the part of the acantosphere above the mantle under 275 km from surface. It is liquid at the part where the olivine changes phase transition to basalt,  granite ,gabbro.

I wrote articles "Olivine inside Earth released a large amount of electrons". There is a lot of electrons there. It is thought that the electromagnetic waves are generated by the rotation of the electrons due to the rotation of Earth. This is the identity of Schumann resonance.
Schumann resonance seems to be closely related to the occurrence of gravity on Earth surface. Also, a large amount of electrons generate a magnetic field.