You know there used to be all kinds of speculation about what was inside the Earth, but now we know the Earth is not hollow, Godzilla and King Kong be damned.
How do we know? Earthquakes and seismometers. Whenever there is an Earthquake the vibrations travel through the Earth and can be detected by seismometers on the other side of the planet. Because sound travels at different speeds through air, liquid and solids, we have a pretty good idea what the inside of Earth looks like.
There’s nobody living inside the planet. We have found microbes fairly deep in the ground, but eventually it just gets too hot for anything to live.