Have you read about the theory of "Agharta" where it claims that there is another world inside this planet?
If not, well I'll just wrap it up for you but I cannot give you the exact details because that theory is very broad.
Let's start with the opinions of the people. I've done some research and people claims that NASA is hiding something from us. Why are they doing that? It is because they do not want the people to panic. (Based on what I have read)
What supports that Agharta is true?
I will ony give one claim why they think Agharta is real.
Antarctica is said to be fully covered with snow but when pioneer Richard E. Byrd of the US explored the pole, he found out something which greatly opposes what we tend to believe. A big hole was seen in the Antarctica.
They theorized that there are both holes in the poles and the image above is the map of Agharta.
What does Agarta looks like?
It is the most civilized world. Their technologies are beyond our knowledge and the creatures living are giants.
But as for now, this remains as a theory or just a supposition because there is no concrete evidence of its existence. If you know about the hollow earth, let me know in the comment box.
I have some Biblical evidence showing that there is a world underneath the earth. The Bible referred to it as Hades/Sheol. The greeks call it as the underworld.