Agartha

This is one part of the hollow earth theory I've never really bought into as much.  It's a lot to take in.  My initial perception of it doesn't have any baring on whether it's true or not, so lets explore it and see what we think.  As widespread as the belief in Agartha is, no discussion of the hollow earth theory would be complete without it.

Agartha is a legendary city that exists within the hollow earth.  Alternative accounts put it in a cave near the earth's core.  Regardless of it's exact location, very few, if any, surface-born humans are reported to have ever visited.  It's a place of perfection and wisdom.  Some sites online offer conference calls to the inhabitants of Agartha.  I don't know much about these sites, and I don't want to encourage their practices by linking to them.  They sell their "conference calls" for obscene prices in a very thinly veiled attempt to cheat people out of their money.  It's an immoral hoax, and also discredits the hollow earth theory as a plausible scientific model.

Agartha was referenced by some officials within the Catholic church some centuries ago.  It's a widely-referenced factoid, and possibly the origination of the hollow earth theory.  It's also popped up several other places in our world's history.  Another consistent theme throughout the history of the hollow earth is the Nazi party of Germany.  They have been widely reported as believers of the hollow earth, and many believe they tried to escape to the hollow earth at the end of the war.  More about how this particular theory relates to Agartha can be read in a  forum post here.