That's interesting that you bring up the Earth and moon both being hollow. I've read that the moon was actually manufactured and is hollow while others say that it is inhabited on the dark side. The thing about the earth being hollow that I'd have my doubts about though is the hot core. The closer you get to the center the hotter it gets making it tough to survive down there. How would you suppose they get around that?
Whatever lives there has technology that might be hard for us to imagine today!
There is a really good book by Ingo Swann one of the creators of remote viewing called "penetration" where he gives a pretty detailed account of working for the government on investigating the aliens on the dark side of the moon. Very interesting stuff if you're into it.