I have read the book abut the Hollow Earth, but not that it is inhabited by aliens, or even by anyone at all.
I remember my mom telling me that when Admiral Byrd was on his expedition, that they played the information that he relayed back, on the radio every day. As he got closer to the pole, he started describing warm water and greenery in the water, and even warmer temperatures. She said that when he started reporting that information, the broadcast was cut off, and they did not play it again after that.
Now, I have to admit, that after being taught all my school years, that the earth was solid, except for a molten core, it is hard for me to imagine how it could be hollow inside. Even harder to imagine how there could be light in there, and people, or even aliens, living there. There are also people that claim that the moon is hollow, and that it is actually an artificial satellite, and that there are inhabitants inside the moon as well.
So far, I have nothing even close to definite proof of either of these theories, but I do think that it is pretty interesting.
It is too late tonite , but I am going to watch the video tomorrow .