Not to confuse the hollow earth theory with advanced beings that live underground. These are two different issues; both in part have been made popular by channelers, science adventurers, writers and ancient lore.
That being said, I don't disagree with some of this. Contemporary science can only theorize about what exists deep down in the earth and when we consider contemporary science; we always need to keep that in mind. All things that have not been proven are still on the table.
The problem with channeling throughout the ages is this: Either the person who has made contact cannot understand what is being said to them and can only interpret to the level of their own personal knowledge or is being lied to. All of these are strong possibilities when someone makes contact. However, with anything that those who learn these things in whatever way they do it, there is probably a little truth to it and science should not be too quick in discounting it.
The U.S. Government knows quite a bit more about this than the general public. They have been contacted by these beings (they are humans) but, they do not work with them. The last thing these beings need is a group of humans thought primitive, to develop advanced technology. So, although some technology has been exchanged, all of it will inevitably lead us away from becoming an even greater threat to them than we already are. (Read article: "Is the President of the United States Aware of Alien/UFO Activity?")
The interior of the earth does have many large hollow places in it but the tunnels and bases that are inhabited by these advanced beings have been carved by them- for the most part.
As far as the U.S, government's involvement with these beings, we are able to contact them but this is not something that happens very often. The U.S. Government sees them to be as much of a threat as they see us. They operate on and under the earth with impunity and there is nothing we can do about it; thus- the secrecy associated with it. Granted, they can be dangerous at times but they need us! If they didn't need us, we would not be here!
The biggest problem I see with regard to all speculation regarding these beings is this: They are not extraterrestrials! They are earthlings-they come from the earth! Not from far away planets and they have evolved on the earth in the same way that we are evolving. They have lived and died and then, they discovered advanced technology and the few select ones that were present at that time, became immortal (in a way). They are still with us today doing what they do; mainly trying to survive and in the process, justifying the radical changes they had to make to themselves to become immortal.
It is true that we have developed technologies that we garnered from our association with these advanced beings. However, this technology will not lead us to the technologies which will allow us to develop advanced technology such as they have.
Thanks so much for your questions-.