Cryptoterrestrials Theory

I think most of us here have bandied this theory around a time or two and I have to say it may be more probable than life traveling from other solar systems. Of course both may be true, the existance of one doesn't exclude the other. Brentin Sawin (Mysteries to search) claimed he experienced beings that lived underground, and they used advanced tech from the time they lived above ground. There was another writer I think by the last name of Sanderson that wrote a book about being kidnaped and held underground by these beings. I sure wish we could get some answers to something.