Great thread. I believe that there is life going on inside planet earth, and has been for many thousands of years. As Bertrand Russell once said, we will believe what we want to believe, regardless of the evidence, but in this case there is no reason not to believe, and there is sound "evidence" to support this, as indicated here.
As a separate point, there are many channelled indications from other planetary systems that there is life below earth : there is a lot of references to it in some detail in Patricia Cori's "Atlantis Rising", which is channelled communications from the Sirian High Council (see
www.sirianrevelations.net). We can't "prove" anything from channelled writings, so it is up to the individual to "believe" or not. The book is a great read anyway.