How foolish is it to say that the Aliens live underwater and in tunnels of mountains since 1000s of years?

Well, ..., it's a gross speculation that has no truth to it whatsoever. When the History and Discovery channels produce shows like this it's purely for the money. They know that the sensational aspects of those TV programs generate revenue. However, the real harm comes from people who can't divorce fantasy from reality and end up believing the bogus programming.

I think its possible that beings could have evolved on this planet deep underground that could be more advanced than us and keep well out of our way . Maybe these beings are actually our aliens .

Beyond foolish and well into wishful thinking.

The people who believe is this hooey are nut cases.

I'd say it's pretty foolish...