Let's think about this for a minute, shall we?
IF I had an "underground bunker", why would I invest the time and money in creating it?
A. To provide defense against natural and/or man made hazards, and to avoid detection.
Now, having said that, WHY would I tell you if I did? That sorta negates the "escape detection" thingy. We do not share DNA, I am not hoping you will become the mother of my children, and you do not have skills (like being a surgeon) that I would want to come live in my bunker.
Sometimes they are called basements, storm shelters, root cellars, wine cellars, or fallout shelters.