I read a blog post from a guy who worked at the pizzeria while in college. One tenant of the "theory" is that the members meet and run the operation from the basement.
The previous employee says... "there isn't a basement in this building - it's on a cement slab." The owner has even confirm this separately in a BBC article.
Of course this employee wasn't privy to the secret bat tunnel entrance in the back parking lot ! :-P
Seriously - google search shows that there's a "Boston Bomber" like search of misinformation going on where somebody has discovered evidence of a "space beneath" the building because....well....there's a picture of a basement in the building next door. OMG. ha ha ha -- somebody else followed up and pointed out that the purported pictures show Round European electrical outlets so the pictures can't possibly be from the USA. Wrong building !! whoa duh.
How uninformed can people be ? -- but doesn't stop them from Post First and Asking Questions Later. (pFAQl ?) Post FAQ. Post Truth. Interesting.