Which serves as a cover for our operations.
The secret underground part is of course only open to members... Philly thinks he's the only one in a basement, when in fact he's just situated in the lowest floors of a basement complex.
Of course, headquarters has several other entrances as well. Rufus, for example, has been known to enter after swimming through the sewer system. Apart from Philly, we all wish he wouldn't do that...