Not secret. Just forgotten and unimportant and not a priority.
Why underground? As has has been pointed out elsewhere, during the Great War Adolf was an infantryman on the Western Front, who associated safety with being in a bunker underground. So the Nazis wasted much effort constructing vast underground complexes in which to do all sorts of things.
Let's assume there are elevated radiation levels at this site, which is 75 acres. Compared to many industrial sites, that's big; compared to the Hanford estate, or Windscale, or Chelyabinsk 65, or Oak Ridge, or
How about: processing pitchblende to recover interesting metals? Such as
 Uranium. Principal metal to be had from pitchblende. The oxide can be used to make fancy yellow glass. If you're one of Adolf's minions, it's a waste product.
 Polonium. Also a waste product.
 Radium. Which can be used to make luminous paint for instrument dials, for use in eg aeroplanes. It's not like the Nazis built any aeroplanes, is it?