In this room behind the barrels is a secret door with some shotgun ammo.

DOOM - Levels 2, 3 and 4: Secrets and Collectibles - YouTube A retro chime will sound. I have played the level through following a secret guide and the only one i am missing is the 3rd question mark secret witch i thought was when u opened the hatch near the consol that gives you the yellow door access. This book, however, is not about science fiction it is about real life. The notion of a parallel universe has intrigued the human mind for millennia.