The new Doom contains 13 secret levels and hidden areas themed after classic Doom retro stages. Finding each of these unlocks the full level for play in the Doom main menu. These levels are each activated by finding and pulling a hidden lever, then navigating to the secret level entrance.
The first Doom hidden level is located midway through the martian landscape just past this distinctive cylinder structure
A small hall beneath leads to the switch.
The entrance is located nearby up the path.
The second hidden area is located in the first large open arena near an armor powerup.
The lever is on an elevated platform above.
The entrance is in the main foyer below.
The third secret level lever is hidden in the center of the main foundry chamber at the top level.
The entrance is located on the landing nearby.
The fourth secret level lever is hidden to the left of this installation just beyond the first Rune.
The lever is secreted in the dark to the left of the glowing vent.
The entrance is significantly further ahead, through a door and down the red-hued hall.
EditArgent Energy Tower
Halfway up Agrent Energy Tower, there are two floating transports. One glows green, the other blue. Ride the blue-hued transport to the top of the stage.