Doom has a neat secret for classic Doom fans. Hidden in each level is a lever that makes a chime noise when pulled, and opens a secret door. Behind these doors, which you will also have to find, are levels designed to replicate popular areas from Doom 2 (1994). While they only have enemies from the new Doom, the textures and layouts of these levels are exactly as they were over 20 years ago.
It's worth noting that finding and completing a secret classic Doom 2 level will allow it to be accessed from the level select menu, allowing you to easily replay it whenever you want.
There is one classic Doom 2 area per level, so there are a handful of them. Below, we will share the locations for each, so you can see them for yourself.
Mission 1: UAC
Open the first door that requires the blue key. Go straight and jump down the small drop. Turn slightly left and go up the short path to the right that faces the direction of the railway. After entering the cave in front of you, you will find a map station in the far corner.
Exit out the cave by following the path to your left. After going right outside the cave, you will encounter a massive drop. Fall down to land next to the secret level. The lever is hidden behind a handrail directly to the left of you. The door will be to the right of where you landed.
Mission 2: Resource Operations
Later in the level you will find yourself in a room with VEGA speaking to you. After the door opens and you are given the quest 'Discover the Cause', follow the path and turn left. Enter the door here by prying it open with your hands. Go straight up the stairs and power the terminal from the panel.
Go back down the stairs and down into the space below you by pressing the button. After dropping down, to the the forward-left corner and climb up the boxes and then two walls to where the green armor is located. Follow the platform a few feet over and jump to the next platform to your left. This will require that you stand on the handrail. Climb up to meet an enemy and find the lever directly in front of you. The secret door will be found a few levels below you, so just drop down a few times.
Mission 3: Foundry
When you arrive at the massive foundry room where molten lava flows, head to the top level where the lava originates from. The lever is located to the side of the square platform hanging high over the lava pool facing the waterfall of lava.
To find the secret door, hop back on the bridge and go straight left. The door will be at the far end where lava is falling.
Mission 4: Argent Facility
Later in the level you will find yourself in a room with green lights posted on the sides of your path and a platform nearby. This platform has some barrels hiding the lever that you need to pull.
After pulling the lever, go right and then left through the door. Turn left and you will see a demonic symbol on the ground with body parts hanging over it. Keep following this path and you will locate the secret door.
Mission 5: Argent Energy Tower
You will find a circular room with an entrance labeled 4-A that has a hovering structure in front of it. Hop on this structure to be taken up several levels. Once it reaches the top level it will move to the outer parameter of the wall, jump up onto the small platform and pull the lever in front of you.
The secret door will be located in the entrance where you originated from labeled 4-A. Be careful that you aren't smashed while entering it.
Mission 6: Kadingir Sanctum
You will eventually arrive at a spot with a tall demon and a cave entrance. Enter the cave and flip the lever inside. Exit the cave and drop down slightly right of the path ahead and you'll find the secret door behind you.
Mission 7: Argent Facility
This one is easy to miss, so we'll explain from the start of the level. Go up and enter the door. Then, turn right and follow the pathway until you reach a ladder. Directly in front of the bottom of the ladder you can climb up several levels until you are on a platform sticking out into the open. Follow this platform to near the end until you see a gap where you can jump upward. Make the jump and you will find the lever on the other side of the boxes to your left.
To get to the secret door, drop down to the platform you just came from, and then drop down to the platform that is directly underneath the lever. The secret door will be directly in front of you.
Mission 8: Advanced Research Complex
You will eventually come to an area with a couple of tall rock pillars and some piping on the far rock wall formations. Find your way onto the top of the first pillar, and then jump to the second one. From there, you need to make a very long jump to where the three pipes and green wall are located. Inside here is a hidden doll.
Go out onto the thin platform in front of the three pipes and jump onto the rock wall on the right side from when you're facing the pipes from outside. This rock wall will not be obviously climbable, so take our word for it. Follow the narrow , orange rock ledge and jump over the white pipe ahead of you to find the lever. You will now need to jump across a few long jump platforms until you're in a large area with the secret door on the far side.
Mission 9: Lazarus Labs
This one is hidden next to a blood-drenched in the ground with dead marines inside, located in a room that you will have to enter near the large, circular room with grey walls and panels with red images. There is a Praetor Token inside, and the lever is right next to the entrance to the hole.
After pulling the lever, the secret door will open up inside the hole.
Mission 10: Titan's Realm
You will arive in a room with a wooden door with four skulls on it, and four health packs in front. A rock formation will fall down and act as an elevator upward. Go up and interact with the demonic skull up top to open a door below. The lever is in this door, which will be a few feet from you near the gate.
From there, enter the gate and go right, and then right again to find the secret door.
Mission 11: Necropolis
From the square room with enemies, a blood pool in the middle, and four pillars, enter the small room to the right of the gate. The lever is inside here. Go left and you will find the secret room to your right.
Mission 12: VEGA Central Processing
From where you obtain the Blue Keycard, enter the blue access door near the body and then turn left. You will find an enemy and several dead bodies through another blue door to your right. Loot one of the bodies, and then pull the lever on the other side of the box that he's propped up against.
Proceed back to the body where you obtained the Blue Keycard from. Turn left and then follow the path straight to enter the secret door.
Mission 13: Argent D'Nur
Locate a dead creature with four horns sticking up at the top of a platform. When you come near, the spirit of a Space Marine will appear. Turn around and go left, and down the stairs before turning right. Interact with the demonic skull on the wall to open a door. Go down the stairs and follow the right wall until you see an open passageway. Enter this and look for the lever to your right. It is thee same color as the surrounding environment, making it hard to spot.
The secret door will be directly next to you.