Critical Mass is the eighth level in The Birth.
This level appears to be centered around a power station. There are variety of areas in this level such as a Storage room with lots of crates, a trash compactor, control rooms, a large generator room and more. Towards the end of the level Duke has to use the Emergency Overide configuration to get the yellow access card which is hidden under a pillar as well as opening the door blocking the path to the exit of the building. Once the Emergency Overide code has been put in the entire power station will become unstable and start shaking. When Duke eventually makes his way out; the entire building will immediately collapse behind him. Once outside; Duke can see a large ship in the background which foreshadows where the next level will take place.
There are 4 secrets in this level.
Secret #1: Homer's control room
This secret can only be visited before one of the room blows up and collapses. After going up a spiral staircase and reaching door, explosions will begin. Open the door, but do not run towards the safe, white place behind the explosions. Instead, wait while the ceiling is collapsing at the door. When it will be very close from squishing you, you should notice that one of the walls opposite to the door where you are is opening. Quickly run into the room it unveils and be careful not to get squished.
You will find yourself inside the room mentioned in the Easter Eggs section on this page. There is an atomic health hidden behind the machinery. Also there is a teleport that will get you out of the room and to the next part of the level, behind the collapsed room.
Secret #2: Bonus first aid
After you pass the room with water, some octabrains and two crushers where you need to avoid getting squished, you will be inside a small room with two first aid cupboards on the wall. One of them includes an atomic health and the other a large medkit.
There is also a monitor. Press use/open at the front of it - a small hidden room will open on your right. There will be portable medkit.
Secret #3: Messy RPG
Inside the room after blue keycard door, simply shoot the oozcans (the metal bottles with green substance that explode upon being shot). Part of the wall will blow up, revealing a small place with RPG.
Secret #4: Fly for a shrinker
This secret place is located in the same room as the previous one. You will however need a jetpack to reach it - unfortunately it cannot be found in this level, so you have to keep one from the previous levels. Just fly up to the ceiling then towards the wall opposite to the door - there will be a small cave with shrinker and rockets for devastator.
- One of the secret places in this level has a large control panel with donuts on it and just behind it is a sign that says "Sector 7G". This is a reference to Homer Simpson and the place he works in.
- The name of this level is also shared by Duke Nukem: Critical Mass which is the first game in the recent Duke Nukem Trilogy . Some fans on youtube have even gotten this level and Duke Nukem: Critical Mass mixed up.
- Due to the nature of some events in this level; in Co-0p mode (Xbox 360 only) some alternative paths previously only available in Dukematch were included for the Xbox 360 version . The reason was because In Co-Op when a player is killed they are returned to the start of the level and if the ceiling has already collapsed there was no way for the player to get through. In Dukematch in the storage room there is a switch(This switch does not appear in Single-Player) on that wall that will reveal a teleporter to allow the player to get past the collapsed ceiling.
- At the end of the hallway with the Red Access Card door is an additional exit that can be used in Co-op mode for the Xbox 360 version. Like the first alternative path for Co-op this is due to the unique nature of this level.