This level occupies the map slot MAP31. For other maps which occupy this slot, see Category:MAP31.
MAP31: Cyberden is the first secret map of The Plutonia Experiment. It can be reached via MAP15, and uses the music track "Sign Of Evil" from Doom. The map is named for the four Cyberdemons it contains. They start in visible but inaccessible cages where they can't be hurt or attack the player; through out the map they will be released. The pre-level text for this level states: You've found the second-hardest level we got. Hope you have a saved game a level or two previous. If not, be prepared to die aplenty. For master Marines only.
Four switches located around the map must be pressed to open the cages releasing the Cyberdemons. Each cage has a switch inside it and when all four are toggled, the yellow key in the center of the map becomes accessible and allows the player to exit. There is only one functioning exit in this level, which leads to the second secret map, MAP32: Go 2 It. There is no normal exit that leads to MAP16: The Omen, so if you previously chose to play this secret map, it'll be mandatory to play the other secret map as well.
In the green marble wall at the south end of the level, climb the stairs and make your way left along the narrow corridor in the back of the room. Approaching the skull switch here opens a room back on the lower floor. There's half of a lion-face on one of the walls in the new room; open it to reveal a teleporter to a secret area.
Both of The Plutonia Experiment's unofficial sequel WADs, Plutonia 2 and Plutonia Revisited, adopt the theme of inevitable battles with Cyberdemons in their MAPs31: their secret levels are respectively titled Cybernation and Cyberdemon Vertigo. Unlike "Cyberden" and "Cybernation," however, "Cyberdemon Vertigo" offers a choice between a normal exit (leading to MAP16) and a secret exit (leading to MAP32).