1: Go to the Upper Palace in the City unit.
2: Find the mine shaft that leads onto a room with gold rocks and yellow lights.
3: Near the green slime pool on the floor is a button, press it and then put on the environment suit.
4: Jump in the pool and follow the passage to a Secret Area.
5: This is John Carmack's secret room with a poster on the wall of his car and a Doom poster.
Hard to find, but this sequence is of several to access the secret room.
See also: Game cheats for all id Software Quake games