Instructions:

1. Launch Quake.
2. Go to "Single Player" > "Level Select" > Select the EPISODE as "Scourge of Armagon", MAP as "HIP3M1: Tue Torment" and set the DIFFICULTY as Easy then Start.
3. As the mission starts, go right, press a button to call a rail-lift. Watch out for an enemy standing on it, just kill him.
4. Hop in, re-call it again to lift off BUT make sure to jump on the roof behind you then jump on rock, the message will appears as "You found a secret area!".
5. Follow the screenshot below that guides you with more details.

Good luck, Rangers.