They are discovered by finding a hidden lever in each level, that opens a secret door, and then entering the 16-bit texture mapped area hidden behind the opened door.
List of secret levels
Where they are found is in brackets.
- Entryway (The UAC)
- Hangar (Resource Operations)
- Nuclear Plant (Foundry)
- Toxin Refinery (Argent Facility)
- Halls of the Damned (Argent Energy Tower)
- Slough of Despair (Kadingir Sanctum)
- Underhalls (Argent Facility (Destroyed))
- Phobos Lab (Advanced Research Complex)
- Tower of Babel (Lazarus Labs)
- Phobos Anomaly (Titan's Realm)
- House of Pain (The Necropolis)
- Command Control (VEGA Central Processing)
- Pandemonium (Argent D'Nur)