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)