Ice Nine

Unlock doors:
When reaching a door or gate that requires a switch to open, hold L + R + Select until the screen flashes the phrase "Trust no one" is spoken.

Cheat mode:
Complete the first fifteen missions under the "Graduate" difficulty setting. A cheat menu with unlimited health, unlimited ammunition, mission select option will now be available.

Veteran difficulty:
Complete the first fifteen missions under the "Graduate" difficulty setting.

Bonus missions:
Complete the first fifteen missions under a difficulty setting greater than "Initiate" to unlock three bonus missions.