Bonus Mission 1 - Get an overall 85% rating on the first campaign to unlock Bonus Mission 1.
Bonus Mission 2 - Get an overall 85% rating on the second campaign to unlock Bonus Mission 2.
Bonus Mission 3 - Get an overall 85% rating on the third campaign to unlock Bonus Mission 3.
Bonus Mission 4 - Get an overall 90% rating on the fourth campaign to unlock Bonus Mission 4.
Hidden introduction sequence - Remain idle at the main menu for approximately five minutes. A hidden sequence will play, revealing a lot about the game.