Hidden Level
In mission 1, phase 2, go south after you complete your objectives. Here you'll find a dead-end canyon with an alternate exit. Take it to enter a bonus underwater mission.
Invulnerability
Enter the password "XBABYX", then start a game. Your energy will decrease, but you won't die.
Level Select
To start on any level, enter the password "ZZZCYX".
Rapid Fire
For rapid fire, enter the password "YAYAYA".