Move south after completing the objectives in mission 1, phase 2. Enter the alternate exit in the dead end canyon to start a bonus underwater mission.
Enter XBABYX as a password. The word "Correct" will confirm correct password entry. You will lose energy, but no lives, during game play.
Enter ZZZCYX as a password.
2-2 - CBYXYC - 3-1 - XABXAB - 3-2 - AZCBXC - 4-1 - YYBBCY - 4-2 - BAXCXX
Enter YAYAYA as a password.