The true final Boss:
When you defeat the final Boss in level 5-8, if you have not found the key in an earlier stage you will go all the way back to level 5-1. You will find the key by shooting at crates in levels 5-1 through 5-7. If you uncover the key, shoot it and your score will be displayed in gold instead of the usual white. Now when you defeat the Boss in level 5-8, you will be able to open the door to the true final boss, El Nino.
Unknown:
Enter 0621 as a password. Note: This password is revealed after the final Boss is defeated.
Please any information regarding this code.