Levels 2, 3, 4, 5, and 7 contain a bonus stage. These require that some sort of condition be met and are entered by flying into a hidden area.

Level 2
Condition: None. Fly into the first ship opening along the bottom of the screen. It is located in the last section of the stage (after the giant carbonated bubbles) before meeting the Boss

Level 3
Condition: Destroy all of the guns on the ground. Fly close to the low rock ceiling toward the end of the level and immediately before the last "mountain".

Level 4
Condition: No power-up boxes at the bottom of the screen may be lit. Fly into the back of the Moai head located in the middle of upright three heads toward the end of the level. This particular head will be facing away from you.

Level 5
Condition: Unknown. Fly through the fire at the corner of the inverted volcano. It is located just before the small passage that leads to the two-headed fire dragon.

Level 7
Condition: Unknown. Fly through low corner just after the four turrets. Your ship would normally have to fly upwards diagonally at this point. Note: Collecting a lot of power-up modules by destroying turrets allows entry into the secret level area. The weapons select bar was on an empty box each time - missiles then lasers.