Enter the following button codes during the "Difficulty Select" screen to get the desired results.

R1, Circle, R1, Square, Left, Left, Right, Left - Alaska Level Complete
Left, Right, Left, Left, Right, R1, R2, Circle - Deep Amazon Level Complete
R2, R1, L1, L2, Square, Circle, Square, Circle - Earth Level Complete
Circle, Square, Circle, Square, Right, Left, Left, Right - Mid Atlantic Level Complete
Left, Left, Right, Circle Circle, Right, Right, Left - Starship Level Complete