start out as a SEAL: use laser weapons that don't rely on ammunition.
complet the game on hard difficulty
Complete the game on Normal Difficulty
Collect the question mark boxes from the ships when the counter reads 272, 267, 232, 222, 171, 161, 131, or 121. When you do this, you will be invincible for a short period of time.
Unlock Expert Difficulty:
Beat the game on Hard Difficulty.
Unlock Hard Difficulty:
Beat the game on Normal Difficulty.