Dig Dug: Digging Strike Codes -

Bonus levels:
Get all dinosaur fossils in levels 1 through 12 to unlock the first two bonus islands (levels 13 and 14). In those stages are the fossils that will get you to level 15.

Hard difficulty:
Successfully complete levels 1 through 12.

Sound test option:
Drown the monster in level 12 to unlock the sound test option.