Unlimited ammunition:
Hold [Option] and select "New Game". Release the key after the weapon
level increases to 999999 in the tool box.

Nuclear difficulty level:
Hold [Option] and select "Difficulty Level".

Nuclear missiles:
Locate a plane and shoot it down. Completely destroy the plane just
before it reaches the ground. A prompt offering nuclear missiles will
appear. To fire a nuclear missile, hold [Option] while firing a
missile.

UFO:
Enable the "Nuclear missiles" code, then fire all missiles as nuclear.
Obtain another set of missiles, and fire them as nuclear. A UFO will
appear while the second set of missiles are being fired.

Super shield:
Select the machine gun and shoot the parachutist twice. Then, shoot
him once more just before he reaches the ground.

Earthquake:
Select the laser and fire the weapon a few times in the lower middle
section of the field. When the correct location is hit, an earthquake
will that destroy everything on the ground