Pause the game, and press 'M'.  You should hear a laser gun
 sound. Type 'MOTHERF**KENKIWIB**TARDS' (fill in the stars...)
 Unlimited lives are yours.
 
 Or, on level 2.4, stand at the entrace to the room with the
 giant octopus is located and fire to the right a few times.
 A warp will appear that will take you the beginning of level 3.1.