There is an unlockable bonus mode within the Extra Missions in The 2nd Runner called ZORADIUS (a spin-off of Konami's Gradius side-scrolling space shooter series). ZORADIUS is essentially a playable Gradius level, except in a full 3D environment!
As per tradition with the Gradius series, you pilot the fighter Vic Viper (Vic Viper's flight mode in ZOE2) and are able to add to your ship's capabilities by destroying enemies and picking up power-ups for your craft. Fans of Gradius will instantly recognize the classic Gradius sound effects and BGM from the early Gradius games.
ZORADIUS contains many of the original Gradius power-ups such as SPEED-UP, MISSILE, LASER, etc., and features at least one boss battle.
ZORADIUS is technically an Extra Mission, and players are taken to the ranking screen after the game is over.