Unlockable Stages
     There  are  12 hidden stages in the game. 8 of them appear on the 
world  map  automatically,  but needs to be unlocked by collecting all 
red  diamonds  in  a  given world. Remaining 4 appear after completing 
certain objectives.

10th stage in each world: Collect all red diamonds

Stage 9-1: Beat the game without seeing "Game Over" screen.

Stage 9-2: Beat stage 9-1.

Stage X-1: In stage 4-3 use a warp block located directly above 
           your starting position. Use bubble gum to get there.

Stage X-2: In stage 6-7 after reaching the first check-point flower 
           continue to the right until you reach a bazooka bee. 
           Bounce of its back to find a warp block.