In order to unlock the hidden game of ZSB, you must make it to the reverse gravity planets on Thrust.
Level 7 extra fuelEdit
On level 7 of the Hard+ mode, it appears to be a very small level with no fuel whatsoever. However, one the way to the cavern with the Klystron pod, you need to shoot the button to open the doors so you can descend down, then right and down again in order to access the cavern.
Upon your first descent, wait for the door to close, then shoot the button to open it again. Go right past the cavern. That door will have opened as well, leading to several fuel tanks.