You can make the world as big as you want. Where the code says GreenfootImage back = new GreenfootImage("Cave.png"); Just replace Cave.png to whatever the name of your file is.

Haha! I have actually thought of doing something like that. Where would the camera be though? Would it be like a first person shooter, or would the player still view the game from above?

You store pretty much everything as static variables, which will not act properly after resetting the game. Just change the score and background music to non-static variables, and you should be good.

Haha! I am currently working on a first person shooter. The dark mode only shows about twenty blocks in each direction, which means the overall world you are traveling through can be quite large without lagging the game. Anyway, It is going to be awesome!