Anyone know what the hell this is?
 

K, when ure on the last stage(Helipad), and it shows ur turtle walking to it, notice on the far right of the screen theres still another section of that level. I dont know what u have to do to get to it. Anyone know what that "secret stage" is?

When you beat the game with all 4 turtles, you get to fight Saki in that level.

alot of things happen when you beat with all 4 turtles.....it opens up the challenge mode (non-gba versions). You get the chance to defeat Saki

sweet, thanks guys. I only have Don left to beat the game so i'll prolly do that sometime today. Does it have to be on hard/normal?

No, it works on easy as well.