So, I have a question...
     So, I decided to make a new, superflat, creative mode, world. But when I spawned, It spawned me in a snowy forest. I flew up just above the trees, and started flying forward. I passed a small desert, some rolling plains, and then another snowy forest. As I kept flying I saw an abrupt hault in the forest. I stopped flying and walked to the place where everything stopped appearing. Then as I looked down, I was really on a giant cliff- Going in a perfectly straight down drop. I flew one way down the massive wall, and there was a corner, I followed another wall-And then found a corner, and reapeting this process twice more, I ended up in the same spot I was in before. It was a giant cube, sitting on top of a superflat world. With normal biomes, caves just stopping at the edge, and water and lava flowing from it. So my question is: Has anyone had this happen before? And if so, what is it?

(If my description is unclear, see picture.)