So, remember when 1.7 first came out with all the new biomes? Well, if you have a singleplayer flat world, you may find what I found here... P.S. I did not make this, nor is it unloaded chunks.
I'm pretty sure its 1.7, because all of my flat worlds are like this around the chunks I never loaded.
It's an error with the chunk loading. If you had a superflat world, but never loaded chunks, it's reverting to the seed and turning into a normal world in those chunks, as it no longer recognizes it as superflat after an update.