I generated an AMPLIFIED world in creative. Then I was a little bit flying around and suddenly I found a square hole. It goes until level 58.
I'm able to break and place blocks, go around and all. It's like a normal part of the world.
Then I duplicated the world and went to the coordinates I foundet the hole in the first world - but there wasn't any hole. So it seems to be an error of generating the world.
I attached a picture...