I'm making an adventure map and I really need to know how I can turn a large area into an island. pretty much, I want to explore around and uncover a good amount of area and biomes, then somehow turn what I have uncovered into an island. I'm trying to do with with as little "fixing" as possible. (flat lines where chuncks were deleted and such)
Use a flatland preset for clay, go into it, place down 1 bedrock, then log out. After that change to version 1.5.2, and all the clay would be gone, just the bedrock sitting there. Use MCedit to make a massive island.