Minecraft wiki entry for Custom_maps, associated text: Use Dinnerbone's Chunk Coordinate Finder and input coordinates of a block in every chunk in intervals of 16. For example, find the chunk file at chunk number 0, 0 and then at chunk number 0, 32. Make sure to find the blocks in positive and negative chunks, too. Once you have a full list of chunks that you want to keep, make a backup of your current world. Once you've made your backup, delete any chunk files that are not in your list. For example, if your list contains the regions: