Minecraft wiki entry for Updating_old_terrain, associated text: From time to time, minecraft world generation is changed to introduce new features. Some of these changes are terrain-breaking (like Java Edition 1.8), while others keep the terrain roughly the same in terms of altitude. In either case, you may want to clear out chunks of your old world, so that the new world-gen kicks in to give you the exciting stuff. This tutorial explains how to delete chunks in an older world for them to regenerate in a newer version, so that you won't need to travel thousands of blocks into unexplored terrain to see the new terrains if the player have a huge world. This action is commonly known as chunk resetting.