Minecraft wiki entry for Terrain_features, associated text: Features are generated for a given chunk after the terrain has been formed. The chunk format includes a tag called TerrainPopulated that indicates whether features whose point of origin is in that chunk have been generated. If it is false or missing, they generate again. Feature generation is based on what is already in the chunk, so (for example) flagging a chunk that has already been populated for repopulation approximately doubles the amount of ore in it.