Minecraft wiki entry for History, associated text: The next 16,384 bytes contains the skylight data. The format is identical to the previous "data" format. If a skylight value is f, that means the sunlight is fully lighting that block (like air or glass). The less sunlight is present in that block, the lower the value (like semi-transparent blocks such as leaves or water). And when the skylight value is 0, no sunlight is present in that block (like stone or dirt). Because the game automatically generates lighting, it is unimportant to modify this part of the file when generating a world via an outside program.