Chunk Properties

Chunks are stored in format, with this structure:

* The root tag.
	+ **Level:** Chunk data.
		- **xPos:** X position of the chunk.
		- **zPos:** Z position of the chunk.
		- **LastUpdate:** Tick when the chunk was last saved.
		- **LightPopulated:** 1 or 0 (true/false) indicates whether the chunk is properly lit or not. Changing this to 0 ( false ) may result in a full light update.
		- **TerrainPopulated:** 1 or not present (true/false) indicate whether the terrain in this chunk was populated with special things. (Ores, special blocks, trees, dungeons, flowers, waterfalls, etc.) If set to zero then Minecraft will regenerate these features in the blocks that already exist.
		- **V:** Currently always saved as 1 and not actually loaded by the game. Likely a chunk version tag.
		- **InhabitedTime:** The cumulative number of ticks players have been in this chunk. Note that this value increases faster when more players are in the chunk. Used for regional difficulty: increases the chances of mobs spawning with equipment, the chances of that equipment having enchantments, the chances of spiders having potion effects, the chances of mobs having the ability to pick up dropped items, and the chances of zombies having the ability to spawn other zombies when attacked. Note that at values 3600000 and above, regional difficulty is effectively maxed for this chunk. At values 0 and below, the difficulty is capped to a minimum (thus, if this is set to a negative number, it will behave identically to being set to 0, apart from taking time to build back up to the positives).
		- **Biomes:** May not exist. 256 bytes of biome data, one byte for each vertical column in the chunk. If this tag does not exist it will be created and filled by Minecraft when the chunk is loaded and saved. If any values in this array are -1, Minecraft will also set them to the correct biome.
		- **HeightMap:** 1024 bytes(256 TAG\_Int) of heightmap data. 16 × 16. Each byte records the lowest level in each column where the light from the sky is at full strength. Speeds computing of the SkyLight.
		- **Sections:** List of Compound tags, each tag is a sub-chunk of sorts.
			* An individual Section.
				+ **Y:** The Y index (not coordinate) of this section. Range 0 to 15 (bottom to top), with no duplicates but some sections may be missing if empty.
				+ **Blocks:** 4096 bytes of block IDs defining the terrain. 8 bits per block, plus the bits from the below Add tag.
				+ **Add:** May not exist. 2048 bytes of additional block ID data. The value to add to (combine with) the above block ID to form the true block ID in the range 0 to 4095. 4 bits per block. Combining is done by shifting this value to the left 8 bits and then adding it to the block ID from above.
				+ **Data:** 2048 bytes of block data additionally defining parts of the terrain. 4 bits per block.
				+ **BlockLight:** 2048 bytes recording the amount of block-emitted light in each block. Makes load times faster compared to recomputing at load time. 4 bits per block.
				+ **SkyLight:** 2048 bytes recording the amount of sunlight or moonlight hitting each block. 4 bits per block.
		- **Entities:** Each TAG\_Compound in this list defines an entity in the chunk. See Entity Format below. If this list is empty it will be a list of Byte tags.
		- **TileEntities:** Each TAG\_Compound in this list defines a tile entity in the chunk. See Tile Entity Format below. If this list is empty it will be a list of Byte tags.
		- **TileTicks:** May not exist. Each TAG\_Compound in this list is an "active" block in this chunk waiting to be updated. These are used to save the state of redstone machines, falling sand or water, and other activity.

Note: This page is almost purely copied from the main minecraft wiki, and it needs som serious editing.