Every Chunk in Minecraft is 16 blocks long, 16 blocks wide, and 384 blocks high. This means that each Chunk is composed of 98,304 blocks total—including blocks of air. Since the bulk of a Chunk is vertical, you can traverse them quickly horizontally.
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Chunk Size and Spawning
When you spawn into a new world, Chunks will load around you. As you explore, new Chunks will be loaded to accommodate you, however, not all uncovered Chunks will remain loaded. The distance from your spawn point in a world to one of its 4 edges is something like 30 million blocks. It would be impossible for the game to keep that much information loaded at once. And so, as you adventure, Chunks you get far away from will unload. They will still be there, and will reload the same (except for most mobs), but will not be physically present. This saves the game from having to dedicate resources to unnecessary assets.
The Vertical Distance of a Chunk
Each Chunk in Minecraft extends from the lowest layer up to the highest layer of the world. These are the vertical limits that you can potentially reach. The lowest layer is where Bedrock is and can be known by different terms. Some people call it “layer 0” or “layer 1” while others refer to it by its Y coordinate “layer -64”. The -64 comes from the lowest layers being about 64 blocks below sea level.
On the opposite end, the highest point of a Chunk is 320 blocks above sea level, most often referred to as the “build limit”. It is named so due to the fact that you cannot place blocks above “layer 320”.
About Spawn Chunks
There is an exception to the Chunk loading and unloading mechanic. In order to ensure that you always have a place to respawn, the game will always keep a group of Chunks around your world spawn point loaded. These Spawn Chunks are a 19 by 19 square of Chunks that generate around your world spawn point.
If something happens to your Bed or another respawn point, you’ll be able to reappear at a designated location. If the game didn’t do this, it’s possible that you could respawn in an unloaded Chunk, which would involve you infinitely falling through the world.
If you have the seed number for a world, you create a new world that mirrors an old one. All of the things you’ve built in the old world will not be carried over, however, everything else will be the same. For example, all Nether Fortresses and Ocean Monuments will appear in the same places.
How Map Seeds Affect Chunk Generation
The only parameters that would change how generating an old seed works are the map generator and the version number of the game. You can set the map generation from the settings menu before creating a new world. The version number simply means what update of the game you’re on. Playing a game on a new update may prevent old worlds from spawning as they did before.
You can use this mechanic to get a fresh start in a world that you are particularly fond of. Knowing where everything is would also make it feel familiar, and probably quite comfy. You could visit your favorite mushroom forests and beaches as long as you have the same world seed. Thankfully, you can’t accidentally feed this seed to any Chickens.
Events in Loaded Chunks
Certain things can only happen in loaded Chunks. These events usually involve mob spawning of all kinds, as creatures cannot exist in unloaded Chunks. Even tamed mobs like your pet Cat and your Horses, as well as named mobs, will not stay generated. However, due to them being flagged by the game thanks to their tagging or naming, they will reload with the Chunk they are in.
However, in worlds where the Chunk spawning is slow, tamed mobs can get temporarily trapped in unloaded Chunks. If you notice that one of your pets is missing, retrace your steps and reenter previously-explored Chunks. This can reload a space with your pet and allow them to teleport back to you (unless they were told to sit, in which case they will load but not teleport back to your side).
Farming also requires you to be nearby, as crops can only grow in loaded Chunks. This is important for planning where you will build your farms. A good choice would be to build farms above where you usually go mining. No matter how deep you go, since Chunks are vertically oriented, a farm that is approximately above you will always stay loaded. Therefore, your crops will continue to grow as you do other things.