Overworld

The Overworld is the starting dimension in Minecraft. As with all dimensions in the game, the Overworld can generate infinitely. The Overworld is limited to 862 by 862 blocks on the Xbox 360 Edition, and 256 by 256 blocks on Old Pocket Edition worlds. At the edge of the Overworld, there is an animated world border. Portals in the Overworld can be used to teleport to the Nether.

The Overworld encompasses the level ceiling down to bedrock and extending in every direction on the horizontal plane. It is generated through a secret process which creates multiple noise maps to create differing elevations, general chunk shapes, and complex mountain and cave systems.

Most mobs in the game can appear in the Overworld: Wolves, Bats, Pigs, Sheep, Cows, Horses, Mooshrooms, Chickens, Chicken Jockeys, Squid, Zombies, Ocelots, Skeletons, Spiders, Villagers, Spider Jockeys, Witches, Endermen, Creepers and Slimes all spawn normally. Zombie Pigmen can also spawn near Nether portals.

Mobs from the Nether may also pass through Nether Portals and enter the Overworld: Zombie Pigmen, Magma Cubes, Wither Skeletons, Ghasts, and Blazes (mobs from the Overworld can also enter the Nether through a Nether Portal). The Ender Dragon however can't enter the Overworld because there is no return portal from the End unless the dragon is defeated.

Seeds
Worlds in Minecraft are generated through a procedural formula that takes a random number as a starting point - a seed, and it will be used to generate all the terrain.

Using a specific seed generates exactly the same world each time, and thus interesting Minecraft worlds could be shared between players. The spawn is not on exactly the same spot, though, so it is wise to give coordinates instead of directions. The seed input is converted into an integer, so, for instance, the word 'Glacier' corresponds to a value of 1772835215, which generates exactly the same world when entered as a seed value.

Many seeds are chosen simply because they spawn the player near desirable resources, a stronghold or some certain special structure, such as a dungeon. Players often like to find seeds with other very important generated structures such as abandoned mine shafts (possibly for the melon seeds and cobwebs) and villages (perhaps for a surplus of wheat, potatoes, and/or carrots, and for the open possibility of trading with villagers.). While Strongholds have a minimum distance from the origin, a seed could also be chosen for a relatively close one. Seeds can also be chosen for useful biomes near the spawn point; notably, a Mushroom Island biome provides a large sanctuary from monsters, even underground. Other particularly useful biomes include Jungle (wood, cats), Desert (NPC villages, sand, open line-of-sight), Mesa (stained clay), Taiga (wolves, snow), Swampland (aboveground slimes), and Extreme Hills (emerald ores). Players can find the current map seed by entering the /seed command.

Sizes
In Classic and Indev, maps can be generated in "small" (128×128×64), "normal" (256×256×64), and "huge" (512×512×64) sizes.

In Infdev, Alpha and Beta, maps are somewhat infinitely big - They are made up of chunks; this means that as the player explores the map distant chunks are generated automatically, leading to theoretically infinite maps. In practice, technical reasons (the limits of 32-bit math ) force the maximum map size, including the Far Lands, to be around 9.3 million times the surface area of Earth, which comes out to about 4.7 quadrillion km2 (The hard limit where chunks are overwritten is at X/Z of ±34,359,738,368, making the world at most 68,719,476,736 meters wide and long, which is about 4,722,366,482,869,645 km2. When compared to Earth's total surface area, 510,072,000 km2, this works out to be about 9,258,235 times that). Whilst the horizontal planes of the maps are vast in size, the vertical plane remains at a fixed (soft limit) 256-block height.

Map limitation


While the map is infinite, the number of blocks the player may walk on is limited. The map, counting air as a block, and not counting blocks beyond and at where block physics fail (32,000,000 from the center) from top to bottom, and all in between, is essentially always 2.62144 × 1017 blocks big. The last point a person may still play normally, which in layman terms the very edge of the map is X/Z: 30,000,000. The player can't move past those coordinates, though in older versions of the game, the player will just fall through the blocks into The Void, but could keep exploring if flying in Creative Mode. There is also a height limit of 255.

Beyond this edge, world generation is significantly simpler. Any chunk after the edge consists of just dirt, stone, sand, gravel and water with absolutely no vegetation. Caves do generate here, but no ores or structures appear inside, and the lighting in some of these caves may appear to make them look brighter with more contrast. Another point to note is that if a person stands on the very border of the edge, the ocean will look empty and 'seemingly' be able to see as though there was no water.

Because of these limitations the maximum blocks that can be generated in a world is approximately 921,600,000,000,000,000. This means that a filled world with no entities or tile entities would be 3686400000000000000 bits (409.27261579781770706 Petabytes) in block data alone due to the fact that each standard block is assigned 4 bits of information.

EDIT: At just under 30,000,000 there is an animated wall of blue stripes. As you get near it, the edges of your screen turn red, and you can not go past it. There is nothing past maybe 10 blocks after the wall, just emptiness. This occurs even if there is no ocean to speak of where you are.

Generated blocks
Entries marked with a D require additional data to fully define the block in a Beta world. Entries marked with an I have a different ID as an inventory item. Entries marked with a B require additional inventory data to fully define the inventory item. Entries marked with a T have tile entities associated with them to store additional data. Blocks with IDs in red cannot be legitimately obtained in the player's inventory in the game; they can only be obtained by "hacking" (such as the use of Inventory editors) or, in multiplayer using the /give server command. Blocks available only in Creative mode are in blue, and blocks available in Survival mode only with the use of a Silk Touch-enchanted tool are in green.

Naturally generated
Naturally Generated includes blocks that are created through the world seed.

Naturally created
Naturally created means a combination of events that cause a new block to be placed by natural causes, not the player. Some of these blocks may also be created as part of world generation.

Structures
Same as naturally generated, but these blocks are only created with the "Generate Structures" option enabled.

Chunks
Chunks are the method used by Notch to divide maps into manageable pieces. They are 16 blocks wide, 16 blocks long, and 256 blocks high, each containing 65,536 blocks. By adjusting the render distance, differing numbers of chunks will be loaded into memory, ranging from 25 to 1089. Only chunks which have been loaded may experience activity such as spawning, despawning, growth, fluid movement, or player interaction. Upon reaching the required distance away from a chunk, it will be unloaded from the memory, however not deleted. Thus, upon re-entering that area, Minecraft will reload the chunk(s).

Biomes
The Overworld is also subdivided into biomes. What biome you are in determines the physical aspects of the land above ground and can entirely change its appearance. It also influences which mobs may spawn and affects the behavior of the weather. Biomes may have varying sizes, and each has its own features. For example, a forest biome will have large quantities of trees, and a snow biome will have much snow and ice.

Natural structures
The Overworld is able to seamlessly create new areas by using patterns found in the surrounding chunks and extend those into the newly created chunk. These patterns, while unique to each world, can be categorized easily by comparing them to a real-world equivalent, such as cliffs or oceans. While Minecraft is based on landforms found on earth, impossible formations, such as floating islands, can be found throughout the Overworld.

Navigating the Overworld
The Overworld can be navigated by use of the position of the sun and the moon, and the movement of the clouds and the stars. All of these rise in the east and go west.

Tools used for navigating the Overworld include the Compass and Map. The compass points to the world spawn point (sleeping in a bed does not change the compass operation), and the Map displays an area around where it was made, and shows an overview at various scales depending on its zoom factor. In the Nether and The End, maps are much less useful.

The key toggles a debug console which shows the player their absolute coordinates, where the X and Z coordinates show longitude and latitude, and the Y coordinate shows height, where Y=63 corresponds to sea level. The 'f' value indicates the direction the player is facing.

Day and night
The Overworld is the only dimension with a day/night cycle. During the daytime, the sun acts as a light source. This light is strong enough to kill Zombies, and Skeletons, make Endermen teleport away from the player and also makes Spiders neutral. The only mobs that survive and stay hostile in the daylight are creepers and slimes. At night time the moon is the only natural light source. However, it provides little light, allowing hostile mobs to spawn.

Far Lands


The Far Lands used to be an area that formed the "edge" of the "infinite" map, but was later removed from a patch on Beta 1.8 due to the change in the terrain generation code. When players made it to the Far Lands, they experienced an excessive amount of lag and the world became severely distorted.

According to Notch, this distortion could be fixed, but since no one was likely to make it to the Far Lands without some form of cheating, he says that he was likely not going to fix it.

In Beta 1.8, instead of a distorted land, there is a seemingly endless ocean. Normal terrain only starts to generate beyond the actual edges of the world.

As of 1.7.2, the player can not go past 30,000,000 in any direction. This was added by placing an Invisible Bedrock-like wall at 30,000,000 blocks from the center of the map in each direction.

History
In addition to this, as new features get added, they will change Overworld generation.

Trivia

 * If someone made a 1:1 (1 block = 1 meter) scale version of the Earth, its area would be 510,000,000,000,000 blocks. Assuming the map is only one block deep and takes 1 byte/block, we get approximately 475,000GB, or 464TB.
 * 461 quadrillion (4.608x1017) blocks would fit in a Minecraft world assuming that it spreads from 30,000,000 to -30,000,000.
 * A Minecraft world is theoretically bigger than the earth, but in practice, an Earth-sized map would be large enough to overwhelm almost any consumer-grade computer. Indeed, nearly all structures within Minecraft, such as strongholds, villages, and mountains, are much smaller than a realistic version would be. The same applies even on larger scales such as biomes, landmasses, oceans, and the vertical space between bedrock and the "sky" (approximated by the build limit).