Overworld

The Overworld is the starting dimension in Minecraft. It is the only dimension that has existed throughout the entire development of Minecraft; The Nether was the second dimension to be added, in the Alpha 1.2.0 Halloween Update, and The End is another dimension that appeared for the first time in Beta 1.9 pre4.

The Overworld is the entire three dimensional space in which in-game construction can occur, encompassing the span from 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.

All but five of the mobs in the game can appear in the Overworld: Wolves, Pigs, Sheep, Cows, Mooshrooms, Chickens, Squid, Zombies, Ocelots, Skeletons, Spiders, Spider Jockeys, Endermen, Creepers and Slimes all spawn normally, and Pigs can, very rarely, be hit by lightning, which turns them into Zombie Pigmen. Also if Creepers get hit by lightning, they can turn into Charged Creepers, though this happens very rarely. Ghasts, Blazes and Magma Cubes can only spawn in the Nether, the Ender Dragon can only spawn in The End, and Giants are unspawnable.

History
Since Alpha 1.2.3, the key toggles a debug console which shows the player their exact coordinates.

As of Beta 1.3 the option was added for the player to specify the world seed.

In Beta 1.7, an 'f' value was added to the performance screen, indicating the direction the player is facing.

As of Beta 1.8, players can find the current map seed by pressing F3.

Since Beta 1.9 (Pre-Release 4), the Compass needle and the Clock will spin randomly within The End.

Since Snapshot 12w07a, new maps have a height of 256 thanks to the new Anvil level format.

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 of the overworld (and the Nether).

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 NPC Villages (perhaps for a surplus of wheat).

Players could find the current map seed by pressing.

As of snapshot 12w21a, the /seed command can also be used.

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 and not fall into the void, which in layman terms the very edge of the map as of 1.2.5 is X/Z: 30,000,000.

Beyond this edge, world generation is significantly simpler. It is interesting to note that if a person installs an x-ray mod, texture pack or just flies through with Creative Mode, he or she will notice that any chunk after the edge consists of just dirt, sand and stone 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. If a person reaches it normally, be sure to notice that the sea floor of the chunks after the edge consists of only dirt. Making it stand out, 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.

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 deep, and encompass 65,536 blocks. By adjusting the render distance, differing numbers of chunks will be loaded into memory, with "far" rendering the maximum of 81 chunks. 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, whatever has been changed will remain changed.

Biomes
Another subdivision of The Overworld is into Biomes. What biome you are in determines the physical aspects of the land from above ground and can entirely change its appearance. It also can determine which mobs spawn and the weather patterns. Biomes may have varying sizes, and each has its own features. For example, a forest biome will have large quantities of trees, where as a snow biome will have 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, the movement of the clouds, and the movement of the stars. Assuming that the sun and moon rise in the east and go west, the clouds can be observed moving west and stars east (not since 1.0.0, see below.)

Tools used for Navigating The Overworld include the Compass and Map. The Compass points to the original spawn point (sleeping in a bed does not change the compass operation), and the Map displays a fixed area around where it was made, and shows you the area very generically. In the Nether, the Compass needle will spin wildly, and thus is useless. The Compass needle and the Clock will spin randomly within The End, similar to the Nether.

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=64 corresponds to sea level. The 'f' value indicates the direction the player is facing, 0 being the direction in which the sun sets, and it goes counter clockwise from there: 1 being the direction in which the clouds move, 2 and 3 are opposite to 0 and 1 respectively.

Since the sun changed position relative to the clouds (they now move in the same direction) in 1.0.0, if you want to assume that the sun rises in the east and sets in the west, the 'f' values are: 1=west, 2=north, 3=east, and 0=south.

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 only a 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 accidentally 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 since he liked the idea of a mysterious world beyond a certain distance.

Ever since 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, but block physics do not work at that point and anyone who enters will fall into the Void, unless they are in Creative Mode in which case they can fly through the illusionary terrain.

Trivia

 * World seeds can be extracted from the level.dat file by various mods or tools, such as http://seedscope.net. Alternatively, it can be found in-game on the performance screen accessed by pressing F3.
 * There is a bug that sometimes causes the files for deleted worlds to not be deleted from your hard drive. If a new world is created with the same name, this can cause the game to reuse the old files, and thus, parts of the old map, rather than generate an entirely new map. On Windows, this can be replicated by having the saves folder in .minecraft open when you delete the save ingame.