Overworld

The Overworld is the starting dimension in Minecraft. It is the only dimension that has existed throughout the development of Minecraft; The Nether was the second dimension to be added, in Alpha 1.2.0, and the Sky Dimension is confirmed but not yet implemented.

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 one of the mobs in the game can appear in the Overworld: Wolves, Pigs, Sheep, Cows, Chickens, Squid, Zombies, Skeletons, Spiders, Spider Jockeys, Creepers and Slimes all spawn normally, and Pigs can be hit by lightning, which turns them into Zombie pigmen however, it is very rare for lightning to strike a pig. Ghasts can only spawn in the Nether. (However, in very rare occasions they can spawn out of a portal; this is intentional although Jeb claims they have not implemented it yet).

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).

In Beta 1.3 the option was added for the player to specify the world seed. 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 co-ordinates 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.

Sizes
In Classic and Indev, maps can be generated in "small" (128x128x64), "normal" (256x256x64), and "huge" (512x512x64) 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 integers ) force the maximum map size to around 1.8 times the surface area of the Earth. Whilst the horizontal planes of the maps are vast in size, the vertical plane remains at a fixed 128-block height.

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(32000000 from the center) from top to bottom, and all in between, is essentially always 131,071,992,000,000,000 blocks big. In multiplayer, chunks will also stop generating after the world reaches a certain file size.

Chunks
Chunks are the method used by Notch to divide maps into manageable pieces. They are 16 blocks wide, 16 blocks long, and 128 blocks deep, and encompass 32,768 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, liquid 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. 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 north and go south, the clouds can be observed moving west and stars north. However, many players prefer to assume that the sun and moon rise in the east and move west, like in the real world; This means clouds move north and stars move west.

Tools used for Navigating The Overworld include the Compass and Map. The Compass points to the spawn point, and the Map shows 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 is thus useless.

Since Alpha 1.2.3, 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. In addition, in Beta 1.7 an 'f' value was added, which 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.

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 and also makes Spiders neutral. The only hostile mob that survives in the daylight are creepers. 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 forms the "edge" of the "infinite" map, but was later removed accidentally from a patch on 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 map was 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.

The far lands may be brought back in a later patch, but it is unlikely.

Trivia

 * World seeds can be extracted from the level.dat file by various modes or seeds, 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.