Overworld

The Overworld is the starting dimension in Minecraft. As with all dimensions in the game, the Overworld can generate infinitely, however, the Overworld is limited to 862 by 862 blocks on the Xbox 360, PlayStation Vita and PlayStation 3 Edition, and 256 by 256 blocks on Old Pocket Edition worlds. At the edge of the Overworld, there is an animated world border. Nether 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 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 spawn 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 (zombie pigmen, magma cubes, wither skeletons, ghasts, and blazes) can use a nether portal to enter the Overworld. Mobs from the Overworld can enter the Nether in the same way. 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.

Map limitation


While the map is infinite, the number of blocks the player may walk on is limited. The map contains a world border at +/- 29,999,984 x/z. The world border 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 except by teleporting. There is nothing past 16 blocks after the wall, just emptiness.

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 3,686,400,000,000,000,000 bits (409.27261579781770706 Petabytes) in block data alone due to the fact that each standard block is assigned 4 bits of information. The total area of this is about 8 times the surface of the Earth.

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.

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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.

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. At night time the moon is the only natural light source. However, it provides little light, allowing hostile mobs to spawn.

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 510,900,000,000,000 blocks. Assuming the map is only one block deep and takes 1 byte/block, we get approximately 475,800 GB, or 464.7 TB. A Moon-sized map would be made of 37,950,000,000,000 blocks and would take 35,340 GB, or 34.52 TB.
 * 921.6 quadrillion (9.216x1017) blocks would fit in a Minecraft world assuming that it spreads from 30,000,000 to -30,000,000.