Seed (level generation)

A seed is a code used in Minecraft's world generation process. Each world has its own seed value which is used to keep generated terrain consistent, as generation is pseudo-random. Prior to Beta 1.3, seeds were automatically generated per-world. Since then, the player can input their own seed, which is not limited to numbers due to the use of the Java function. For instance, a seed of "abc" is converted to the numeric value 97&times;312 + 98&times;311 + 99&times;310 = 96354.

Contrary to common belief, using a biome name (i.e. tundra, desert, forest, etc.) as the seed does NOT always result in the creation of a world with primarily that biome, nor is it guaranteed you will spawn in said biome when the map is generated. A great example of this is the seed Glacier. Seeds also determine the entire landscape, not just the area near the spawn.

Using the same seed to generate two separate worlds will generate the same terrain consistently, even on different systems. As such, seeds can be shared by players to generate the same world between them. The player's initial spawn usually is the same, but depends on computer, mods, and version.

If you leave the seed field blank, the game will use the time (computer clock) as a seed. Leaving the seed field blank will not produce the same map every time. Using seed 0 in singleplayer will also be ignored, as though it were left blank.

The seed will only influence the numbers returned by the terrain generator, and won't influence its underlying behavior. For instance, there is no random seed that will give you a world with lava instead of water, or an entirely desert world. Similarly, it is impossible to create a world that is one big cave.

It should be noted, however, that if you use words or phrases as a seed input, you'll be restricted to a subset of the possible worlds in Minecraft, due to the datatypes used for the seed and Java  function. You'll be able to access around 4 billion possible worlds by using string input. Using pure numbers or letting Minecraft generate the world will give you access to the full set of possible worlds, which is around 18 quintilion. See the discussion page for details. However, it should be noted that, even if you explored 1000 seeds a day, it would still take you 10,960 years before you used all the seeds.

Since the 1.8.1 update the world seed is viewable when the debug screen is toggled on, but only as a numeric value. It has also been confirmed that seeds for pre-1.8 worlds (such as Glacier or gargamel) will not generate the same land in 1.8 and later versions of Minecraft as the terrain generator code has been changed in the update.

Future updates
Future updates complete the seed level making it possible to see some new added biomes.